// -------------------------------------------------------------------------------------------------
// Copyright (c) Microsoft Corporation. All rights reserved.
// Licensed under the MIT License (MIT). See LICENSE in the repo root for license information.
// -------------------------------------------------------------------------------------------------

using System.Collections.Generic;
using System.Linq;
using EnsureThat;

namespace Microsoft.Health.Core.Extensions;

public static class EnumerableExtensions
{
    /// <summary>
    /// Creates the cartesian product.
    /// </summary>
    /// <typeparam name="TSource">The type of the elements of the input sequences.</typeparam>
    /// <param name="sequences">The input sequence.</param>
    /// <returns>An <see cref="IEnumerable{T}"/> that contains the cartesian product of the input sequences.</returns>
    public static IEnumerable<IEnumerable<TSource>> CartesianProduct<TSource>(this IEnumerable<IEnumerable<TSource>> sequences)
    {
        EnsureArg.IsNotNull(sequences, nameof(sequences));

        IEnumerable<IEnumerable<TSource>> emptyProduct = new[] { Enumerable.Empty<TSource>() };

        return sequences.Aggregate(
            emptyProduct,
            (accumulator, sequence) =>
            {
                return accumulator.SelectMany(a => sequence.Select(s => a.Concat(Enumerable.Repeat(s, 1))));
            });
    }

    /// <summary>
    /// Generates a sequence that contains one value.
    /// </summary>
    /// <typeparam name="TResult">The element type.</typeparam>
    /// <param name="element">The element to return.</param>
    /// <returns>An <see cref="IEnumerable{T}"/> that contains one value.</returns>
    public static IEnumerable<TResult> AsEnumerable<TResult>(this TResult element)
    {
        yield return element;
    }
}