// Copyright (c) Microsoft Corporation.
// Licensed under the MIT License.

use crate::error::Error;
use crate::storage::VmgsStorage;
#[cfg(with_encryption)]
use anyhow::anyhow;
#[cfg(with_encryption)]
use anyhow::Context;
use disk_backend::Disk;
#[cfg(feature = "inspect")]
use inspect::Inspect;
#[cfg(feature = "inspect")]
use inspect_counters::Counter;
use std::collections::HashMap;
use std::num::NonZeroU32;
use vmgs_format::EncryptionAlgorithm;
use vmgs_format::FileAttribute;
use vmgs_format::FileId;
use vmgs_format::VmgsAuthTag;
use vmgs_format::VmgsDatastoreKey;
use vmgs_format::VmgsEncryptionKey;
use vmgs_format::VmgsExtendedFileTable;
use vmgs_format::VmgsFileTable;
use vmgs_format::VmgsHeader;
use vmgs_format::VmgsNonce;
use vmgs_format::VMGS_BYTES_PER_BLOCK;
use vmgs_format::VMGS_EXTENDED_FILE_TABLE_BLOCK_SIZE;
use vmgs_format::VMGS_FILE_TABLE_BLOCK_SIZE;
use vmgs_format::VMGS_MIN_FILE_BLOCK_OFFSET;
use vmgs_format::VMGS_SIGNATURE;
use vmgs_format::VMGS_VERSION_3_0;
use zerocopy::AsBytes;
use zerocopy::FromBytes;
use zerocopy::FromZeroes;

/// Info about a specific VMGS file.
#[derive(Debug)]
pub struct VmgsFileInfo {
    /// Number of bytes allocated in the file.
    pub allocated_bytes: u64,
    /// Number of valid bytes in the file.
    pub valid_bytes: u64,
}

// Aggregates fully validated data from the FILE_TABLE and EXTENDED_FILE_TABLE
// control blocks.
#[derive(Clone, Copy, PartialEq, Eq, Debug)]
#[cfg_attr(feature = "inspect", derive(Inspect))]
struct ResolvedFileControlBlock {
    // FILE_TABLE data
    // ---------------
    block_offset: u32,
    #[cfg_attr(feature = "inspect", inspect(with = "|x| x.get()"))]
    allocated_blocks: NonZeroU32,
    valid_bytes: u64,

    nonce: VmgsNonce,
    authentication_tag: VmgsAuthTag,

    // EXTENDED_FILE_TABLE data
    // ---------------
    attributes: FileAttribute,
    encryption_key: VmgsDatastoreKey,
}

/// Implementation of the VMGS file format, backed by a generic [`Disk`]
/// device.
#[cfg_attr(not(with_encryption), allow(dead_code))]
#[cfg_attr(feature = "inspect", derive(Inspect))]
pub struct Vmgs {
    storage: VmgsStorage,

    #[cfg(feature = "inspect")]
    stats: vmgs_inspect::VmgsStats,

    active_header_index: usize,
    active_header_sequence_number: u32,
    version: u32,
    #[cfg_attr(feature = "inspect", inspect(with = "vmgs_inspect::fcbs"))]
    fcbs: HashMap<FileId, ResolvedFileControlBlock>,
    encryption_algorithm: EncryptionAlgorithm,
    datastore_key_count: u8,
    active_datastore_key_index: Option<usize>,
    #[cfg_attr(feature = "inspect", inspect(iter_by_index))]
    datastore_keys: [VmgsDatastoreKey; 2],
    metadata_key: VmgsDatastoreKey,
    #[cfg_attr(feature = "inspect", inspect(iter_by_index))]
    encrypted_metadata_keys: [VmgsEncryptionKey; 2],
}

#[cfg(feature = "inspect")]
mod vmgs_inspect {
    use super::*;

    #[derive(Default)]
    pub struct IoStat {
        pub attempt: Counter,
        pub resolved: Counter,
    }

    // explicit inspect implementation, since we want to massage the data's
    // presentation a bit
    impl Inspect for IoStat {
        fn inspect(&self, req: inspect::Request<'_>) {
            let mut resp = req.respond();
            resp.counter("ok", self.resolved.get())
                .counter("err", self.attempt.get() - self.resolved.get());
        }
    }

    #[derive(Inspect, Default)]
    pub struct VmgsStats {
        #[inspect(with = "stat_map")]
        pub read: HashMap<FileId, IoStat>,
        #[inspect(with = "stat_map")]
        pub write: HashMap<FileId, IoStat>,
    }

    pub(super) fn fcbs(fcbs: &HashMap<FileId, ResolvedFileControlBlock>) -> impl Inspect + '_ {
        inspect::adhoc(|req| {
            let mut res = req.respond();
            for (id, fcb) in fcbs.iter() {
                res.field(&format!("{}-{:?}", id.0, id), fcb);
            }
        })
    }

    pub fn stat_map(map: &HashMap<FileId, IoStat>) -> impl Inspect + '_ {
        inspect::iter_by_key(map).map_key(|x| format!("{:?}", x))
    }
}

impl Vmgs {
    /// Format and open a new VMGS file.
    pub async fn format_new(disk: Disk) -> Result<Self, Error> {
        let mut storage = VmgsStorage::new(disk);
        tracing::debug!("formatting and initializing VMGS datastore");
        // Errors from validate_file are fatal, as they involve invalid device metadata
        Vmgs::validate_file(&storage)?;

        let active_header = Self::format(&mut storage, VMGS_VERSION_3_0).await?;

        Self::finish_open(storage, active_header, 0).await
    }

    /// Open the VMGS file.
    pub async fn open(disk: Disk) -> Result<Self, Error> {
        tracing::debug!("opening VMGS datastore");
        let mut storage = VmgsStorage::new(disk);
        // Errors from validate_file are fatal, as they involve invalid device metadata
        Vmgs::validate_file(&storage)?;

        let (header_1, header_2) = read_headers_inner(&mut storage).await?;

        let empty_header = VmgsHeader::new_zeroed();

        if header_1.as_bytes() == empty_header.as_bytes()
            && header_2.as_bytes() == empty_header.as_bytes()
        {
            return Err(Error::EmptyFile);
        }

        let active_header_index =
            get_active_header(validate_header(&header_1), validate_header(&header_2))?;

        let active_header = if active_header_index == 0 {
            header_1
        } else {
            header_2
        };

        Self::finish_open(storage, active_header, active_header_index).await
    }

    async fn finish_open(
        mut storage: VmgsStorage,
        active_header: VmgsHeader,
        active_header_index: usize,
    ) -> Result<Vmgs, Error> {
        let version = active_header.version;
        let (encryption_algorithm, encrypted_metadata_keys, datastore_key_count) =
            if version >= VMGS_VERSION_3_0 {
                let encryption_algorithm =
                    if active_header.encryption_algorithm == EncryptionAlgorithm::AES_GCM {
                        EncryptionAlgorithm::AES_GCM
                    } else {
                        EncryptionAlgorithm::NONE
                    };
                let encrypted_metadata_keys = active_header.metadata_keys;

                let is_key_zero_empty = is_empty_key(&encrypted_metadata_keys[0].encryption_key);
                let is_key_one_empty = is_empty_key(&encrypted_metadata_keys[1].encryption_key);
                let datastore_key_count = {
                    if is_key_zero_empty && is_key_one_empty {
                        0
                    } else if !is_key_zero_empty && !is_key_one_empty {
                        encrypted_metadata_keys.len() as u8
                    } else {
                        1
                    }
                };
                (
                    encryption_algorithm,
                    active_header.metadata_keys,
                    datastore_key_count,
                )
            } else {
                (
                    EncryptionAlgorithm::NONE,
                    [VmgsEncryptionKey::new_zeroed(); 2],
                    0,
                )
            };

        // Read the file table and initialize the internal file metadata.
        let file_table_size_bytes = block_count_to_byte_count(active_header.file_table_size);
        let file_table_offset_bytes = block_count_to_byte_count(active_header.file_table_offset);

        let mut file_table_buffer = vec![0; file_table_size_bytes as usize];

        if let Err(e) = storage
            .read_block(file_table_offset_bytes, file_table_buffer.as_mut_slice())
            .await
        {
            return Err(Error::CorruptFormat(format!(
                "Error reading file table: {:?}",
                e
            )));
        }

        let file_table = VmgsFileTable::ref_from_prefix(&file_table_buffer).unwrap();

        let file_control_blocks =
            initialize_file_metadata(file_table, version, storage.block_capacity())?;

        Ok(Self {
            storage,

            active_header_index,
            active_header_sequence_number: active_header.sequence,
            version,
            fcbs: file_control_blocks,
            encryption_algorithm,
            datastore_key_count,
            active_datastore_key_index: None,
            datastore_keys: [VmgsDatastoreKey::new_zeroed(); 2],
            metadata_key: VmgsDatastoreKey::new_zeroed(),
            encrypted_metadata_keys,

            #[cfg(feature = "inspect")]
            stats: Default::default(),
        })
    }

    /// Formats the backing store with initial metadata, and sets active header.
    async fn format(storage: &mut VmgsStorage, version: u32) -> Result<VmgsHeader, Error> {
        tracing::info!("Formatting new VMGS file.");
        let aligned_header_size = round_up_count(size_of::<VmgsHeader>(), storage.sector_size());

        // The second header is initialized as invalid (all zeros).
        let mut header = VmgsHeader::new_zeroed();

        storage
            .write_block(aligned_header_size, header.as_bytes())
            .await
            .map_err(Error::WriteDisk)?;

        // Write an empty file table at min offset. All entries are zeroed except
        // for the first one, which is the file table itself
        let mut file_table = VmgsFileTable::new_zeroed();
        file_table.entries[FileId::FILE_TABLE].offset = VMGS_MIN_FILE_BLOCK_OFFSET;
        file_table.entries[FileId::FILE_TABLE].allocation_size = VMGS_FILE_TABLE_BLOCK_SIZE;
        file_table.entries[FileId::FILE_TABLE].valid_data_size =
            block_count_to_byte_count(VMGS_FILE_TABLE_BLOCK_SIZE);
        if version >= VMGS_VERSION_3_0 {
            file_table.entries[FileId::EXTENDED_FILE_TABLE].offset =
                VMGS_MIN_FILE_BLOCK_OFFSET + VMGS_FILE_TABLE_BLOCK_SIZE;
            file_table.entries[FileId::EXTENDED_FILE_TABLE].allocation_size =
                VMGS_EXTENDED_FILE_TABLE_BLOCK_SIZE;
            file_table.entries[FileId::EXTENDED_FILE_TABLE].valid_data_size =
                block_count_to_byte_count(VMGS_EXTENDED_FILE_TABLE_BLOCK_SIZE);
        }

        storage
            .write_block(
                block_count_to_byte_count(VMGS_MIN_FILE_BLOCK_OFFSET),
                file_table.as_bytes(),
            )
            .await
            .map_err(Error::WriteDisk)?;

        initialize_file_metadata(&file_table, VMGS_VERSION_3_0, storage.block_capacity())?;

        // Write an empty extended file table if the datastore supports V3.
        if version >= VMGS_VERSION_3_0 {
            let extended_file_table = VmgsExtendedFileTable::new_zeroed();
            storage
                .write_block(
                    block_count_to_byte_count(
                        VMGS_MIN_FILE_BLOCK_OFFSET + VMGS_FILE_TABLE_BLOCK_SIZE,
                    ),
                    extended_file_table.as_bytes(),
                )
                .await
                .map_err(Error::WriteDisk)?;
        }

