using System.Runtime.InteropServices;
using System.Security.Cryptography;

namespace Just.Core;

public static class GuidV8
{
    private const long TicksPrecision = TimeSpan.TicksPerMillisecond / 10; // 100-microsecond units

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    [ExcludeFromCodeCoverage]
    public static Guid NewGuid(RngEntropy entropy = RngEntropy.Strong) => NewGuid(DateTime.UtcNow, entropy);

    public static Guid NewGuid(DateTime dateTime, RngEntropy entropy = RngEntropy.Strong)
    {
        var epoch = dateTime.Subtract(DateTime.UnixEpoch);
        var timestamp = epoch.Ticks / TicksPrecision;

        // Negative timestamps can't be encoded correctly due to unsigned bit operations
        if (timestamp < 0)
        {
            throw new ArgumentException("Timestamp must be on or after UnixEpoch (1970-01-01 UTC).", nameof(dateTime));
        }

        uint tsHigh = (uint)((timestamp >> 16) & 0xFFFFFFFF);
        ushort tsLow = (ushort)(timestamp & 0x0000FFFF);

        Span<byte> bytes = stackalloc byte[10];

        if (entropy == RngEntropy.Strong)
        {
            RandomNumberGenerator.Fill(bytes);
        }
        else
        {
            Random.Shared.NextBytes(bytes);
        }

        bytes[0] = (byte)((bytes[0] & 0x0F) | 0x80); // Version 8
        bytes[2] = (byte)((bytes[2] & 0x1F) | 0x80); // Variant 0b1000

        ushort version = (ushort)((bytes[0] << 8) | bytes[1]);

        return new Guid(
            tsHigh,
            tsLow,
            version,
            bytes[2], bytes[3], bytes[4], bytes[5], bytes[6], bytes[7], bytes[8], bytes[9]);
    }

    /// <summary>
    /// Extracts the timestamp from a UUID v8 generated by <see cref="NewGuid(DateTime, RngEntropy)"/>.
    /// </summary>
    /// <param name="guid">The UUID v8 to extract the timestamp from.</param>
    /// <returns>The <see cref="DateTime"/> (UTC) encoded in the GUID's timestamp fields.</returns>
    /// <exception cref="ArgumentException">
    /// Thrown if <paramref name="guid"/> is not a UUID v8.
    /// </exception>
    /// <remarks>
    /// The returned timestamp has 100-microsecond precision (the resolution used by
    /// <see cref="NewGuid(DateTime, RngEntropy)"/>). Sub-100µs components of the original
    /// <see cref="DateTime"/> are not recoverable.
    /// </remarks>
    [Pure]
    public static DateTime ExtractTimestamp(Guid guid)
    {
        Span<byte> bytes = stackalloc byte[16];
        guid.TryWriteBytes(bytes);

        // Version is the high nibble of byte 7 (parameter 'c' high byte, little-endian byte 7)
        var version = bytes[7] >> 4;
        if (version != 8)
            throw new ArgumentException($"The provided GUID is not a UUID v8 (version={version}).", nameof(guid));

        // tsHigh is stored as Int32 at bytes 0-3
        uint tsHigh = MemoryMarshal.Read<uint>(bytes);
        // tsLow is stored as Int16 at bytes 4-5
        ushort tsLow = MemoryMarshal.Read<ushort>(bytes[4..]);

        long timestamp = ((long)tsHigh << 16) | tsLow;
        return DateTime.UnixEpoch.AddTicks(timestamp * TicksPrecision);
    }
}