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;
    }
}