// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

using System.Runtime.CompilerServices;

namespace CommunityToolkit.HighPerformance.Helpers.Internals;

/// <summary>
/// Helpers to process sequences of values by reference with a given step.
/// </summary>
internal static class RefEnumerableHelper
{
    /// <summary>
    /// Clears a target memory area.
    /// </summary>
    /// <typeparam name="T">The type of values to clear.</typeparam>
    /// <param name="r0">A <typeparamref name="T"/> reference to the start of the memory area.</param>
    /// <param name="length">The number of items in the memory area.</param>
    /// <param name="step">The number of items between each consecutive target value.</param>
    public static void Clear<T>(ref T r0, nint length, nint step)
    {
        nint offset = 0;

        // Main loop with 8 unrolled iterations
        while (length >= 8)
        {
            Unsafe.Add(ref r0, offset) = default!;
            Unsafe.Add(ref r0, offset += step) = default!;
            Unsafe.Add(ref r0, offset += step) = default!;
            Unsafe.Add(ref r0, offset += step) = default!;
            Unsafe.Add(ref r0, offset += step) = default!;
            Unsafe.Add(ref r0, offset += step) = default!;
            Unsafe.Add(ref r0, offset += step) = default!;
            Unsafe.Add(ref r0, offset += step) = default!;

            length -= 8;
            offset += step;
        }

        if (length >= 4)
        {
            Unsafe.Add(ref r0, offset) = default!;
            Unsafe.Add(ref r0, offset += step) = default!;
            Unsafe.Add(ref r0, offset += step) = default!;
            Unsafe.Add(ref r0, offset += step) = default!;

            length -= 4;
            offset += step;
        }

        // Clear the remaining values
        while (length > 0)
        {
            Unsafe.Add(ref r0, offset) = default!;

            length -= 1;
            offset += step;
        }
    }

    /// <summary>
    /// Copies a sequence of discontiguous items from one memory area to another.
    /// </summary>
    /// <typeparam name="T">The type of items to copy.</typeparam>
    /// <param name="sourceRef">The source reference to copy from.</param>
    /// <param name="destinationRef">The target reference to copy to.</param>
    /// <param name="length">The total number of items to copy.</param>
    /// <param name="sourceStep">The step between consecutive items in the memory area pointed to by <paramref name="sourceRef"/>.</param>
    public static void CopyTo<T>(ref T sourceRef, ref T destinationRef, nint length, nint sourceStep)
    {
        nint sourceOffset = 0;
        nint destinationOffset = 0;

        while (length >= 8)
        {
            Unsafe.Add(ref destinationRef, destinationOffset + 0) = Unsafe.Add(ref sourceRef, sourceOffset);
            Unsafe.Add(ref destinationRef, destinationOffset + 1) = Unsafe.Add(ref sourceRef, sourceOffset += sourceStep);
            Unsafe.Add(ref destinationRef, destinationOffset + 2) = Unsafe.Add(ref sourceRef, sourceOffset += sourceStep);
            Unsafe.Add(ref destinationRef, destinationOffset + 3) = Unsafe.Add(ref sourceRef, sourceOffset += sourceStep);
            Unsafe.Add(ref destinationRef, destinationOffset + 4) = Unsafe.Add(ref sourceRef, sourceOffset += sourceStep);
            Unsafe.Add(ref destinationRef, destinationOffset + 5) = Unsafe.Add(ref sourceRef, sourceOffset += sourceStep);
            Unsafe.Add(ref destinationRef, destinationOffset + 6) = Unsafe.Add(ref sourceRef, sourceOffset += sourceStep);
            Unsafe.Add(ref destinationRef, destinationOffset + 7) = Unsafe.Add(ref sourceRef, sourceOffset += sourceStep);

            length -= 8;
            sourceOffset += sourceStep;
            destinationOffset += 8;
        }

        if (length >= 4)
        {
            Unsafe.Add(ref destinationRef, destinationOffset + 0) = Unsafe.Add(ref sourceRef, sourceOffset);
            Unsafe.Add(ref destinationRef, destinationOffset + 1) = Unsafe.Add(ref sourceRef, sourceOffset += sourceStep);
            Unsafe.Add(ref destinationRef, destinationOffset + 2) = Unsafe.Add(ref sourceRef, sourceOffset += sourceStep);
            Unsafe.Add(ref destinationRef, destinationOffset + 3) = Unsafe.Add(ref sourceRef, sourceOffset += sourceStep);

            length -= 4;
            sourceOffset += sourceStep;
            destinationOffset += 4;
        }

        while (length > 0)
        {
            Unsafe.Add(ref destinationRef, destinationOffset) = Unsafe.Add(ref sourceRef, sourceOffset);

            length -= 1;
            sourceOffset += sourceStep;
            destinationOffset += 1;
        }
    }

