using System.Security.Cryptography;

namespace Fleet.Core;

/// <summary>26 个字符的 ULID（Crockford base32，前 48 位为毫秒时间戳），按时间可排序。</summary>
public static class Ulid
{
    private const string Alphabet = "0123456789ABCDEFGHJKMNPQRSTVWXYZ";

    public static string New(TimeProvider time)
    {
        long ms = time.GetUtcNow().ToUnixTimeMilliseconds();
        Span<byte> random = stackalloc byte[10];
        RandomNumberGenerator.Fill(random);

        Span<char> chars = stackalloc char[26];
        for (int i = 9; i >= 0; i--)
        {
            chars[i] = Alphabet[(int)(ms & 31)];
            ms >>= 5;
        }

        // 80 位随机数 → 16 个 base32 字符
        UInt128 r = 0;
        foreach (byte b in random)
        {
            r = (r << 8) | b;
        }

        for (int i = 25; i >= 10; i--)
        {
            chars[i] = Alphabet[(int)(r & 31)];
            r >>= 5;
        }

        return new string(chars);
    }

    /// <summary>带类型前缀的 ID，例如 <c>agt_01J…</c>。</summary>
    public static string NewPrefixed(string prefix, TimeProvider time) => prefix + "_" + New(time);
}