// From a discussion on mastodon https://mastodon.social/@antonz@c.im/112620103070756332
// This version:
// * simplifies the setup (an inline array is already a Span)
// * uses BinaryPrimitives.ReverseEndianness to batch writing the long to the buffer
// * uses SkipLocalsInit on the ctor to avoid some zero init
// Should be ~ 15% faster than the revised version with Span
// Anyway most of the time is spent in DateTimeOffset.UtcNow and RandomNumberGenerator.Fill
using System.Buffers.Binary;
using System.Diagnostics.CodeAnalysis;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using System.Security.Cryptography;

public readonly struct UUIDv7Fast
{
    private readonly InteriorStruct _interior;

    [SkipLocalsInit]
    public UUIDv7Fast()
    {
        long timestamp = DateTimeOffset.UtcNow.ToUnixTimeMilliseconds();
        Unsafe.As<InteriorStruct, long>(ref _interior) = BitConverter.IsLittleEndian ? BinaryPrimitives.ReverseEndianness(timestamp << 16) : timestamp << 16;
        Span<byte> value = _interior;
        RandomNumberGenerator.Fill(value.Slice(6));
        // version and variant
        value[6] = (byte)((value[6] & 0x0F) | 0x70);
        value[8] = (byte)((value[8] & 0x3F) | 0x80);
    }

    public byte[] GetBytes() => ((ReadOnlySpan<byte>)_interior).ToArray();

    public void CopyTo(Span<byte> destination) => ((ReadOnlySpan<byte>)_interior).CopyTo(destination);

    public override string ToString() => Convert.ToHexString(_interior);

    public override int GetHashCode()
    {
        HashCode hc = new();
        hc.AddBytes(_interior);
        return hc.ToHashCode();
    }

    [InlineArray(16)]
    private struct InteriorStruct
    {
        private byte _element;
    }
}