social-media/backend/src/Web/Common/GuidExtensions.cs

namespace Hutopy.Web.Common;

/// <summary>
/// Adapted from https://raw.githubusercontent.com/uuidjs/uuid/main/src/v7.ts.
/// to match the uuid v7 generated on the client
/// </summary>
public static class GuidHelper
{
    private class V7State
    {
        public long Msecs { get; set; } = long.MinValue;
        public int Seq { get; set; }
    }

    private static readonly V7State State = new();
    private static readonly Random Random = new();

    public static Guid GenerateUuidV7()
    {
        byte[] randomValues = new byte[16];
        Random.NextBytes(randomValues);

        UpdateV7State(
            State,
            DateTimeOffset.UtcNow.ToUnixTimeMilliseconds(),
            randomValues);

        var values = V7Bytes(randomValues, State.Msecs, State.Seq);

        return new Guid(values);
    }

    private static void UpdateV7State(V7State state, long now, byte[] randomBytes)
    {
        if (now > state.Msecs)
        {
            state.Seq = (randomBytes[6] << 23) | (randomBytes[7] << 16) | (randomBytes[8] << 8) | randomBytes[9];
            state.Msecs = now;
        }
        else
        {
            state.Seq = (state.Seq + 1) | 0;
            if (state.Seq == 0)
            {
                state.Msecs++;
            }
        }
    }

    private static byte[] V7Bytes(byte[] randomBytes, long? msecs = null, int? seq = null, byte[]? buf = null, int offset = 0)
    {
        if (buf == null)
        {
            buf = new byte[16];
            offset = 0;
        }

        // Defaults
        msecs ??= DateTimeOffset.UtcNow.ToUnixTimeMilliseconds();
        seq ??= ((randomBytes[6] & 0x7f) << 24) | (randomBytes[7] << 16) | (randomBytes[8] << 8) | randomBytes[9];

        // byte 0-5: timestamp (48 bits)
        buf[offset++] = (byte)((msecs.Value / 0x10000000000) & 0xff);
        buf[offset++] = (byte)((msecs.Value / 0x100000000) & 0xff);
        buf[offset++] = (byte)((msecs.Value / 0x1000000) & 0xff);
        buf[offset++] = (byte)((msecs.Value / 0x10000) & 0xff);
        buf[offset++] = (byte)((msecs.Value / 0x100) & 0xff);
        buf[offset++] = (byte)(msecs.Value & 0xff);
        
        // byte 6: `version` (4 bits) | sequence bits 28-31 (4 bits)
        buf[offset++] = (byte)(0x70 | ((seq.Value >> 28) & 0x0f));
        
        // byte 7: sequence bits 20-27 (8 bits)
        buf[offset++] = (byte)((seq.Value >> 20) & 0xff);
        
        // byte 8: `variant` (2 bits) | sequence bits 14-19 (6 bits)
        buf[offset++] = (byte)(0x80 | ((seq.Value >> 14) & 0x3f));
        
        // byte 9: sequence bits 6-13 (8 bits)
        buf[offset++] = (byte)((seq.Value >> 6) & 0xff);
        
        // byte 10: sequence bits 0-5 (6 bits) | random (2 bits)
        buf[offset++] = (byte)(((seq.Value << 2) & 0xff) | (randomBytes[10] & 0x03));

        // bytes 11-15: random (40 bits)
        buf[offset++] = randomBytes[11];
        buf[offset++] = randomBytes[12];
        buf[offset++] = randomBytes[13];
        buf[offset++] = randomBytes[14];
        buf[offset] = randomBytes[15];

        return buf;
    }
}