        // Write the first header as the valid header
        header.signature = VMGS_SIGNATURE;
        header.version = VMGS_VERSION_3_0;
        header.sequence = 1;
        header.header_size = size_of::<VmgsHeader>() as u32;
        header.file_table_offset = VMGS_MIN_FILE_BLOCK_OFFSET;
        header.file_table_size = VMGS_FILE_TABLE_BLOCK_SIZE;
        header.checksum = compute_crc32(header.as_bytes());
        header.encryption_algorithm = EncryptionAlgorithm::NONE;

        storage
            .write_block(0, header.as_bytes())
            .await
            .map_err(Error::WriteDisk)?;

        // Flush the device to persist changes
        storage.flush().await.map_err(Error::FlushDisk)?;

        Ok(header)
    }

    fn validate_file(storage: &VmgsStorage) -> Result<(), Error> {
        let sector_count = storage.sector_count();
        let sector_size = storage.sector_size();

        // Don't need to parse MBR/GPT table, VMGS uses RAW file format

        // Validate capacity and max transfer size. This also enesures that there are no arithmetic
        // overflows when converting from sector counts to byte counts.
        if sector_count == 0 || sector_count > u64::MAX / 4096 {
            return Err(Error::Initialization(format!(
                "Invalid sector count of {}",
                sector_count,
            )));
        }

        // Any power-of-2 sector size up to 4096 bytes works, but in practice only 512 and 4096
        // indicate a supported (tested) device configuration.
        if sector_size != 512 && sector_size != 4096 {
            return Err(Error::Initialization(format!(
                "Invalid sector size {}",
                sector_size
            )));
        }

        Ok(())
    }

    /// Get allocated and valid bytes from File Control Block for file_id.
    ///
    /// When reading data from a file, the buffer must be at least `valid_bytes` long.
    pub fn get_file_info(&self, file_id: FileId) -> Result<VmgsFileInfo, Error> {
        let fcb = self.fcbs.get(&file_id).ok_or(Error::FileInfoAllocated)?;

        Ok(VmgsFileInfo {
            allocated_bytes: block_count_to_byte_count(fcb.allocated_blocks.get()),
            valid_bytes: fcb.valid_bytes,
        })
    }

    /// maps out the used/unused space in the file and finds the smallest unused space to allocate new data.
    /// Appends the newly allocated FileControlBlock to the end of temp_fcbs.
    ///
    /// # Arguments
    ///
    /// * 'block_count' - allocated_blocks to allocate for new FCB
    fn allocate_space(
        &mut self,
        block_count: u32,
        temp_fcbs: &mut Vec<ResolvedFileControlBlock>,
        valid_bytes: u64,
    ) -> Result<(), Error> {
        struct AllocationBlock {
            block_offset: u32,
            allocated_blocks: u32,
        }

        if block_count == 0 {
            return Err(Error::AllocateZero);
        }
        // map out file offets/sizes to see what space is unused
        let mut allocation_list = Vec::new();
        for (_, fcb) in self.fcbs.iter() {
            allocation_list.push(AllocationBlock {
                block_offset: fcb.block_offset,
                allocated_blocks: fcb.allocated_blocks.get(),
            });
        }

        for temp_fcb in temp_fcbs.iter() {
            allocation_list.push(AllocationBlock {
                block_offset: temp_fcb.block_offset,
                allocated_blocks: temp_fcb.allocated_blocks.get(),
            });
        }
        // TODO: this will get removed when allocation_list is re-written
        // sort by block offset
        allocation_list.sort_by_key(|a| a.block_offset);

        let mut best_offset = 0;
        let mut best_free_count = 0;
        let mut last_allocation_end_offset = VMGS_MIN_FILE_BLOCK_OFFSET;
        let mut found = false;

        // find smallest set of blocks that will fit the data we're allocating
        for fcb in allocation_list.iter() {
            if fcb.block_offset < last_allocation_end_offset {
                return Err(Error::AllocateOffset);
            }
            let free_count = fcb.block_offset - last_allocation_end_offset;
            if free_count >= block_count && (best_free_count == 0 || free_count < best_free_count) {
                best_free_count = free_count;
                best_offset = last_allocation_end_offset;
                found = true;
            }
            last_allocation_end_offset = fcb.block_offset + fcb.allocated_blocks;
        }
        if last_allocation_end_offset < self.storage.block_capacity() {
            let free_count = self.storage.block_capacity() - last_allocation_end_offset;
            if free_count >= block_count && (best_free_count == 0 || free_count < best_free_count) {
                best_offset = last_allocation_end_offset;
                found = true;
            }
        }
        if !found {
            return Err(Error::InsufficientResources);
        }
        let new_fcb = ResolvedFileControlBlock {
            block_offset: best_offset,
            allocated_blocks: NonZeroU32::new(block_count).unwrap(),
            valid_bytes,
            attributes: FileAttribute::new(),
            nonce: VmgsNonce::new_zeroed(),
            authentication_tag: VmgsAuthTag::new_zeroed(),
            encryption_key: VmgsDatastoreKey::new_zeroed(),
        };
        temp_fcbs.push(new_fcb);

        Ok(())
    }

    /// Writes `buf` to file, then updates file table to point to updated data.
    async fn write_file_internal(
        &mut self,
        file_id: FileId,
        buf: &[u8],
        file_table_fcb: &mut ResolvedFileControlBlock,
        data_fcb: &mut ResolvedFileControlBlock,
        should_encrypt: bool,
        should_write_file_table: bool,
    ) -> Result<(), Error> {
        let data_nonce_auth_tag = if should_encrypt {
            let data_encryption_key = {
                let mut encryption_key = VmgsDatastoreKey::new_zeroed();
                getrandom::getrandom(&mut encryption_key).expect("rng failure");
                encryption_key
            };
            let data_nonce = generate_nonce();
            let mut data_auth_tag = VmgsAuthTag::new_zeroed();

            self.write_encrypted_data(
                data_fcb.block_offset,
                &data_encryption_key,
                &data_nonce,
                buf,
                &mut data_auth_tag,
            )
            .await?;

            // Update the data file control block.
            data_fcb.nonce.copy_from_slice(&data_nonce);
            data_fcb
                .encryption_key
                .copy_from_slice(&data_encryption_key);
            data_fcb.authentication_tag.copy_from_slice(&data_auth_tag);
            Some((data_nonce, data_auth_tag))
        } else {
            // Write the file contents to the newly allocated space
            self.storage
                .write_block(block_count_to_byte_count(data_fcb.block_offset), buf)
                .await
                .map_err(Error::WriteDisk)?;
            None
        };

        // Initialize the new file table with current metadata for all files.
        let mut new_file_table = VmgsFileTable::new_zeroed();
        for (file_id, fcb) in self.fcbs.iter() {
            let new_file_entry = &mut new_file_table.entries[*file_id];

            new_file_entry.offset = fcb.block_offset;
            new_file_entry.allocation_size = fcb.allocated_blocks.get();
            new_file_entry.valid_data_size = fcb.valid_bytes;

            if self.version >= VMGS_VERSION_3_0 {
                new_file_entry.nonce.copy_from_slice(&fcb.nonce);
                new_file_entry
                    .authentication_tag
                    .copy_from_slice(&fcb.authentication_tag);
            }
        }

        // Fill in the metadata for the file being written.
        let file_entry = &mut new_file_table.entries[file_id];
        *file_entry = vmgs_format::VmgsFileEntry {
            offset: data_fcb.block_offset,
            allocation_size: data_fcb.allocated_blocks.get(),
            valid_data_size: buf.len() as u64,
            ..vmgs_format::VmgsFileEntry::new_zeroed()
        };

        if let Some((data_nonce, data_auth_tag)) = data_nonce_auth_tag {
            // Fill in the V3 fields of the file table entry.
            file_entry.nonce.copy_from_slice(&data_nonce);
            file_entry
                .authentication_tag
                .copy_from_slice(&data_auth_tag);
        }

        // Fill in the metadata for the new file table itself (file ID 0)
        let file_table_entry = &mut new_file_table.entries[FileId::FILE_TABLE];
        *file_table_entry = vmgs_format::VmgsFileEntry {
            offset: file_table_fcb.block_offset,
            allocation_size: file_table_fcb.allocated_blocks.get(),
            valid_data_size: file_table_fcb.valid_bytes,
            ..vmgs_format::VmgsFileEntry::new_zeroed()
        };

        if should_write_file_table {
            // Write out the new file table.
            self.storage
                .write_block(
                    block_count_to_byte_count(file_table_fcb.block_offset),
                    new_file_table.as_bytes(),
                )
                .await
                .map_err(Error::WriteDisk)?;
        }

        // Update the in-memory file control blocks. Updating file_control_block last ensures
        // operation to be atomic, in case the program crashes on the write above or any intermediate operation

        self.fcbs.insert(FileId::FILE_TABLE, *file_table_fcb);
        self.fcbs.insert(file_id, *data_fcb);

        Ok(())
    }

    /// Copies current file metadata to an extended file table structure.
    fn fill_extended_file_table(
        &mut self,
        new_extended_file_table: &mut VmgsExtendedFileTable,
    ) -> Result<(), Error> {
        *new_extended_file_table = VmgsExtendedFileTable::new_zeroed();
        for (file_id, fcb) in self.fcbs.iter_mut() {
            let extended_file_entry = &mut new_extended_file_table.entries[*file_id];
            extended_file_entry.attributes = fcb.attributes;
            extended_file_entry
                .encryption_key
                .copy_from_slice(&fcb.encryption_key);
        }

        Ok(())
    }

    /// Update and write new header to storage device.
    async fn update_header(&mut self, new_header: &mut VmgsHeader) -> Result<(), Error> {
        // Wrapping prevents integer overflow checks
        new_header.sequence = self.active_header_sequence_number.wrapping_add(1);
        new_header.checksum = 0;
        new_header.checksum = compute_crc32(new_header.as_bytes());

        let new_header_index = if self.active_header_index == 0 { 1 } else { 0 };

        self.storage
            .write_block(
                new_header_index as u64 * self.storage.aligned_header_size(),
                new_header.as_bytes(),
            )
            .await
            .map_err(Error::WriteDisk)?;
        self.set_active_header(new_header_index, new_header.sequence);
        Ok(())
    }

    /// Sets the active header index and sequence number.
    fn set_active_header(
        &mut self,
        active_header_index: usize,
        active_header_sequence_number: u32,
    ) {
        assert!(active_header_index < 2);
        self.active_header_index = active_header_index;
        self.active_header_sequence_number = active_header_sequence_number;
    }

    /// Reads the specified `file_id`, decrypting its contents.
    pub async fn read_file(&mut self, file_id: FileId) -> Result<Vec<u8>, Error> {
        self.read_file_inner(file_id, true).await
    }

