chore: initial project import

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

@@ -0,0 +1,66 @@
+using System.Buffers.Binary;
+
+namespace Fleet.Core;
+
+/// <summary>LEB128 变长整数（无符号），以及 zigzag 有符号映射。</summary>
+public static class VarInt
+{
+    public const int MaxBytes = 10;
+
+    public static ulong ZigZag(long value) => (ulong)((value << 1) ^ (value >> 63));
+
+    public static long UnZigZag(ulong value) => (long)(value >> 1) ^ -(long)(value & 1);
+
+    public static int Write(Span<byte> destination, ulong value)
+    {
+        int i = 0;
+        while (value >= 0x80)
+        {
+            destination[i++] = (byte)(value | 0x80);
+            value >>= 7;
+        }
+
+        destination[i++] = (byte)value;
+        return i;
+    }
+
+    public static void Write(Stream destination, ulong value)
+    {
+        Span<byte> buffer = stackalloc byte[MaxBytes];
+        int n = Write(buffer, value);
+        destination.Write(buffer[..n]);
+    }
+
+    /// <summary>读取失败（数据被截断或超过 10 字节）时返回 false。</summary>
+    public static bool TryRead(ReadOnlySpan<byte> source, out ulong value, out int consumed)
+    {
+        value = 0;
+        consumed = 0;
+        int shift = 0;
+        while (consumed < source.Length && consumed < MaxBytes)
+        {
+            byte b = source[consumed++];
+            value |= (ulong)(b & 0x7F) << shift;
+            if ((b & 0x80) == 0)
+            {
+                return true;
+            }
+
+            shift += 7;
+        }
+
+        return false;
+    }
+
+    public static bool TryReadUInt32LittleEndian(ReadOnlySpan<byte> source, out uint value)
+    {
+        if (source.Length < 4)
+        {
+            value = 0;
+            return false;
+        }
+
+        value = BinaryPrimitives.ReadUInt32LittleEndian(source);
+        return true;
+    }
+}