using System;
using System.IO;
using System.Text;

namespace AsarSharp.PickleTools
{
    public class Pickle
    {
        public const int SIZE_INT32 = 4;
        public const int SIZE_UINT32 = 4;
        public const int SIZE_INT64 = 8;
        public const int SIZE_UINT64 = 8;
        public const int SIZE_FLOAT = 4;
        public const int SIZE_DOUBLE = 8;

        // Initial payload allocation. Bumped from 64 — large headers used to
        // realloc many times when growing geometrically from 64.
        public const int PAYLOAD_UNIT = 4096;

        public const long CAPACITY_READ_ONLY = 9007199254740992;

        private byte[] _header;
        private int _headerSize;
        private long _capacityAfterHeader;
        private int _writeOffset;

        private Pickle(byte[] buffer = null)
        {
            if (buffer != null)
            {
                if (buffer.Length < SIZE_UINT32)
                    throw new ArgumentException("Buffer is too small.", nameof(buffer));

                _header = buffer;
                int payloadSize = GetPayloadSize();
                if (payloadSize > buffer.Length)
                    throw new ArgumentException("Payload size exceeds buffer length.", nameof(buffer));

                _headerSize = buffer.Length - payloadSize;
                _capacityAfterHeader = CAPACITY_READ_ONLY;
                _writeOffset = 0;

                if (_headerSize != AlignInt(_headerSize, SIZE_UINT32))
                {
                    _headerSize = 0;
                }

                if (_headerSize == 0)
                {
                    _header = new byte[0];
                }
            }
            else
            {
                _header = new byte[0];
                _headerSize = SIZE_UINT32;
                _capacityAfterHeader = 0;
                _writeOffset = 0;
                Resize(PAYLOAD_UNIT);
                SetPayloadSize(0);
            }
        }

        public static Pickle CreateEmpty() => new Pickle();
        public static Pickle CreateFromBuffer(byte[] buffer) => new Pickle(buffer);

        public byte[] GetHeader() => _header;
        public int GetHeaderSize() => _headerSize;

        public PickleIterator CreateIterator() => new PickleIterator(this);

        /// <summary>Total byte length of the serialised pickle (header + payload).</summary>
        public int GetTotalSize() => _headerSize + GetPayloadSize();

        /// <summary>Materialise the pickle into a fresh byte array (allocates).</summary>
        public byte[] ToBuffer()
        {
            int resultSize = GetTotalSize();
            byte[] result = new byte[resultSize];
            Buffer.BlockCopy(_header, 0, result, 0, resultSize);
            return result;
        }

        /// <summary>Write the serialised pickle straight to <paramref name="stream"/> — no extra copy.</summary>
        public void WriteTo(Stream stream)
        {
            stream.Write(_header, 0, GetTotalSize());
        }




        public bool WriteInt(int value)
        {
            const int dataLength = SIZE_INT32; // already 4-byte aligned
            int newSize = _writeOffset + dataLength;

            if (newSize > _capacityAfterHeader)
            {
                Resize(Math.Max((int)_capacityAfterHeader * 2, newSize));
            }

            WriteInt32LE(value, _headerSize + _writeOffset);
            SetPayloadSize(newSize);
            _writeOffset = newSize;
            return true;
        }


        public bool WriteUInt32(uint value)
        {
            const int dataLength = SIZE_UINT32;
            int newSize = _writeOffset + dataLength;

            if (newSize > _capacityAfterHeader)
            {
                Resize(Math.Max((int)_capacityAfterHeader * 2, newSize));
            }

            WriteUInt32LE(value, _headerSize + _writeOffset);
            SetPayloadSize(newSize);
            _writeOffset = newSize;
            return true;
        }



        public bool WriteString(string value)
        {
            int byteLen = Encoding.UTF8.GetByteCount(value);

            if (!WriteInt(byteLen))
            {
                return false;
            }

            int aligned = AlignInt(byteLen, SIZE_UINT32);
            int newSize = _writeOffset + aligned;

            if (newSize > _capacityAfterHeader)
            {
                Resize(Math.Max((int)_capacityAfterHeader * 2, newSize));
            }

            int writeStart = _headerSize + _writeOffset;
            Encoding.UTF8.GetBytes(value, 0, value.Length, _header, writeStart);

            // zero alignment padding
            for (int i = writeStart + byteLen; i < writeStart + aligned; i++)
            {
                _header[i] = 0;
            }

            SetPayloadSize(newSize);
            _writeOffset = newSize;
            return true;
        }

        public void SetPayloadSize(int payloadSize)
        {
            WriteUInt32LE((uint)payloadSize, 0);
        }

        public int GetPayloadSize()
        {
            uint size = ReadUInt32LE(0);
            if (size > int.MaxValue)
                throw new InvalidOperationException("Payload size exceeds maximum allowed (2GB).");
            return (int)size;
        }

        private void Resize(int newCapacity)
        {
            newCapacity = AlignInt(newCapacity, PAYLOAD_UNIT);
            // The backing array must hold the header plus the full advertised
            // payload capacity (matches Chromium's realloc(header_size_ + new_capacity)).
            // Sizing it from _header.Length under-allocates by _headerSize on the
            // first growth (when _header is still empty), leaving the payload region
            // _headerSize bytes short of _capacityAfterHeader and overrunning the
            // buffer when a write fills the payload.
            byte[] newHeader = new byte[_headerSize + newCapacity];
            Buffer.BlockCopy(_header, 0, newHeader, 0, Math.Min(_header.Length, newHeader.Length));
            _header = newHeader;
            _capacityAfterHeader = newCapacity;
        }

        public static int AlignInt(int i, int alignment)
        {
            return i + ((alignment - (i % alignment)) % alignment);
        }

        #region Auxiliary methods for reading/writing values in Little Endian

        private uint ReadUInt32LE(int offset)
        {
            // _header is allocated by us so always little-endian-friendly when on LE host.
            return (uint)(_header[offset] |
                          (_header[offset + 1] << 8) |
                          (_header[offset + 2] << 16) |
                          (_header[offset + 3] << 24));
        }

        private void WriteInt32LE(int value, int offset)
        {
            _header[offset]     = (byte)value;
            _header[offset + 1] = (byte)(value >> 8);
            _header[offset + 2] = (byte)(value >> 16);
            _header[offset + 3] = (byte)(value >> 24);
        }

        private void WriteUInt32LE(uint value, int offset)
        {
            _header[offset]     = (byte)value;
            _header[offset + 1] = (byte)(value >> 8);
            _header[offset + 2] = (byte)(value >> 16);
            _header[offset + 3] = (byte)(value >> 24);
        }




        #endregion
    }
}