    /// Reads the specified `file_id`, but does not decrypt the contents.
    pub async fn read_file_raw(&mut self, file_id: FileId) -> Result<Vec<u8>, Error> {
        self.read_file_inner(file_id, false).await
    }

    async fn read_file_inner(&mut self, file_id: FileId, decrypt: bool) -> Result<Vec<u8>, Error> {
        #[cfg(feature = "inspect")]
        self.stats
            .read
            .entry(file_id)
            .or_default()
            .attempt
            .increment();

        let file_info = self.get_file_info(file_id)?;
        if file_id == FileId::FILE_TABLE {
            return Err(Error::FileId);
        }

        let fcb = self.fcbs[&file_id];

        let mut buf = vec![0; file_info.valid_bytes as usize];

        let file_is_encrypted = fcb.attributes.encrypted() || fcb.attributes.authenticated();

        if decrypt
            && self.version >= VMGS_VERSION_3_0
            && self.encryption_algorithm != EncryptionAlgorithm::NONE
            && file_is_encrypted
            && self.active_datastore_key_index.is_some()
        {
            self.read_decrypted_data(
                fcb.block_offset,
                &fcb.encryption_key,
                &fcb.nonce,
                &fcb.authentication_tag,
                &mut buf,
            )
            .await?;
        } else if file_is_encrypted && decrypt {
            return Err(Error::ReadEncrypted);
        } else {
            let byte_offset = block_count_to_byte_count(fcb.block_offset);
            self.storage
                .read_block(byte_offset, &mut buf)
                .await
                .map_err(Error::ReadDisk)?;
        }

        #[cfg(feature = "inspect")]
        self.stats
            .read
            .entry(file_id)
            .or_default()
            .resolved
            .increment();

        Ok(buf)
    }

    /// Writes `buf` to a file_id without encrypting it.
    ///
    /// If the file is already encrypted, this will return a failure. Use
    /// [`Self::write_file_allow_overwrite_encrypted`] if you want to allow
    /// this.
    ///
    /// To write encrypted data, use `write_file_encrypted` instead.
    pub async fn write_file(&mut self, file_id: FileId, buf: &[u8]) -> Result<(), Error> {
        self.write_file_inner(file_id, buf, false).await
    }

    /// Writes `buf` to a file_id without encrypting it, allowing overrites of
    /// an already-encrypted file.
    pub async fn write_file_allow_overwrite_encrypted(
        &mut self,
        file_id: FileId,
        buf: &[u8],
    ) -> Result<(), Error> {
        self.write_file_inner(file_id, buf, true).await
    }

    async fn write_file_inner(
        &mut self,
        file_id: FileId,
        buf: &[u8],
        overwrite_encrypted: bool,
    ) -> Result<(), Error> {
        #[cfg(feature = "inspect")]
        self.stats
            .write
            .entry(file_id)
            .or_default()
            .attempt
            .increment();

        if file_id == FileId::FILE_TABLE {
            return Err(Error::FileId);
        }
        if buf.len() > vmgs_format::VMGS_MAX_FILE_SIZE_BYTES as usize {
            return Err(Error::WriteFileLength);
        }
        let mut blocks_to_allocate =
            (round_up_count(buf.len(), VMGS_BYTES_PER_BLOCK) / VMGS_BYTES_PER_BLOCK as u64) as u32;
        // Always allocate at least one block, to allow for zero sized data buffers
        if blocks_to_allocate == 0 {
            blocks_to_allocate = 1;
        }
        if blocks_to_allocate as u64 > vmgs_format::VMGS_MAX_FILE_SIZE_BLOCKS {
            return Err(Error::WriteFileBlocks);
        }
        if self
            .fcbs
            .get(&file_id)
            .map(|fcb| fcb.attributes.encrypted())
            .unwrap_or(false)
        {
            if overwrite_encrypted {
                tracing::warn!("overwriting encrypted file with plaintext data!")
            } else {
                return Err(Error::OverwriteEncrypted);
            }
        }

        // Allocate space for the new file contents and the new file table.
        // On success, the contents of the temporary FCBs are copied to the existing FCBs.
        let mut temp_fcbs: Vec<ResolvedFileControlBlock> = Vec::new();
        // file_table_fcb
        self.allocate_space(
            VMGS_FILE_TABLE_BLOCK_SIZE,
            &mut temp_fcbs,
            block_count_to_byte_count(VMGS_FILE_TABLE_BLOCK_SIZE),
        )?;
        // data_fcb
        self.allocate_space(blocks_to_allocate, &mut temp_fcbs, buf.len() as u64)?;

        // extended_file_table_fcb is Some() if we should write to extended file table.
        let extended_file_table_fcb = if self.encryption_algorithm == EncryptionAlgorithm::NONE
            || self
                .fcbs
                .get(&file_id)
                .map(|f| f.attributes == FileAttribute::new())
                .unwrap_or(true)
        {
            None
        } else {
            self.allocate_space(
                VMGS_EXTENDED_FILE_TABLE_BLOCK_SIZE,
                &mut temp_fcbs,
                block_count_to_byte_count(VMGS_EXTENDED_FILE_TABLE_BLOCK_SIZE),
            )?;
            temp_fcbs.last_mut().unwrap().attributes = FileAttribute::new()
                .with_encrypted(true)
                .with_authenticated(true);

            Some(temp_fcbs.pop().unwrap())
        };

        // the C++ code originally implemented gsl::finally to deallocate these from the allocation list
        // on exception. However, currently with this being a single threaded Rust crate, that shouldn't be
        // needed. When switching to multithreading, that may change.
        let mut data_fcb = temp_fcbs.pop().unwrap();
        let mut file_table_fcb = temp_fcbs.pop().unwrap();

        data_fcb.attributes = FileAttribute::new();

        // Write the file contents to the newly allocated space.
        self.write_file_internal(
            file_id,
            buf,
            &mut file_table_fcb,
            &mut data_fcb,
            false,
            // Write the file table to the storage since there is no need to manipulate the extended file table.
            extended_file_table_fcb.is_none(),
        )
        .await?;

        if let Some(mut extended_table_fcb) = extended_file_table_fcb {
            // Initialize the new extended file table with current metadata for all files.
            let mut new_extended_file_table = VmgsExtendedFileTable::new_zeroed();
            self.fill_extended_file_table(&mut new_extended_file_table)?;

            // Fill in the metadata for the new extended table.
            let extended_file_entry = &mut new_extended_file_table.entries[file_id];
            extended_file_entry.attributes = data_fcb.attributes;
            extended_file_entry
                .encryption_key
                .copy_from_slice(&data_fcb.encryption_key);

            // Write the extended file table to the newly allocated space.
            self.write_file_internal(
                FileId::EXTENDED_FILE_TABLE,
                new_extended_file_table.as_bytes(),
                &mut file_table_fcb,
                &mut extended_table_fcb,
                true,
                true,
            )
            .await?;
        }

        // Data must be hardened on persistent storage before the header is updated.
        self.storage.flush().await.map_err(Error::FlushDisk)?;

        // Prepare a new header.
        let mut new_header = self.prepare_new_header(&file_table_fcb);

        if self.encryption_algorithm != EncryptionAlgorithm::NONE {
            if let Some(extended_table_fcb) = extended_file_table_fcb {
                let mut metadata_key_auth_tag = VmgsAuthTag::new_zeroed();
                self.metadata_key
                    .copy_from_slice(&extended_table_fcb.encryption_key);

                let current_index = self.active_datastore_key_index.unwrap();

                increment_nonce(&mut self.encrypted_metadata_keys[current_index].nonce)?;

                let encrypted_metadata_key = encrypt_metadata_key(
                    &self.datastore_keys[current_index],
                    &self.encrypted_metadata_keys[current_index].nonce,
                    &self.metadata_key,
                    &mut metadata_key_auth_tag,
                )?;

                self.encrypted_metadata_keys[current_index]
                    .authentication_tag
                    .copy_from_slice(&metadata_key_auth_tag);
                self.encrypted_metadata_keys[current_index]
                    .encryption_key
                    .copy_from_slice(&encrypted_metadata_key);
            }

            new_header.encryption_algorithm = self.encryption_algorithm;
            new_header
                .metadata_keys
                .copy_from_slice(&self.encrypted_metadata_keys);
        }

        self.update_header(&mut new_header).await?;

        #[cfg(feature = "inspect")]
        self.stats
            .write
            .entry(file_id)
            .or_default()
            .resolved
            .increment();

        Ok(())
    }

    /// Encrypts `buf` and writes the encrypted payload to a file_id if the VMGS file has encryption configured.
    /// If the VMGS doesn't have encryption configured, will do a plaintext write instead.
    #[cfg(with_encryption)]
    pub async fn write_file_encrypted(&mut self, file_id: FileId, buf: &[u8]) -> Result<(), Error> {
        if file_id == FileId::FILE_TABLE {
            return Err(Error::FileId);
        }
        if buf.len() > vmgs_format::VMGS_MAX_FILE_SIZE_BYTES as usize {
            return Err(Error::WriteFileLength);
        }
        let mut blocks_to_allocate =
            (round_up_count(buf.len(), VMGS_BYTES_PER_BLOCK) / VMGS_BYTES_PER_BLOCK as u64) as u32;
        // Always allocate at least one block, to allow for zero sized data buffers
        if blocks_to_allocate == 0 {
            blocks_to_allocate = 1;
        }
        if blocks_to_allocate as u64 > vmgs_format::VMGS_MAX_FILE_SIZE_BLOCKS {
            return Err(Error::WriteFileBlocks);
        }
        if self.encryption_algorithm == EncryptionAlgorithm::NONE {
            tracing::trace!("VMGS file not encrypted, performing plaintext write");
            return self.write_file(file_id, buf).await;
        }

        // Allocate space for the new file contents and the new file table.
        // On success, the contents of the temporary FCBs are copied to the existing FCBs.
        let mut temp_fcbs: Vec<ResolvedFileControlBlock> = Vec::new();
        // file_table_fcb
        self.allocate_space(
            VMGS_FILE_TABLE_BLOCK_SIZE,
            &mut temp_fcbs,
            block_count_to_byte_count(VMGS_FILE_TABLE_BLOCK_SIZE),
        )?;
        // data_fcb
        self.allocate_space(blocks_to_allocate, &mut temp_fcbs, buf.len() as u64)?;

        let mut extended_file_table_fcb = {
            self.allocate_space(
                VMGS_EXTENDED_FILE_TABLE_BLOCK_SIZE,
                &mut temp_fcbs,
                block_count_to_byte_count(VMGS_EXTENDED_FILE_TABLE_BLOCK_SIZE),
            )?;
            temp_fcbs.last_mut().unwrap().attributes = FileAttribute::new()
                .with_encrypted(true)
                .with_authenticated(true);
            temp_fcbs.pop().unwrap()
        };

        // the C++ code originally implemented gsl::finally to deallocate these from the allocation list
        // on exception. However, currently with this being a single threaded Rust crate, that shouldn't be
        // needed. When switching to multithreading, that may change.
        let mut data_fcb = temp_fcbs.pop().unwrap();
        let mut file_table_fcb = temp_fcbs.pop().unwrap();

        data_fcb.attributes = FileAttribute::new()
            .with_encrypted(true)
            .with_authenticated(true);

        // Write the file contents to the newly allocated space.
        self.write_file_internal(
            file_id,
            buf,
            &mut file_table_fcb,
            &mut data_fcb,
            true,
            false,
        )
        .await?;

        // Initialize the new extended file table with current metadata for all files.
        let mut new_extended_file_table = VmgsExtendedFileTable::new_zeroed();
        self.fill_extended_file_table(&mut new_extended_file_table)?;

