using System;
using System.Text;

namespace XWorld.Framework.Protocol
{
    // 与 PacketWriter 对偶；任何越界/畸形长度抛 FormatException（容错入口统一）
    public sealed class PacketReader
    {
        private readonly byte[] _buf;
        private int _pos;

        public PacketReader(byte[] data)
        {
            _buf = data ?? Array.Empty<byte>();
            _pos = 0;
        }

        public int Position => _pos;
        public bool HasMore => _pos < _buf.Length;

        public byte ReadByte()
        {
            if (_pos >= _buf.Length) throw new FormatException("PacketReader: read past end");
            return _buf[_pos++];
        }

        public bool ReadBool() => ReadByte() != 0;

        public ushort ReadUInt16()
        {
            byte lo = ReadByte();
            byte hi = ReadByte();
            return (ushort)(lo | (hi << 8));
        }

        public uint ReadVarUInt()
        {
            uint result = 0;
            int shift = 0;
            while (true)
            {
                if (shift > 28) throw new FormatException("PacketReader: varuint too long");
                byte b = ReadByte();
                // 第 5 字节(shift==28)只有低 4 位有效，超出即为越界 varuint
                if (shift == 28 && (b & 0x70) != 0)
                    throw new FormatException("PacketReader: varuint overflow");
                result |= (uint)(b & 0x7F) << shift;
                if ((b & 0x80) == 0) break;
                shift += 7;
            }
            return result;
        }

        public int ReadVarInt()
        {
            uint zig = ReadVarUInt();
            return (int)(zig >> 1) ^ -(int)(zig & 1); // zigzag decode
        }

        public long ReadVarLong()
        {
            ulong result = 0;
            int shift = 0;
            while (true)
            {
                if (shift > 63) throw new FormatException("PacketReader: varlong too long");
                byte b = ReadByte();
                // 第 10 字节(shift==63)只有最低 1 位有效，超出即为越界 varlong
                if (shift == 63 && (b & 0x7E) != 0)
                    throw new FormatException("PacketReader: varlong overflow");
                result |= (ulong)(b & 0x7F) << shift;
                if ((b & 0x80) == 0) break;
                shift += 7;
            }
            return (long)(result >> 1) ^ -(long)(result & 1);
        }

        public float ReadSingle()
        {
            byte[] b = new byte[4];
            b[0] = ReadByte(); b[1] = ReadByte(); b[2] = ReadByte(); b[3] = ReadByte();
            if (!BitConverter.IsLittleEndian) Array.Reverse(b);
            return BitConverter.ToSingle(b, 0);
        }

        public byte[] ReadBytes()
        {
            uint len = ReadVarUInt();
            if (len == 0) return Array.Empty<byte>();
            if (_pos + len > _buf.Length) throw new FormatException("PacketReader: bytes length exceeds buffer");
            byte[] outv = new byte[len];
            Array.Copy(_buf, _pos, outv, 0, (int)len);
            _pos += (int)len;
            return outv;
        }

        public string ReadString()
        {
            uint len = ReadVarUInt();
            if (len == 0) return string.Empty;
            if (_pos + len > _buf.Length) throw new FormatException("PacketReader: string length exceeds buffer");
            string s = Encoding.UTF8.GetString(_buf, _pos, (int)len);
            _pos += (int)len;
            return s;
        }
    }
}