chore: initial project import

backend/fleet/src/Fleet.Core/Ulid.cs

@@ -0,0 +1,41 @@
+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);
+}