        // Fill in the metadata for the new extended table.
        let extended_file_entry = &mut new_extended_file_table.entries[file_id];
        extended_file_entry.attributes = data_fcb.attributes;
        extended_file_entry
            .encryption_key
            .copy_from_slice(&data_fcb.encryption_key);

        // Write the extended file table to the newly allocated space.
        self.write_file_internal(
            FileId::EXTENDED_FILE_TABLE,
            new_extended_file_table.as_bytes(),
            &mut file_table_fcb,
            &mut extended_file_table_fcb,
            true,
            true,
        )
        .await?;

        // Data must be hardened on persistent storage before the header is updated.
        self.storage.flush().await.map_err(Error::FlushDisk)?;

        // Prepare a new header.
        let mut new_header = self.prepare_new_header(&file_table_fcb);

        if self.encryption_algorithm != EncryptionAlgorithm::NONE {
            let mut metadata_key_auth_tag = VmgsAuthTag::new_zeroed();
            self.metadata_key
                .copy_from_slice(&extended_file_table_fcb.encryption_key);

            let active_key = self.active_datastore_key_index.unwrap();
            increment_nonce(&mut self.encrypted_metadata_keys[active_key].nonce)?;

            let encrypted_metadata_key = encrypt_metadata_key(
                &self.datastore_keys[active_key],
                &self.encrypted_metadata_keys[active_key].nonce,
                &self.metadata_key,
                &mut metadata_key_auth_tag,
            )?;

            self.encrypted_metadata_keys[active_key]
                .authentication_tag
                .copy_from_slice(&metadata_key_auth_tag);
            self.encrypted_metadata_keys[active_key]
                .encryption_key
                .copy_from_slice(&encrypted_metadata_key);

            new_header.encryption_algorithm = self.encryption_algorithm;
            new_header
                .metadata_keys
                .copy_from_slice(&self.encrypted_metadata_keys);
        }

        self.update_header(&mut new_header).await
    }

    /// Decrypts the extended file table by the encryption_key and
    /// updates the related metadata in memory.
    #[cfg(with_encryption)]
    pub async fn unlock_with_encryption_key(
        &mut self,
        encryption_key: &[u8],
    ) -> Result<usize, Error> {
        if self.version < VMGS_VERSION_3_0 {
            return Err(Error::Other(anyhow!(
                "unlock_with_encryption_key() not supported with VMGS version"
            )));
        }
        if self.encryption_algorithm == EncryptionAlgorithm::NONE {
            return Err(Error::Other(anyhow!(
                "unlock_with_encryption_key() not supported with None EncryptionAlgorithm"
            )));
        }

        // Iterate through two metadata keys and get the index of the valid key which can be successfully
        // decrypted by the encryption_key, as well as set the decrypted key as the VMGS's metadata key
        let mut valid_index = None;
        let mut errs = [None, None];

        for (i, key) in self.encrypted_metadata_keys.iter().enumerate() {
            let result = decrypt_metadata_key(
                encryption_key,
                &key.nonce,
                &key.encryption_key,
                &key.authentication_tag,
            );

            match result {
                Ok(metadata_key) => {
                    self.metadata_key.copy_from_slice(&metadata_key);
                    valid_index = Some(i);
                    break;
                }
                Err(err) => {
                    errs[i] = Some(err);
                }
            }
        }

        let valid_index = match valid_index {
            Some(idx) => idx,
            None => {
                tracing::error!(
                    error = &errs[0].take().unwrap() as &dyn std::error::Error,
                    "first index failed to decrypt",
                );
                tracing::error!(
                    error = &errs[1].take().unwrap() as &dyn std::error::Error,
                    "second index failed to decrypt",
                );
                return Err(Error::Other(anyhow::anyhow!(
                    "failed to use the root key provided to decrypt VMGS metadata key"
                )));
            }
        };
        let extended_file_header = self.fcbs[&FileId::EXTENDED_FILE_TABLE];
        let extended_file_table_size_bytes =
            block_count_to_byte_count(extended_file_header.allocated_blocks.get());
        let mut extended_file_table_buffer = vec![0; extended_file_table_size_bytes as usize];
        let self_metadata_key = self.metadata_key;

        // Read and decrypt the extended file table
        self.read_decrypted_data(
            extended_file_header.block_offset,
            &self_metadata_key,
            &extended_file_header.nonce,
            &extended_file_header.authentication_tag,
            &mut extended_file_table_buffer,
        )
        .await
        .context("failed to decrypt extended file table")?;

        // Update the cached extended file table
        let extended_file_table =
            VmgsExtendedFileTable::read_from_prefix(&extended_file_table_buffer[..])
                .context("Invalid decrypted extended file table")?;
        for (file_id, fcb) in self.fcbs.iter_mut() {
            fcb.attributes = extended_file_table.entries[*file_id].attributes;
            fcb.encryption_key = extended_file_table.entries[*file_id].encryption_key;
        }

        self.datastore_keys[valid_index].copy_from_slice(encryption_key);
        self.active_datastore_key_index = Some(valid_index);

        Ok(valid_index)
    }

    /// Encrypts the plaintext data and writes the encrypted data to the storage.
    #[cfg_attr(not(with_encryption), allow(unused_variables))]
    async fn write_encrypted_data(
        &mut self,
        block_offset: u32,
        encryption_key: &[u8],
        nonce: &[u8],
        plaintext_data: &[u8],
        authentication_tag: &mut [u8],
    ) -> Result<(), Error> {
        #[cfg(not(with_encryption))]
        unreachable!("Encryption requires the encryption feature");
        #[cfg(with_encryption)]
        {
            let encrypted_text = crate::encrypt::vmgs_encrypt(
                encryption_key,
                nonce,
                plaintext_data,
                authentication_tag,
            )?;

            // Write the encrypted file contents to the newly allocated space.
            self.storage
                .write_block(block_count_to_byte_count(block_offset), &encrypted_text)
                .await
                .map_err(Error::WriteDisk)?;

            Ok(())
        }
    }

    /// Decrypts the encrypted data and reads it to the buffer.
    #[cfg_attr(not(with_encryption), allow(unused_variables))]
    async fn read_decrypted_data(
        &mut self,
        block_offset: u32,
        decryption_key: &[u8],
        nonce: &[u8],
        authentication_tag: &[u8],
        plaintext_data: &mut [u8],
    ) -> Result<(), Error> {
        #[cfg(not(with_encryption))]
        unreachable!("Encryption requires the encryption feature");
        #[cfg(with_encryption)]
        {
            // Read and decrypt the encrypted file contents.
            let mut buf = vec![0; plaintext_data.len()];

            self.storage
                .read_block(block_count_to_byte_count(block_offset), &mut buf)
                .await
                .map_err(Error::ReadDisk)?;

            // sanity check: encrypted data should never be all zeros. if we
            // find that it is all-zeroes, then that's indicative of some kind
            // of logic error / data corruption
            if buf.iter().all(|x| *x == 0) {
                return Err(Error::InvalidFormat("encrypted data is all-zeros".into()));
            }

            let decrypted_text =
                crate::encrypt::vmgs_decrypt(decryption_key, nonce, &buf, authentication_tag)?;
            if decrypted_text.len() != plaintext_data.len() {
                return Err(Error::Other(anyhow!(
                    "Decrypt error, slice sizes should match."
                )));
            }
            plaintext_data.copy_from_slice(&decrypted_text);

            Ok(())
        }
    }

    /// Associates a new root key with the data store. Returns the index of the newly associated key.
    #[cfg(with_encryption)]
    pub async fn add_new_encryption_key(
        &mut self,
        encryption_key: &[u8],
        encryption_algorithm: EncryptionAlgorithm,
    ) -> Result<usize, Error> {
        if self.version < VMGS_VERSION_3_0 {
            return Err(Error::Other(anyhow!(
                "add_new_encryption_key() not supported with VMGS version"
            )));
        }
        if self.encryption_algorithm != EncryptionAlgorithm::NONE
            && self.active_datastore_key_index.is_none()
        {
            return Err(Error::Other(anyhow!(
                "add_new_encryption_key() invalid datastore key index"
            )));
        }
        if self.datastore_key_count == self.datastore_keys.len() as u8 {
            return Err(Error::Other(anyhow!(
                "add_new_encryption_key() no space to add new encryption key"
            )));
        }
        if is_empty_key(encryption_key) {
            return Err(Error::Other(anyhow!("Trying to add empty encryption key")));
        }
        if encryption_algorithm == EncryptionAlgorithm::NONE {
            return Err(Error::Other(anyhow!(
                "Encryption not supported for VMGS file"
            )));
        }
        if self.encryption_algorithm != EncryptionAlgorithm::NONE
            && encryption_algorithm != self.encryption_algorithm
        {
            return Err(Error::Other(anyhow!("Encryption algorithm provided to add_new_encryption_key does not match VMGS's encryption algorithm.")));
        }

        let mut new_key_index = 0;
        let mut new_metadata_key = self.metadata_key;
        if self.datastore_key_count == 0 {
            // Allocate space for the new file table and the new extended file table.
            // Two temporary FCBs will be added to the allocation list, and will be unlinked
            // from the allocation list no matter whether the function succeeds or fails.
            // On success, the contents of the temporary FCBs are copied to the existing FCBs.
            let mut temp_fcbs: Vec<ResolvedFileControlBlock> = Vec::new();
            self.allocate_space(
                VMGS_FILE_TABLE_BLOCK_SIZE,
                &mut temp_fcbs,
                block_count_to_byte_count(VMGS_FILE_TABLE_BLOCK_SIZE),
            )?;

            self.allocate_space(
                VMGS_EXTENDED_FILE_TABLE_BLOCK_SIZE,
                &mut temp_fcbs,
                block_count_to_byte_count(VMGS_EXTENDED_FILE_TABLE_BLOCK_SIZE),
            )?;

            let mut extended_file_table_fcb = temp_fcbs.pop().unwrap();
            let mut file_table_fcb = temp_fcbs.pop().unwrap();

            extended_file_table_fcb.attributes = FileAttribute::new()
                .with_encrypted(true)
                .with_authenticated(true);

            // Initialize a new extended file table.
            let new_extended_file_table = VmgsExtendedFileTable::new_zeroed();
            // Write the extende file table to the newly allocated space
            self.write_file_internal(
                FileId::EXTENDED_FILE_TABLE,
                new_extended_file_table.as_bytes(),
                &mut file_table_fcb,
                &mut extended_file_table_fcb,
                true,
                true,
            )
            .await?;

            new_metadata_key
                .copy_from_slice(&self.fcbs[&FileId::EXTENDED_FILE_TABLE].encryption_key);
        } else if self.active_datastore_key_index == Some(0) {
            new_key_index = 1;
        }

        // Prepare a new header.
        let mut new_header = self.prepare_new_header(&self.fcbs[&FileId::FILE_TABLE]);
        new_header.encryption_algorithm = EncryptionAlgorithm::AES_GCM;

