/// # Sample
/// Deutsch–Jozsa algorithm
///
/// # Description
/// Deutsch–Jozsa is a quantum algorithm that determines whether a given Boolean
/// function 𝑓 is constant (0 on all inputs or 1 on all inputs) or balanced
/// (1 for exactly half of the input domain and 0 for the other half).
///
/// This Q# program implements the Deutsch–Jozsa algorithm.
namespace Sample {
    open Microsoft.Quantum.Diagnostics;
    open Microsoft.Quantum.Math;
    open Microsoft.Quantum.Measurement;

    @EntryPoint()
    operation Main() : (String, Bool)[] {
        // A Boolean function is a function that maps bitstrings to a bit:
        //     𝑓 : {0, 1}^n → {0, 1}.

        // We say that 𝑓 is constant if 𝑓(𝑥⃗) = 𝑓(𝑦⃗) for all bitstrings 𝑥⃗ and
        // 𝑦⃗, and that 𝑓 is balanced if 𝑓 evaluates to true for exactly half of
        // its inputs.

        // If we are given a function 𝑓 as a quantum operation 𝑈 |𝑥〉|𝑦〉 =
        // |𝑥〉|𝑦 ⊕ 𝑓(𝑥)〉, and are promised that 𝑓 is either constant or is
        // balanced, then the Deutsch–Jozsa algorithm decides between these
        // cases with a single application of 𝑈.

        // Here, we demonstrate the use of the Deutsch-Jozsa algorithm by
        // determining the type (constant or balanced) of various functions.
        let nameFunctionTypeTuples = [
            ("SimpleConstantBoolF", SimpleConstantBoolF, true),
            ("SimpleBalancedBoolF", SimpleBalancedBoolF, false),
            ("ConstantBoolF", ConstantBoolF, true),
            ("BalancedBoolF", BalancedBoolF, false)
        ];

        mutable results = [];
        for (name, fn, shouldBeConstant) in nameFunctionTypeTuples {
            let isConstant = DeutschJozsa(fn, 5);
            if (isConstant != shouldBeConstant) {
                let shouldBeConstantStr = shouldBeConstant ? "constant" | "balanced";
                fail $"{name} should be detected as {shouldBeConstantStr}";
            }

            let isConstantStr = isConstant ? "constant" | "balanced";
            Message($"{name} is {isConstantStr}");
            set results += [(name, isConstant)];
        }

        return results;
    }

    /// # Summary
    /// This operation implements the DeutschJozsa algorithm.
    /// It returns the Boolean value `true` if the function is constant and
    /// `false` if it is not.
    /// It is assumed that the function is either constant or balanced.
    ///
    /// # Input
    /// ## Uf
    /// A quantum operation that implements |𝑥〉|𝑦〉 ↦ |𝑥〉|𝑦 ⊕ 𝑓(𝑥)〉, where 𝑓 is a
    /// Boolean function, 𝑥 is an 𝑛 bit register and 𝑦 is a single qubit.
    /// ## n
    /// The number of bits in the input register |𝑥〉.
    ///
    /// # Output
    /// A boolean value `true` that indicates that the function is constant and
    /// `false` that indicates that the function is balanced.
    ///
    /// # See Also
    /// - For details see Section 1.4.3 of Nielsen & Chuang.
    ///
    /// # References
    /// - [ *Michael A. Nielsen , Isaac L. Chuang*,
    ///     Quantum Computation and Quantum Information ]
    /// (http://doi.org/10.1017/CBO9780511976667)
    operation DeutschJozsa(Uf : ((Qubit[], Qubit) => Unit), n : Int) : Bool {
        // We allocate n + 1 clean qubits. Note that the function `Uf` is defined
        // on inputs of the form (x, y), where x has n bits and y has 1 bit.
        use queryRegister = Qubit[n];
        use target = Qubit();

        // The last qubit needs to be flipped so that the function will actually
        // be computed into the phase when Uf is applied.
        X(target);

        // Now, a Hadamard transform is applied to each of the qubits.
        H(target);
        // We use a within-apply block to ensure that the Hadamard transform is
        // correctly inverted on the |𝑥〉 register.
        within {
            for q in queryRegister {
                H(q);
            }
        } apply {
            // We apply Uf to the n+1 qubits, computing |𝑥, 𝑦〉 ↦ |𝑥, 𝑦 ⊕ 𝑓(𝑥)〉.
            Uf(queryRegister, target);
        }

        // The following for-loop measures all qubits and resets them to the |0〉
        // state so that they can be safely deallocated at the end of the block.
        // The loop also sets `result` to `true` if all measurement results are
        // `Zero`, i.e. if the function is a constant function, and sets
        // `result` to `false` if not, which according to the assumption on 𝑓
        // means that it must be balanced.
        mutable result = true;
        for q in queryRegister {
            if MResetZ(q) == One {
                set result = false;
            }
        }

        // Finally, the last qubit, which held the 𝑦-register, is reset.
        Reset(target);
        return result;
    }

    // Simple constant Boolean function
    operation SimpleConstantBoolF(args : Qubit[], target : Qubit) : Unit {
        X(target);
    }

    // Simple balanced Boolean function
    operation SimpleBalancedBoolF(args : Qubit[], target : Qubit) : Unit {
        CX(args[0], target);
    }

    // A more complex constant Boolean function.
    // It applies X to every input basis vector.
    operation ConstantBoolF(args : Qubit[], target : Qubit) : Unit {
        for i in 0..(2^Length(args)) - 1 {
            ApplyControlledOnInt(i, X, args, target);
        }
    }

    // A more complex balanced Boolean function.
    // It applies X to half of the input basis vectors.
    operation BalancedBoolF(args : Qubit[], target : Qubit) : Unit {
        for i in 0..2..(2^Length(args)) - 1 {
            ApplyControlledOnInt(i, X, args, target);
        }
    }
}