    /// <summary>
    /// Copies a sequence of discontiguous items from one memory area to another.
    /// </summary>
    /// <typeparam name="T">The type of items to copy.</typeparam>
    /// <param name="sourceRef">The source reference to copy from.</param>
    /// <param name="destinationRef">The target reference to copy to.</param>
    /// <param name="length">The total number of items to copy.</param>
    /// <param name="sourceStep">The step between consecutive items in the memory area pointed to by <paramref name="sourceRef"/>.</param>
    /// <param name="destinationStep">The step between consecutive items in the memory area pointed to by <paramref name="destinationRef"/>.</param>
    public static void CopyTo<T>(ref T sourceRef, ref T destinationRef, nint length, nint sourceStep, nint destinationStep)
    {
        nint sourceOffset = 0;
        nint destinationOffset = 0;

        while (length >= 8)
        {
            Unsafe.Add(ref destinationRef, destinationOffset) = Unsafe.Add(ref sourceRef, sourceOffset);
            Unsafe.Add(ref destinationRef, destinationOffset += destinationStep) = Unsafe.Add(ref sourceRef, sourceOffset += sourceStep);
            Unsafe.Add(ref destinationRef, destinationOffset += destinationStep) = Unsafe.Add(ref sourceRef, sourceOffset += sourceStep);
            Unsafe.Add(ref destinationRef, destinationOffset += destinationStep) = Unsafe.Add(ref sourceRef, sourceOffset += sourceStep);
            Unsafe.Add(ref destinationRef, destinationOffset += destinationStep) = Unsafe.Add(ref sourceRef, sourceOffset += sourceStep);
            Unsafe.Add(ref destinationRef, destinationOffset += destinationStep) = Unsafe.Add(ref sourceRef, sourceOffset += sourceStep);
            Unsafe.Add(ref destinationRef, destinationOffset += destinationStep) = Unsafe.Add(ref sourceRef, sourceOffset += sourceStep);
            Unsafe.Add(ref destinationRef, destinationOffset += destinationStep) = Unsafe.Add(ref sourceRef, sourceOffset += sourceStep);

            length -= 8;
            sourceOffset += sourceStep;
            destinationOffset += destinationStep;
        }

        if (length >= 4)
        {
            Unsafe.Add(ref destinationRef, destinationOffset) = Unsafe.Add(ref sourceRef, sourceOffset);
            Unsafe.Add(ref destinationRef, destinationOffset += destinationStep) = Unsafe.Add(ref sourceRef, sourceOffset += sourceStep);
            Unsafe.Add(ref destinationRef, destinationOffset += destinationStep) = Unsafe.Add(ref sourceRef, sourceOffset += sourceStep);
            Unsafe.Add(ref destinationRef, destinationOffset += destinationStep) = Unsafe.Add(ref sourceRef, sourceOffset += sourceStep);

            length -= 4;
            sourceOffset += sourceStep;
            destinationOffset += destinationStep;
        }

        while (length > 0)
        {
            Unsafe.Add(ref destinationRef, destinationOffset) = Unsafe.Add(ref sourceRef, sourceOffset);

            length -= 1;
            sourceOffset += sourceStep;
            destinationOffset += destinationStep;
        }
    }