        // Use the new datastore key to encrypt the metadata key.
        let metadata_key_nonce = generate_nonce();
        let mut metadata_key_auth_tag = VmgsAuthTag::new_zeroed();
        let encrypted_metadata_key = encrypt_metadata_key(
            encryption_key,
            &metadata_key_nonce,
            &new_metadata_key,
            &mut metadata_key_auth_tag,
        )?;

        self.encrypted_metadata_keys[new_key_index]
            .nonce
            .copy_from_slice(&metadata_key_nonce);
        self.encrypted_metadata_keys[new_key_index]
            .authentication_tag
            .copy_from_slice(&metadata_key_auth_tag);
        self.encrypted_metadata_keys[new_key_index]
            .encryption_key
            .copy_from_slice(&encrypted_metadata_key);

        new_header
            .metadata_keys
            .copy_from_slice(&self.encrypted_metadata_keys);

        // Update the header on the storage device
        self.update_header(&mut new_header).await?;

        // Update the cached DataStore key.
        self.datastore_keys[new_key_index].copy_from_slice(encryption_key);
        self.metadata_key.copy_from_slice(&new_metadata_key);
        self.datastore_key_count += 1;
        self.encryption_algorithm = encryption_algorithm;
        self.active_datastore_key_index = Some(new_key_index);

        Ok(new_key_index)
    }

    /// Disassociates the root key at the specified index from the data store.
    #[cfg(with_encryption)]
    pub async fn remove_encryption_key(&mut self, key_index: usize) -> Result<(), Error> {
        if self.version < VMGS_VERSION_3_0 {
            return Err(Error::Other(anyhow!(
                "remove_encryption_key() not supported with VMGS version."
            )));
        }
        if self.encryption_algorithm != EncryptionAlgorithm::NONE
            && self.active_datastore_key_index.is_none()
        {
            return Err(Error::Other(anyhow!(
                "remove_encryption_key() invalid datastore key index or encryption algorithm."
            )));
        }
        if self.datastore_key_count != self.datastore_keys.len() as u8
            && self.active_datastore_key_index != Some(key_index)
        {
            return Err(Error::Other(anyhow!(
                "remove_encryption_key() invalid key_index"
            )));
        }

        // Remove the corresponding datastore_key
        self.datastore_keys[key_index].fill(0);

        // Remove the corresponding metadata_key
        self.encrypted_metadata_keys[key_index] = VmgsEncryptionKey::new_zeroed();

        // Prepare a new header
        let mut new_header = self.prepare_new_header(&self.fcbs[&FileId::FILE_TABLE]);
        new_header
            .metadata_keys
            .copy_from_slice(&self.encrypted_metadata_keys);

        // Set the encryption algorithm to none, when there is only one valid metadata key before removal
        if self.datastore_key_count == 1 {
            new_header.encryption_algorithm = EncryptionAlgorithm::NONE;
        } else {
            new_header.encryption_algorithm = self.encryption_algorithm;
        }

        // Update the header on the storage device
        self.update_header(&mut new_header).await?;

        // Update cached metadata
        if self.datastore_key_count == 1 {
            self.encryption_algorithm = EncryptionAlgorithm::NONE;
            self.datastore_key_count = 0;
            self.active_datastore_key_index = None;
        } else {
            self.datastore_key_count = 1;

            let new_active_datastore_key_index = if key_index == 0 { 1 } else { 0 };
            if is_empty_key(&self.datastore_keys[new_active_datastore_key_index]) {
                self.active_datastore_key_index = None;
            } else {
                self.active_datastore_key_index = Some(new_active_datastore_key_index);
            }
        }

        Ok(())
    }

    /// Gets the encryption algorithm of the VMGS
    pub fn get_encryption_algorithm(&self) -> EncryptionAlgorithm {
        self.encryption_algorithm
    }

    /// Whether the VMGS file is encrypted
    pub fn is_encrypted(&self) -> bool {
        self.encryption_algorithm != EncryptionAlgorithm::NONE
    }

    /// Get the active datastore key index
    pub fn get_active_datastore_key_index(&self) -> Option<usize> {
        self.active_datastore_key_index
    }

    fn prepare_new_header(&self, file_table_fcb: &ResolvedFileControlBlock) -> VmgsHeader {
        VmgsHeader {
            signature: VMGS_SIGNATURE,
            version: self.version,
            header_size: size_of::<VmgsHeader>() as u32,
            file_table_offset: file_table_fcb.block_offset,
            file_table_size: file_table_fcb.allocated_blocks.get(),
            ..VmgsHeader::new_zeroed()
        }
    }
}

/// Read both headers. For compatibility with the V1 format, the headers are
/// at logical sectors 0 and 1
pub async fn read_headers(disk: Disk) -> Result<(VmgsHeader, VmgsHeader), Error> {
    read_headers_inner(&mut VmgsStorage::new(disk)).await
}

async fn read_headers_inner(storage: &mut VmgsStorage) -> Result<(VmgsHeader, VmgsHeader), Error> {
    // Read both headers, and determine the active one. For compatibility with
    // the V1 format, the headers are at logical sectors 0 and 1
    let mut first_two_blocks = [0; (VMGS_BYTES_PER_BLOCK * 2) as usize];
    storage
        .read_block(0, &mut first_two_blocks)
        .await
        .map_err(Error::ReadDisk)?;

    // first_two_blocks will contain enough bytes to read the first two headers
    let header_1 = VmgsHeader::read_from_prefix(&first_two_blocks).unwrap();
    let header_2 =
        VmgsHeader::read_from_prefix(&first_two_blocks[storage.aligned_header_size() as usize..])
            .unwrap();

    Ok((header_1, header_2))
}

/// Determines which header to use given the results of checking the
/// validity of each of the headers.
pub fn get_active_header(
    header_1: Result<&VmgsHeader, Error>,
    header_2: Result<&VmgsHeader, Error>,
) -> Result<usize, Error> {
    let active_header_index =
        if let (Ok(header_1), Ok(header_2)) = (header_1.as_deref(), header_2.as_deref()) {
            // If both headers are valid, find the header with the larger sequence number.
            // The header with the most recent sequence number is considered
            // the current copy. To handle integer overflow, a header with sequence number 0
            // is considered the current copy if and only if the other header contains 0xFFFFFFFF.
            if header_1.sequence == header_2.sequence.wrapping_add(1) {
                0
            } else if header_2.sequence == header_1.sequence.wrapping_add(1) {
                1
            } else {
                return Err(Error::CorruptFormat(format!(
                    "Invalid header sequence numbers. Header 1: {}, Header 2: {}",
                    header_1.sequence, header_2.sequence
                )));
            }
        } else if header_1.is_ok() {
            0
        } else if header_2.is_ok() {
            1
        } else {
            return Err(Error::InvalidFormat(format!(
                "No valid header: Header 1: {} Header 2: {}",
                header_1.err().unwrap(),
                header_2.err().unwrap()
            )));
        };

    Ok(active_header_index)
}

/// Validate the contents of header match VMGS file type.
pub fn validate_header(header: &VmgsHeader) -> Result<&VmgsHeader, Error> {
    if header.signature != VMGS_SIGNATURE {
        return Err(Error::InvalidFormat(String::from(
            "Invalid header signature",
        )));
    }
    if header.version != VMGS_VERSION_3_0 {
        return Err(Error::InvalidFormat(String::from("Invalid header version")));
    }
    if header.header_size != size_of::<VmgsHeader>() as u32 {
        return Err(Error::InvalidFormat(String::from("Invalid header size")));
    }
    if header.file_table_offset < VMGS_MIN_FILE_BLOCK_OFFSET {
        return Err(Error::InvalidFormat(String::from(
            "Invalid file table offset",
        )));
    }
    if header.file_table_size != VMGS_FILE_TABLE_BLOCK_SIZE {
        return Err(Error::InvalidFormat(String::from(
            "Invalid file table size",
        )));
    }

    let stored_checksum = header.checksum;
    let mut zero_checksum_header = *header;
    zero_checksum_header.checksum = 0;
    let computed_checksum = compute_crc32(zero_checksum_header.as_bytes());
    if stored_checksum != computed_checksum {
        return Err(Error::CorruptFormat(String::from(
            "Invalid header checksum",
        )));
    }
    Ok(header)
}

/// Initializes cached file metadata from the specified header. (File control blocks)
fn initialize_file_metadata(
    file_table: &VmgsFileTable,
    version: u32,
    block_capacity: u32,
) -> Result<HashMap<FileId, ResolvedFileControlBlock>, Error> {
    let file_entries = file_table.entries;
    let mut file_control_blocks = HashMap::new();

    for (file_id, file_entry) in file_entries.iter().enumerate() {
        let file_id = FileId(file_id as u32);

        // Check if the file is allocated.
        let Some(allocated_blocks) = NonZeroU32::new(file_entry.allocation_size) else {
            continue;
        };

        // Validate the file offset.
        if file_entry.offset < VMGS_MIN_FILE_BLOCK_OFFSET || file_entry.offset >= block_capacity {
            return Err(Error::CorruptFormat(format!(
                "Invalid file offset {} for file_id {:?} \n{:?}",
                file_entry.offset, file_id, file_entry
            )));
        }

        // The file must entirely fit in the available space.
        let file_allocation_end_block = file_entry.offset + file_entry.allocation_size;
        if file_allocation_end_block > block_capacity {
            return Err(Error::CorruptFormat(String::from(
                "Invalid file allocation end block",
            )));
        }

        // Validate the valid data size.
        let file_allocation_size_bytes = block_count_to_byte_count(file_entry.allocation_size);
        if file_entry.valid_data_size > file_allocation_size_bytes {
            return Err(Error::CorruptFormat(String::from("Invalid data size")));
        }

        // Initialize the file control block for this file ID
        file_control_blocks.insert(file_id, {
            let (nonce, authentication_tag) = if version >= VMGS_VERSION_3_0 {
                (file_entry.nonce, file_entry.authentication_tag)
            } else {
                Default::default()
            };

            ResolvedFileControlBlock {
                block_offset: file_entry.offset,
                allocated_blocks,
                valid_bytes: file_entry.valid_data_size,

                nonce,
                authentication_tag,

                attributes: FileAttribute::new(),
                encryption_key: VmgsDatastoreKey::new_zeroed(),
            }
        });
    }

    Ok(file_control_blocks)
}

/// Convert block count to byte count.
fn block_count_to_byte_count(block_count: u32) -> u64 {
    block_count as u64 * VMGS_BYTES_PER_BLOCK as u64
}

fn round_up_count(count: usize, pow2: u32) -> u64 {
    (count as u64 + pow2 as u64 - 1) & !(pow2 as u64 - 1)
}

/// Generates a nonce for the encryption. First 4 bytes are a random seed, and last 8 bytes are zero's.
fn generate_nonce() -> VmgsNonce {
    let mut nonce = VmgsNonce::new_zeroed();
    // Generate a 4-byte random seed for nonce
    getrandom::getrandom(&mut nonce[..4]).expect("rng failure");
    nonce
}

/// Increment Nonce by one.
fn increment_nonce(nonce: &mut VmgsNonce) -> Result<(), Error> {
    // Update the random seed of nonce
    getrandom::getrandom(&mut nonce[..vmgs_format::VMGS_NONCE_RANDOM_SEED_SIZE])
        .expect("rng failure");

    // Increment the counter of nonce by 1.
    for i in &mut nonce[vmgs_format::VMGS_NONCE_RANDOM_SEED_SIZE..] {
        *i = i.wrapping_add(1);

        if *i != 0 {
            break;
        }
    }

    Ok(())
}

/// Checks whether an encryption key is all zero's.
fn is_empty_key(encryption_key: &[u8]) -> bool {
    encryption_key.iter().all(|&x| x == 0)
}

