using System.Buffers.Binary;

namespace AcDream.Core.Net.Cryptography;

/// <summary>
/// Simple 32-bit checksum used by the AC packet header. The algorithm is:
/// <list type="bullet">
///   <item>Seed the accumulator with <c>length &lt;&lt; 16</c>.</item>
///   <item>Sum the input as little-endian uint32s, four bytes at a time.</item>
///   <item>Fold any tail bytes (length not divisible by 4) into the top of
///         the accumulator with a descending left-shift per byte (24, 16, 8, 0).</item>
/// </list>
/// Reimplemented from reading ACE's AGPL reference
/// (<c>references/ACE/Source/ACE.Common/Cryptography/Hash32.cs</c>). See
/// <c>NOTICE.md</c> for the attribution policy.
/// </summary>
public static class Hash32
{
    public static uint Calculate(ReadOnlySpan<byte> data)
    {
        int length = data.Length;
        uint checksum = (uint)length << 16;

        int wordAligned = length & ~3;
        for (int i = 0; i < wordAligned; i += 4)
        {
            checksum += BinaryPrimitives.ReadUInt32LittleEndian(data.Slice(i));
        }

        // Tail bytes (0, 1, 2, or 3 bytes) with descending shift: first tail
        // byte goes into bits 24..31, second into 16..23, third into 8..15.
        int shift = 24;
        for (int j = wordAligned; j < length; j++)
        {
            checksum += (uint)data[j] << shift;
            shift -= 8;
        }

        return checksum;
    }
}