ULID Generator — C# source

// ulid-generator - 26-char Crockford-base32 sortable ID: 10 chars of 48-bit ms timestamp + 16 chars of 80-bit randomness. Language: C# (.NET 8, BCL only). Port of src/lib/ulid.ts (Go twin: cli/ulid-generator) - encode/generate/decode-time core; inspectUlid (date/random-hex view) lives in this dir's javascript.js (80-line budget).
using System;
using System.Security.Cryptography;

internal static class Ulid
{
    private const string Crockford = "0123456789ABCDEFGHJKMNPQRSTVWXYZ"; // no I, L, O, U

    // Divide the big-endian value in b (mutated in place) by 32; return the
    // remainder 0-31. Long division in base 256, exactly as in the Go twin.
    private static int DivBy32(byte[] b)
    {
        int rem = 0;
        for (int i = 0; i < b.Length; i++)
        {
            int cur = (rem << 8) | b[i];
            b[i] = (byte)(cur >> 5);
            rem = cur & 0x1F;
        }
        return rem;
    }

    // Base32-encode the 128-bit big-endian [6 time bytes | 10 random bytes] into
    // the canonical 26 chars. Because 32^16 == 2^80, the first 10 chars equal the
    // TS/Go twins' separate time encoding - the cross-language lock-step anchor.
    public static string Encode(byte[] timeBytes, byte[] randomBytes)
    {
        var b = new byte[16];
        Array.Copy(timeBytes, 0, b, 0, 6);
        Array.Copy(randomBytes, 0, b, 6, 10);
        var outChars = new char[26];
        for (int i = 25; i >= 0; i--) outChars[i] = Crockford[DivBy32(b)];
        return new string(outChars);
    }

    // 48-bit ms timestamp -> 6 big-endian bytes (MSB first), like TS timeBytes().
    public static byte[] TimeBytes(long ms)
    {
        var outBytes = new byte[6];
        for (int i = 5; i >= 0; i--, ms >>= 8) outBytes[i] = (byte)(ms & 0xFF);
        return outBytes;
    }

    // New ULID for the epoch-ms. RandomNumberGenerator is the BCL CSPRNG -
    // the analogue of Go's crypto/rand.
    public static string Generate(long ms) =>
        Encode(TimeBytes(ms), RandomNumberGenerator.GetBytes(10));

    // Extract the 48-bit ms timestamp from the first 10 chars; throws on
    // malformed input (wrong length, or a char outside the alphabet such as I/L/O/U).
    public static long DecodeTime(string id)
    {
        if (id.Length != 26) throw new ArgumentException("ulid: malformed id: want 26 chars");
        long ts = 0;
        for (int i = 0; i < 10; i++)
        {
            int v = Crockford.IndexOf(id[i]);
            if (v < 0) throw new ArgumentException($"ulid: invalid character '{id[i]}'");
            ts = ts * 32 + v;
        }
        return ts;
    }
}

internal static class Program
{
    private static void Main()
    {
        string ulid = Ulid.Generate(1_722_000_000_000);
        Console.WriteLine($"ulid:     {ulid}");
        Console.WriteLine($"time(ms): {Ulid.DecodeTime(ulid)}  (round-trip ok)");
    }
}