    /// <summary>
    /// Copies a sequence of discontiguous items from one memory area to another. This mirrors
    /// <see cref="CopyTo{T}(ref T,ref T,nint,nint)"/>, but <paramref name="sourceStep"/> refers to <paramref name="destinationRef"/> instead.
    /// </summary>
    /// <typeparam name="T">The type of items to copy.</typeparam>
    /// <param name="sourceRef">The source reference to copy from.</param>
    /// <param name="destinationRef">The target reference to copy to.</param>
    /// <param name="length">The total number of items to copy.</param>
    /// <param name="sourceStep">The step between consecutive items in the memory area pointed to by <paramref name="sourceRef"/>.</param>
    public static void CopyFrom<T>(ref T sourceRef, ref T destinationRef, nint length, nint sourceStep)
    {
        nint sourceOffset = 0;
        nint destinationOffset = 0;

        while (length >= 8)
        {
            Unsafe.Add(ref destinationRef, destinationOffset) = Unsafe.Add(ref sourceRef, sourceOffset);
            Unsafe.Add(ref destinationRef, destinationOffset += sourceStep) = Unsafe.Add(ref sourceRef, sourceOffset + 1);
            Unsafe.Add(ref destinationRef, destinationOffset += sourceStep) = Unsafe.Add(ref sourceRef, sourceOffset + 2);
            Unsafe.Add(ref destinationRef, destinationOffset += sourceStep) = Unsafe.Add(ref sourceRef, sourceOffset + 3);
            Unsafe.Add(ref destinationRef, destinationOffset += sourceStep) = Unsafe.Add(ref sourceRef, sourceOffset + 4);
            Unsafe.Add(ref destinationRef, destinationOffset += sourceStep) = Unsafe.Add(ref sourceRef, sourceOffset + 5);
            Unsafe.Add(ref destinationRef, destinationOffset += sourceStep) = Unsafe.Add(ref sourceRef, sourceOffset + 6);
            Unsafe.Add(ref destinationRef, destinationOffset += sourceStep) = Unsafe.Add(ref sourceRef, sourceOffset + 7);

            length -= 8;
            sourceOffset += 8;
            destinationOffset += sourceStep;
        }

        if (length >= 4)
        {
            Unsafe.Add(ref destinationRef, destinationOffset) = Unsafe.Add(ref sourceRef, sourceOffset);
            Unsafe.Add(ref destinationRef, destinationOffset += sourceStep) = Unsafe.Add(ref sourceRef, sourceOffset + 1);
            Unsafe.Add(ref destinationRef, destinationOffset += sourceStep) = Unsafe.Add(ref sourceRef, sourceOffset + 2);
            Unsafe.Add(ref destinationRef, destinationOffset += sourceStep) = Unsafe.Add(ref sourceRef, sourceOffset + 3);

            length -= 4;
            sourceOffset += 4;
            destinationOffset += sourceStep;
        }

        while (length > 0)
        {
            Unsafe.Add(ref destinationRef, destinationOffset) = Unsafe.Add(ref sourceRef, sourceOffset);

            length -= 1;
            sourceOffset += 1;
            destinationOffset += sourceStep;
        }
    }

    /// <summary>
    /// Fills a target memory area.
    /// </summary>
    /// <typeparam name="T">The type of values to fill.</typeparam>
    /// <param name="r0">A <typeparamref name="T"/> reference to the start of the memory area.</param>
    /// <param name="length">The number of items in the memory area.</param>
    /// <param name="step">The number of items between each consecutive target value.</param>
    /// <param name="value">The value to assign to every item in the target memory area.</param>
    public static void Fill<T>(ref T r0, nint length, nint step, T value)
    {
        nint offset = 0;

        while (length >= 8)
        {
            Unsafe.Add(ref r0, offset) = value;
            Unsafe.Add(ref r0, offset += step) = value;
            Unsafe.Add(ref r0, offset += step) = value;
            Unsafe.Add(ref r0, offset += step) = value;
            Unsafe.Add(ref r0, offset += step) = value;
            Unsafe.Add(ref r0, offset += step) = value;
            Unsafe.Add(ref r0, offset += step) = value;
            Unsafe.Add(ref r0, offset += step) = value;

            length -= 8;
            offset += step;
        }

        if (length >= 4)
        {
            Unsafe.Add(ref r0, offset) = value;
            Unsafe.Add(ref r0, offset += step) = value;
            Unsafe.Add(ref r0, offset += step) = value;
            Unsafe.Add(ref r0, offset += step) = value;

            length -= 4;
            offset += step;
        }

        while (length > 0)
        {
            Unsafe.Add(ref r0, offset) = value;

            length -= 1;
            offset += step;
        }
    }
}