/// Encrypts MetadataKey. Returns encrypted_metadata_key.
#[cfg_attr(not(with_encryption), allow(unused_variables))]
fn encrypt_metadata_key(
    encryption_key: &[u8],
    nonce: &[u8],
    metadata_key: &[u8],
    authentication_tag: &mut [u8],
) -> Result<Vec<u8>, Error> {
    #[cfg(not(with_encryption))]
    unreachable!("Encryption requires the encryption feature");
    #[cfg(with_encryption)]
    {
        let encrypted_metadata_key =
            crate::encrypt::vmgs_encrypt(encryption_key, nonce, metadata_key, authentication_tag)?;

        if encrypted_metadata_key.len() != metadata_key.len() {
            return Err(Error::Other(anyhow!(format!(
                "encrypted metadata key length ({:?}) doesn't match metadata key length ({:?})",
                encrypted_metadata_key, metadata_key
            ))));
        }
        Ok(encrypted_metadata_key)
    }
}

/// Decrypts metadata_key. Returns decrypted_metadata_key.
#[cfg_attr(not(with_encryption), allow(unused_variables), allow(dead_code))]
fn decrypt_metadata_key(
    datastore_key: &[u8],
    nonce: &[u8],
    metadata_key: &[u8],
    authentication_tag: &[u8],
) -> Result<Vec<u8>, Error> {
    #[cfg(not(with_encryption))]
    unreachable!("Encryption requires the encryption feature");
    #[cfg(with_encryption)]
    {
        let decrypted_metadata_key =
            crate::encrypt::vmgs_decrypt(datastore_key, nonce, metadata_key, authentication_tag)?;
        if decrypted_metadata_key.len() != metadata_key.len() {
            return Err(Error::Other(anyhow!(format!(
                "decrypted metadata key length ({:?}) doesn't match metadata key length ({:?})",
                decrypted_metadata_key, metadata_key
            ))));
        }

        Ok(decrypted_metadata_key)
    }
}

/// Computes the cr32 checksum for a given byte stream.
fn compute_crc32(buf: &[u8]) -> u32 {
    let mut hasher = crc32fast::Hasher::new();
    hasher.update(buf);
    hasher.finalize()
}

#[cfg(feature = "save_restore")]
#[allow(missing_docs)]
pub mod save_restore {
    use super::*;

    pub mod state {
        use mesh_protobuf::Protobuf;
        use std::num::NonZeroU32;

        pub type SavedVmgsNonce = [u8; 12];
        pub type SavedVmgsAuthTag = [u8; 16];
        pub type SavedVmgsDatastoreKey = [u8; 32];

        #[derive(Protobuf)]
        #[mesh(package = "vmgs")]
        pub struct SavedResolvedFileControlBlock {
            #[mesh(1)]
            pub block_offset: u32,
            #[mesh(2)]
            pub allocated_blocks: NonZeroU32,
            #[mesh(3)]
            pub valid_bytes: u64,
            #[mesh(4)]
            pub nonce: SavedVmgsNonce,
            #[mesh(5)]
            pub authentication_tag: SavedVmgsAuthTag,
            #[mesh(6)]
            pub attributes: u32,
            #[mesh(7)]
            pub encryption_key: SavedVmgsDatastoreKey,
        }

        #[derive(Protobuf)]
        #[mesh(package = "vmgs")]
        pub struct SavedVmgsEncryptionKey {
            #[mesh(1)]
            pub nonce: SavedVmgsNonce,
            #[mesh(2)]
            pub authentication_tag: SavedVmgsAuthTag,
            #[mesh(3)]
            pub encryption_key: SavedVmgsDatastoreKey,
        }

        #[derive(Protobuf)]
        #[mesh(package = "vmgs")]
        pub struct SavedVmgsState {
            #[mesh(1)]
            pub active_header_index: usize,
            #[mesh(2)]
            pub active_header_sequence_number: u32,
            #[mesh(3)]
            pub version: u32,
            #[mesh(4)]
            pub fcbs: Vec<(u32, SavedResolvedFileControlBlock)>,
            #[mesh(5)]
            pub encryption_algorithm: u16,
            #[mesh(6)]
            pub datastore_key_count: u8,
            #[mesh(7)]
            pub active_datastore_key_index: Option<usize>,
            #[mesh(8)]
            pub datastore_keys: [SavedVmgsDatastoreKey; 2],
            #[mesh(9)]
            pub metadata_key: SavedVmgsDatastoreKey,
            #[mesh(10)]
            pub encrypted_metadata_keys: [SavedVmgsEncryptionKey; 2],
        }
    }

    impl Vmgs {
        /// Construct a [`Vmgs`] instance, re-using existing saved-state from an
        /// earlier instance.
        ///
        /// # Safety
        ///
        /// `open_from_saved` does NOT perform ANY validation on the provided
        /// `state`, and will blindly assume that it matches the underlying
        /// `storage` instance!
        ///
        /// Callers MUST ensure that the provided `state` matches the provided
        /// `storage`, and that no external entities have modified `storage` between
        /// the call to `save` and `open_from_saved`.
        ///
        /// Failing to do so may result in data corruption/loss, read/write
        /// failures, encryption errors, etc... (though, notably: it will _not_
        /// result in any memory-unsafety, hence why the function isn't marked
        /// `unsafe`).
        pub fn open_from_saved(disk: Disk, state: state::SavedVmgsState) -> Self {
            let state::SavedVmgsState {
                active_header_index,
                active_header_sequence_number,
                version,
                fcbs,
                encryption_algorithm,
                datastore_key_count,
                active_datastore_key_index,
                datastore_keys,
                metadata_key,
                encrypted_metadata_keys,
            } = state;

            Self {
                storage: VmgsStorage::new(disk),
                #[cfg(feature = "inspect")]
                stats: Default::default(),

                active_header_index,
                active_header_sequence_number,
                version,
                fcbs: fcbs
                    .into_iter()
                    .map(|(file_id, fcb)| {
                        let state::SavedResolvedFileControlBlock {
                            block_offset,
                            allocated_blocks,
                            valid_bytes,
                            nonce,
                            authentication_tag,
                            attributes,
                            encryption_key,
                        } = fcb;

                        (
                            FileId(file_id),
                            ResolvedFileControlBlock {
                                block_offset,
                                allocated_blocks,
                                valid_bytes,
                                nonce,
                                authentication_tag,
                                attributes: FileAttribute::from(attributes),
                                encryption_key,
                            },
                        )
                    })
                    .collect(),
                encryption_algorithm: EncryptionAlgorithm(encryption_algorithm),
                datastore_key_count,
                active_datastore_key_index,
                datastore_keys,
                metadata_key,
                encrypted_metadata_keys: encrypted_metadata_keys.map(|k| {
                    let state::SavedVmgsEncryptionKey {
                        nonce,
                        authentication_tag,
                        encryption_key,
                    } = k;

                    VmgsEncryptionKey {
                        nonce,
                        reserved: 0,
                        authentication_tag,
                        encryption_key,
                    }
                }),
            }
        }

        /// Save the in-memory Vmgs file metadata.
        ///
        /// This saved state can be used alongside `open_from_saved` to obtain a
        /// new `Vmgs` instance _without_ needing to invoke any IOs on the
        /// underlying storage.
        pub fn save(&self) -> state::SavedVmgsState {
            let Self {
                storage: _,

                #[cfg(feature = "inspect")]
                    stats: _,

                active_header_index,
                active_header_sequence_number,
                version,
                fcbs,
                encryption_algorithm,
                datastore_key_count,
                active_datastore_key_index,
                datastore_keys,
                metadata_key,
                encrypted_metadata_keys,
            } = self;

            state::SavedVmgsState {
                active_header_index: *active_header_index,
                active_header_sequence_number: *active_header_sequence_number,
                version: *version,
                fcbs: fcbs
                    .iter()
                    .map(|(file_id, fcb)| {
                        let ResolvedFileControlBlock {
                            block_offset,
                            allocated_blocks,
                            valid_bytes,
                            nonce,
                            authentication_tag,
                            attributes,
                            encryption_key,
                        } = fcb;

                        (
                            file_id.0,
                            state::SavedResolvedFileControlBlock {
                                block_offset: *block_offset,
                                allocated_blocks: *allocated_blocks,
                                valid_bytes: *valid_bytes,
                                nonce: *nonce,
                                authentication_tag: *authentication_tag,
                                attributes: (*attributes).into(),
                                encryption_key: *encryption_key,
                            },
                        )
                    })
                    .collect(),
                encryption_algorithm: encryption_algorithm.0,
                datastore_key_count: *datastore_key_count,
                active_datastore_key_index: *active_datastore_key_index,
                datastore_keys: *datastore_keys,
                metadata_key: *metadata_key,
                encrypted_metadata_keys: encrypted_metadata_keys.map(|k| {
                    let VmgsEncryptionKey {
                        nonce,
                        reserved: _,
                        authentication_tag,
                        encryption_key,
                    } = k;

                    state::SavedVmgsEncryptionKey {
                        nonce,
                        authentication_tag,
                        encryption_key,
                    }
                }),
            }
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use pal_async::async_test;
    #[cfg(with_encryption)]
    use vmgs_format::VMGS_ENCRYPTION_KEY_SIZE;

    const ONE_MEGA_BYTE: u64 = 1024 * 1024;

    fn new_test_file() -> Disk {
        disklayer_ram::ram_disk(4 * ONE_MEGA_BYTE, false).unwrap()
    }

    #[async_test]
    async fn empty_vmgs() {
        let disk = new_test_file();

        let result = Vmgs::open(disk).await;
        assert!(matches!(result, Err(Error::EmptyFile)));
    }

    #[async_test]
    async fn format_empty_vmgs() {
        let disk = new_test_file();
        let result = Vmgs::format_new(disk).await;
        assert!(result.is_ok());
    }

    #[async_test]
    async fn basic_read_write() {
        let disk = new_test_file();
        let mut vmgs = Vmgs::format_new(disk).await.unwrap();
        assert_eq!(vmgs.active_header_index, 0);
        assert_eq!(vmgs.active_header_sequence_number, 1);
        assert_eq!(vmgs.version, VMGS_VERSION_3_0);

        // write
        let buf = b"hello world";
        vmgs.write_file(FileId::BIOS_NVRAM, buf).await.unwrap();

        assert_eq!(vmgs.active_header_index, 1);
        assert_eq!(vmgs.active_header_sequence_number, 2);

        // read
        let read_buf = vmgs.read_file(FileId::BIOS_NVRAM).await.unwrap();

        assert_eq!(buf, &*read_buf);
        assert_eq!(vmgs.active_header_index, 1);
        assert_eq!(vmgs.active_header_sequence_number, 2);
    }

    #[async_test]
    async fn basic_read_write_large() {
        let disk = new_test_file();
        let mut vmgs = Vmgs::format_new(disk).await.unwrap();

        // write
        let buf: Vec<u8> = (0..).map(|x| x as u8).take(1024 * 4 + 1).collect();

        vmgs.write_file(FileId::BIOS_NVRAM, &buf).await.unwrap();

        assert_eq!(vmgs.active_header_index, 1);
        assert_eq!(vmgs.active_header_sequence_number, 2);

        // read
        let read_buf = vmgs.read_file(FileId::BIOS_NVRAM).await.unwrap();

        assert_eq!(buf, read_buf);
        assert_eq!(vmgs.active_header_index, 1);
        assert_eq!(vmgs.active_header_sequence_number, 2);

        // write
        let buf: Vec<u8> = (0..).map(|x| x as u8).take(1024 * 4 * 4 + 1).collect();

        vmgs.write_file(FileId::TPM_PPI, &buf).await.unwrap();

        assert_eq!(vmgs.active_header_index, 0);
        assert_eq!(vmgs.active_header_sequence_number, 3);

        // read
        let read_buf = vmgs.read_file(FileId::TPM_PPI).await.unwrap();

        assert_eq!(buf, read_buf);
        assert_eq!(vmgs.active_header_index, 0);
        assert_eq!(vmgs.active_header_sequence_number, 3);

        // write
        let buf: Vec<u8> = (0..).map(|x| x as u8).take(1024 * 4 * 4 * 4 + 1).collect();

        vmgs.write_file(FileId::GUEST_FIRMWARE, &buf).await.unwrap();

        assert_eq!(vmgs.active_header_index, 1);
        assert_eq!(vmgs.active_header_sequence_number, 4);

        // read
        let read_buf = vmgs.read_file(FileId::GUEST_FIRMWARE).await.unwrap();

        assert_eq!(buf, read_buf);
        assert_eq!(vmgs.active_header_index, 1);
        assert_eq!(vmgs.active_header_sequence_number, 4);
    }

    #[async_test]
    async fn open_existing_file() {
        let buf_1 = b"hello world";
        let buf_2 = b"short sentence";
        let buf_3 = b"funny joke";

        // Create VMGS file and write to different FileId's
        let disk = new_test_file();
        let mut vmgs = Vmgs::format_new(disk.clone()).await.unwrap();

        vmgs.write_file(FileId::BIOS_NVRAM, buf_1).await.unwrap();

        assert_eq!(vmgs.active_header_index, 1);
        assert_eq!(vmgs.active_header_sequence_number, 2);
        assert_eq!(vmgs.fcbs[&FileId(0)].block_offset, 4);
        assert_eq!(vmgs.fcbs[&FileId(1)].block_offset, 5);

        vmgs.write_file(FileId::TPM_PPI, buf_2).await.unwrap();

        assert_eq!(vmgs.active_header_index, 0);
        assert_eq!(vmgs.active_header_sequence_number, 3);
        assert_eq!(vmgs.fcbs[&FileId(0)].block_offset, 2);
        assert_eq!(vmgs.fcbs[&FileId(1)].block_offset, 5);
        assert_eq!(vmgs.fcbs[&FileId(2)].block_offset, 6);

        vmgs.write_file(FileId::BIOS_NVRAM, buf_3).await.unwrap();

        assert_eq!(vmgs.active_header_index, 1);
        assert_eq!(vmgs.active_header_sequence_number, 4);
        assert_eq!(vmgs.fcbs[&FileId(0)].block_offset, 4);
        assert_eq!(vmgs.fcbs[&FileId(1)].block_offset, 7);
        assert_eq!(vmgs.fcbs[&FileId(2)].block_offset, 6);

        // Re-open VMGS file and read from the same FileId's
        drop(vmgs);

        let mut vmgs = Vmgs::open(disk).await.unwrap();

        assert_eq!(vmgs.fcbs[&FileId(0)].block_offset, 4);
        assert_eq!(vmgs.fcbs[&FileId(1)].block_offset, 7);
        assert_eq!(vmgs.fcbs[&FileId(2)].block_offset, 6);
        let read_buf_1 = vmgs.read_file(FileId::BIOS_NVRAM).await.unwrap();

        assert_eq!(buf_3, &*read_buf_1);
        assert_eq!(vmgs.active_header_index, 1);
        assert_eq!(vmgs.active_header_sequence_number, 4);

        let read_buf_2 = vmgs.read_file(FileId::TPM_PPI).await.unwrap();

        assert_eq!(buf_2, &*read_buf_2);
        assert_eq!(vmgs.fcbs[&FileId(0)].block_offset, 4);
        assert_eq!(vmgs.fcbs[&FileId(1)].block_offset, 7);
        assert_eq!(vmgs.fcbs[&FileId(2)].block_offset, 6);
    }

    #[async_test]
    async fn multiple_read_write() {
        let disk = new_test_file();
        let mut vmgs = Vmgs::format_new(disk).await.unwrap();

        let buf_1 = b"Data data data";
        let buf_2 = b"password";
        let buf_3 = b"other data data";

        vmgs.write_file(FileId::BIOS_NVRAM, buf_1).await.unwrap();
        let read_buf_1 = vmgs.read_file(FileId::BIOS_NVRAM).await.unwrap();
        assert_eq!(buf_1, &*read_buf_1);
        assert_eq!(vmgs.active_header_index, 1);
        assert_eq!(vmgs.active_header_sequence_number, 2);

        vmgs.write_file(FileId::TPM_PPI, buf_2).await.unwrap();
        let info = vmgs.get_file_info(FileId::TPM_PPI).unwrap();
        assert_eq!(info.valid_bytes as usize, buf_2.len());
        let read_buf_2 = vmgs.read_file(FileId::TPM_PPI).await.unwrap();
        assert_eq!(buf_2, &*read_buf_2);
        assert_eq!(vmgs.active_header_index, 0);
        assert_eq!(vmgs.active_header_sequence_number, 3);

        vmgs.write_file(FileId::BIOS_NVRAM, buf_3).await.unwrap();
        let read_buf_3 = vmgs.read_file(FileId::BIOS_NVRAM).await.unwrap();
        assert_eq!(buf_3, &*read_buf_3);
        assert_eq!(vmgs.active_header_index, 1);
        assert_eq!(vmgs.active_header_sequence_number, 4);

        vmgs.write_file(FileId::BIOS_NVRAM, buf_1).await.unwrap();
        let read_buf_1 = vmgs.read_file(FileId::BIOS_NVRAM).await.unwrap();
        assert_eq!(buf_1, &*read_buf_1);
        assert_eq!(vmgs.active_header_index, 0);
        assert_eq!(vmgs.active_header_sequence_number, 5);

        vmgs.write_file(FileId::TPM_PPI, buf_2).await.unwrap();
        let read_buf_2 = vmgs.read_file(FileId::TPM_PPI).await.unwrap();
        assert_eq!(buf_2, &*read_buf_2);
        assert_eq!(vmgs.active_header_index, 1);
        assert_eq!(vmgs.active_header_sequence_number, 6);

        vmgs.write_file(FileId::BIOS_NVRAM, buf_3).await.unwrap();
        let read_buf_3 = vmgs.read_file(FileId::BIOS_NVRAM).await.unwrap();
        assert_eq!(buf_3, &*read_buf_3);
        assert_eq!(vmgs.active_header_index, 0);
        assert_eq!(vmgs.active_header_sequence_number, 7);
    }

    #[async_test]
    async fn test_insufficient_resources() {
        let disk = new_test_file();
        let mut vmgs = Vmgs::format_new(disk).await.unwrap();

        let buf: Vec<u8> = vec![1; ONE_MEGA_BYTE as usize * 5];
        let result = vmgs.write_file(FileId::BIOS_NVRAM, &buf).await;
        assert!(result.is_err());
        if let Err(e) = result {
            match e {
                Error::InsufficientResources => (),
                _ => panic!("Wrong error returned"),
            }
        } else {
            panic!("Should have returned Insufficient resources error");
        }
    }

    #[async_test]
    async fn test_empty_write() {
        let disk = new_test_file();
        let mut vmgs = Vmgs::format_new(disk).await.unwrap();

        let buf: Vec<u8> = Vec::new();
        vmgs.write_file(FileId::BIOS_NVRAM, &buf).await.unwrap();

        // read
        let read_buf = vmgs.read_file(FileId::BIOS_NVRAM).await.unwrap();

        assert_eq!(buf, read_buf);
        assert_eq!(read_buf.len(), 0);
        assert_eq!(vmgs.active_header_index, 1);
        assert_eq!(vmgs.active_header_sequence_number, 2);
    }

    // general functions
    #[test]
    fn test_block_count_to_byte_count() {
        let block_count = 10;
        let byte_count = block_count_to_byte_count(block_count);
        assert!(byte_count == block_count as u64 * VMGS_BYTES_PER_BLOCK as u64);
    }

    #[test]
    fn test_validate_header() {
        let mut header = VmgsHeader::new_zeroed();
        header.signature = VMGS_SIGNATURE;
        header.version = VMGS_VERSION_3_0;
        header.header_size = size_of::<VmgsHeader>() as u32;
        header.file_table_offset = VMGS_MIN_FILE_BLOCK_OFFSET;
        header.file_table_size = VMGS_FILE_TABLE_BLOCK_SIZE;
        header.checksum = compute_crc32(header.as_bytes());

        let result = validate_header(&header);
        assert!(result.is_ok());

        let mut header_signature = header;
        header_signature.signature = 0;
        header_signature.checksum = 0;
        header_signature.checksum = compute_crc32(header_signature.as_bytes());
        let result = validate_header(&header_signature);
        match result {
            Err(Error::InvalidFormat(err)) => assert_eq!(err, "Invalid header signature"),
            _ => panic!(),
        };

        let mut header_version = header;
        header_version.version = 0;
        header_version.checksum = 0;
        header_version.checksum = compute_crc32(header_version.as_bytes());
        match validate_header(&header_version) {
            Err(Error::InvalidFormat(err)) => assert_eq!(err, "Invalid header version"),
            _ => panic!(),
        };

        let mut header_header_size = header;
        header_header_size.header_size = 0;
        header_header_size.checksum = 0;
        header_header_size.checksum = compute_crc32(header_header_size.as_bytes());
        match validate_header(&header_header_size) {
            Err(Error::InvalidFormat(err)) => assert_eq!(err, "Invalid header size"),
            _ => panic!(),
        };

        let mut header_ft_offset = header;
        header_ft_offset.file_table_offset = 0;
        header_ft_offset.checksum = 0;
        header_ft_offset.checksum = compute_crc32(header_ft_offset.as_bytes());
        match validate_header(&header_ft_offset) {
            Err(Error::InvalidFormat(err)) => assert_eq!(err, "Invalid file table offset"),
            _ => panic!(),
        };

        let mut header_ft_size = header;
        header_ft_size.file_table_size = 0;
        header_ft_size.checksum = 0;
        header_ft_size.checksum = compute_crc32(header_ft_size.as_bytes());
        match validate_header(&header_ft_size) {
            Err(Error::InvalidFormat(err)) => assert_eq!(err, "Invalid file table size"),
            _ => panic!(),
        };
    }

    #[test]
    fn test_initialize_file_metadata() {
        let mut file_table = VmgsFileTable::new_zeroed();

        file_table.entries[0].offset = 6;
        file_table.entries[0].allocation_size = 1;
        file_table.entries[1].offset = 2;
        file_table.entries[1].allocation_size = 1;
        file_table.entries[2].offset = 4;
        file_table.entries[2].allocation_size = 5;
        file_table.entries[3].offset = 3;
        file_table.entries[3].allocation_size = 3;

        let block_capacity = 1000;

        let fcbs = initialize_file_metadata(&file_table, VMGS_VERSION_3_0, block_capacity).unwrap();
        // assert VmgsFileEntry correctly converted to FileControlBlock
        assert!(fcbs[&FileId(0)].block_offset == 6);
        assert!(fcbs[&FileId(0)].allocated_blocks.get() == 1);
        assert!(fcbs[&FileId(1)].block_offset == 2);
        assert!(fcbs[&FileId(1)].allocated_blocks.get() == 1);
        assert!(fcbs[&FileId(2)].block_offset == 4);
        assert!(fcbs[&FileId(2)].allocated_blocks.get() == 5);
        assert!(fcbs[&FileId(3)].block_offset == 3);
        assert!(fcbs[&FileId(3)].allocated_blocks.get() == 3);
    }

    #[test]
    fn test_round_up_count() {
        assert!(round_up_count(0, 4096) == 0);
        assert!(round_up_count(1, 4096) == 4096);
        assert!(round_up_count(4095, 4096) == 4096);
        assert!(round_up_count(4096, 4096) == 4096);
        assert!(round_up_count(4097, 4096) == 8192);
    }

    #[async_test]
    async fn test_header_sequence_overflow() {
        let disk = new_test_file();
        let mut vmgs = Vmgs::format_new(disk).await.unwrap();

        vmgs.active_header_sequence_number = u32::MAX;

        // write
        let buf = b"hello world";
        vmgs.write_file(FileId::BIOS_NVRAM, buf).await.unwrap();

        assert_eq!(vmgs.active_header_index, 1);
        assert_eq!(vmgs.active_header_sequence_number, 0);

        vmgs.set_active_header(0, u32::MAX);

        let mut new_header = VmgsHeader::new_zeroed();
        vmgs.update_header(&mut new_header).await.unwrap();

        assert_eq!(vmgs.active_header_index, 1);
        assert_eq!(vmgs.active_header_sequence_number, 0);
        assert_eq!(new_header.sequence, 0);
    }

    #[cfg(with_encryption)]
    #[async_test]
    async fn write_file_v3() {
        let disk = new_test_file();
        let mut vmgs = Vmgs::format_new(disk.clone()).await.unwrap();
        let encryption_key = [12; VMGS_ENCRYPTION_KEY_SIZE];

        // write
        let buf = b"hello world";
        let buf_1 = b"hello universe";
        vmgs.write_file(FileId::BIOS_NVRAM, buf).await.unwrap();
        vmgs.add_new_encryption_key(&encryption_key, EncryptionAlgorithm::AES_GCM)
            .await
            .unwrap();
        vmgs.write_file_encrypted(FileId::TPM_PPI, buf_1)
            .await
            .unwrap();

        // read
        let read_buf = vmgs.read_file(FileId::BIOS_NVRAM).await.unwrap();
        assert_eq!(buf, &*read_buf);
        let info = vmgs.get_file_info(FileId::TPM_PPI).unwrap();
        assert_eq!(info.valid_bytes as usize, buf_1.len());
        let read_buf = vmgs.read_file(FileId::TPM_PPI).await.unwrap();
        assert_eq!(buf_1, &*read_buf);

        // Read the file after re-opening the vmgs file
        drop(vmgs);
        let mut vmgs = Vmgs::open(disk).await.unwrap();
        let read_buf = vmgs.read_file(FileId::BIOS_NVRAM).await.unwrap();
        assert_eq!(buf, read_buf.as_bytes());
        let info = vmgs.get_file_info(FileId::TPM_PPI).unwrap();
        assert_eq!(info.valid_bytes as usize, buf_1.len());
        let read_buf = vmgs.read_file(FileId::TPM_PPI).await.unwrap();
        assert_ne!(buf_1, read_buf.as_bytes());

        // Unlock datastore
        vmgs.unlock_with_encryption_key(&encryption_key)
            .await
            .unwrap();
        let info = vmgs.get_file_info(FileId::TPM_PPI).unwrap();
        assert_eq!(info.valid_bytes as usize, buf_1.len());
        let read_buf = vmgs.read_file(FileId::TPM_PPI).await.unwrap();
        assert_eq!(buf_1, &*read_buf);
    }

    #[cfg(with_encryption)]
    #[async_test]
    async fn overwrite_file_v3() {
        let disk = new_test_file();
        let mut vmgs = Vmgs::format_new(disk).await.unwrap();
        let encryption_key = [1; VMGS_ENCRYPTION_KEY_SIZE];
        let buf = vec![1; 8 * 1024];
        let buf_1 = vec![2; 8 * 1024];

        // Add root key.
        let key_index = vmgs
            .add_new_encryption_key(&encryption_key, EncryptionAlgorithm::AES_GCM)
            .await
            .unwrap();
        assert_eq!(key_index, 0);

        // Write a file to the store.
        vmgs.write_file_encrypted(FileId::BIOS_NVRAM, &buf)
            .await
            .unwrap();

        // Encrypt and overwrite the original file.
        vmgs.write_file_encrypted(FileId::BIOS_NVRAM, &buf_1)
            .await
            .unwrap();

        // Verify new file contents
        let read_buf = vmgs.read_file(FileId::BIOS_NVRAM).await.unwrap();
        assert_eq!(buf_1, read_buf);
    }

    #[cfg(with_encryption)]
    #[async_test]
    async fn file_encryption() {
        let buf: Vec<u8> = (0..255).collect();
        let encryption_key = [1; VMGS_ENCRYPTION_KEY_SIZE];

        let disk = new_test_file();
        let mut vmgs = Vmgs::format_new(disk.clone()).await.unwrap();

        // Add datastore key.
        let key_index = vmgs
            .add_new_encryption_key(&encryption_key, EncryptionAlgorithm::AES_GCM)
            .await
            .unwrap();
        assert_eq!(key_index, 0);

        // Write a file to the store.
        vmgs.write_file_encrypted(FileId::BIOS_NVRAM, &buf)
            .await
            .unwrap();

        // Read the file, without closing the datastore
        let read_buf = vmgs.read_file(FileId::BIOS_NVRAM).await.unwrap();
        assert_eq!(buf, read_buf);

        drop(vmgs);

        // Read the file, after closing and reopening the data store.
        let mut vmgs = Vmgs::open(disk).await.unwrap();

        let info = vmgs.get_file_info(FileId::BIOS_NVRAM).unwrap();
        assert_eq!(info.valid_bytes as usize, buf.len());

        // Unlock the store.

        let key_index = vmgs
            .unlock_with_encryption_key(&encryption_key)
            .await
            .unwrap();

        assert_eq!(key_index, 0);

        // Change to a new datastore key.
        let new_encryption_key = [2; VMGS_ENCRYPTION_KEY_SIZE];
        let key_index = vmgs
            .add_new_encryption_key(&new_encryption_key, EncryptionAlgorithm::AES_GCM)
            .await
            .unwrap();
        assert_eq!(key_index, 1);
        vmgs.remove_encryption_key(0).await.unwrap();

        let read_buf = vmgs.read_file(FileId::BIOS_NVRAM).await.unwrap();
        assert_eq!(buf, read_buf);
    }

    #[cfg(with_encryption)]
    #[async_test]
    async fn add_new_encryption_key() {
        let buf: Vec<u8> = (0..255).collect();
        let encryption_key = [1; VMGS_ENCRYPTION_KEY_SIZE];
        let new_encryption_key = [5; VMGS_ENCRYPTION_KEY_SIZE];

        // Initialize version 3 data store
        let disk = new_test_file();
        let mut vmgs = Vmgs::format_new(disk.clone()).await.unwrap();

        // Add datastore key.
        let key_index = vmgs
            .add_new_encryption_key(&encryption_key, EncryptionAlgorithm::AES_GCM)
            .await
            .unwrap();
        assert_eq!(key_index, 0);

        // Write a file to the store.
        vmgs.write_file_encrypted(FileId::BIOS_NVRAM, &buf)
            .await
            .unwrap();

        // Read the file, after closing and reopening the data store.
        drop(vmgs);
        let mut vmgs = Vmgs::open(disk.clone()).await.unwrap();
        let key_index = vmgs
            .unlock_with_encryption_key(&encryption_key)
            .await
            .unwrap();
        assert_eq!(key_index, 0);
        let read_buf = vmgs.read_file(FileId::BIOS_NVRAM).await.unwrap();
        assert_eq!(read_buf, buf);

        // Add new datastore key.
        let key_index = vmgs
            .add_new_encryption_key(&new_encryption_key, EncryptionAlgorithm::AES_GCM)
            .await
            .unwrap();
        assert_eq!(key_index, 1);

        // Read the file by using two different datastore keys, after closing and reopening the data store.
        drop(vmgs);
        let mut vmgs = Vmgs::open(disk).await.unwrap();
        let key_index = vmgs
            .unlock_with_encryption_key(&encryption_key)
            .await
            .unwrap();
        assert_eq!(key_index, 0);
        let key_index = vmgs
            .unlock_with_encryption_key(&new_encryption_key)
            .await
            .unwrap();
        assert_eq!(key_index, 1);
        let read_buf = vmgs.read_file(FileId::BIOS_NVRAM).await.unwrap();
        assert_eq!(read_buf, buf);

        // Remove the newly added datastore key and add it again.
        vmgs.remove_encryption_key(key_index).await.unwrap();
        let key_index = vmgs
            .add_new_encryption_key(&new_encryption_key, EncryptionAlgorithm::AES_GCM)
            .await
            .unwrap();
        assert_eq!(key_index, 1);

        // Remove the old datastore key
        vmgs.remove_encryption_key(0).await.unwrap();
        let result = vmgs.unlock_with_encryption_key(&encryption_key).await;
        assert!(matches!(result, Err(Error::Other(_))));

        // Try to remove the old datastore key again
        let result = vmgs.remove_encryption_key(0).await;
        assert!(matches!(result, Err(Error::Other(_))));

        // Remove the new datastore key and try to read file content, which should be in encrypted state
        vmgs.remove_encryption_key(1).await.unwrap();
        let read_buf = vmgs.read_file_raw(FileId::BIOS_NVRAM).await;
        assert_ne!(read_buf.unwrap(), buf);
    }

    #[cfg(with_encryption)]
    #[async_test]
    async fn test_write_file_encrypted() {
        // Call write_file_encrypted on an unencrypted VMGS and check that plaintext was written

        // Initialize version 3 data store
        let disk = new_test_file();
        let mut vmgs = Vmgs::format_new(disk.clone()).await.unwrap();
        let buf = b"This is plaintext";

        // call write file encrypted
        vmgs.write_file_encrypted(FileId::BIOS_NVRAM, buf)
            .await
            .unwrap();

        // Read
        let read_buf = vmgs.read_file(FileId::BIOS_NVRAM).await.unwrap();
        assert_eq!(vmgs.encryption_algorithm, EncryptionAlgorithm::NONE);
        assert_eq!(buf, &*read_buf);

        // ensure that when we re-create the VMGS object, we can still read the
        // FileId as plaintext
        drop(vmgs);
        let mut vmgs = Vmgs::open(disk).await.unwrap();

        let read_buf = vmgs.read_file(FileId::BIOS_NVRAM).await.unwrap();
        assert_eq!(vmgs.encryption_algorithm, EncryptionAlgorithm::NONE);
        assert_eq!(buf, &*read_buf);
    }
}
