using System.IO;

namespace BizHawk.MultiClient
{
    public partial class MainForm
    {
        private MruStack<MemoryStream> RewindBuf = new MruStack<MemoryStream>(15000);
        private byte[] LastState;

        private void CaptureRewindState()
        {
            if (LastState == null)
            {
                // This is the first frame. Capture the state, and put it in LastState for future deltas to be compared against.
                LastState = Global.Emulator.SaveStateBinary();
                //System.Console.WriteLine(LastState.Length);
                return;
            }

            // Otherwise, it's not the first frame, so build a delta.
            if (LastState.Length <= 0x10000)
                CaptureRewindState64K();
            else
                CaptureRewindStateLarge();
        }

        /// <summary>
        /// Builds a delta for states that are <= 64K in size.
        /// </summary>
        private void CaptureRewindState64K()
        {
            byte[] CurrentState = Global.Emulator.SaveStateBinary();
            int beginChangeSequence = -1;
            bool inChangeSequence = false;
            var ms = new MemoryStream();
            var writer = new BinaryWriter(ms);
            for (int i = 0; i < CurrentState.Length; i++)
            {
                if (inChangeSequence == false)
                {
                    if (CurrentState[i] == LastState[i])
                        continue;

                    inChangeSequence = true;
                    beginChangeSequence = i;
                    continue;
                }

                if (i - beginChangeSequence == 254 || i == CurrentState.Length - 1)
                {
                    writer.Write((byte)(i - beginChangeSequence + 1));
                    writer.Write((ushort)beginChangeSequence);
                    writer.Write(LastState, beginChangeSequence, i - beginChangeSequence + 1);
                    inChangeSequence = false;
                    continue;
                }

                if (CurrentState[i] == LastState[i])
                {
                    writer.Write((byte)(i - beginChangeSequence));
                    writer.Write((ushort)beginChangeSequence);
                    writer.Write(LastState, beginChangeSequence, i - beginChangeSequence);
                    inChangeSequence = false;
                }
            }
            LastState = CurrentState;
            ms.Position = 0;
            RewindBuf.Push(ms);
        }

        /// <summary>
        /// Builds a delta for states that are > 64K in size.
        /// </summary>
        private void CaptureRewindStateLarge()
        {
            byte[] CurrentState = Global.Emulator.SaveStateBinary();
            int beginChangeSequence = -1;
            bool inChangeSequence = false;
            var ms = new MemoryStream();
            var writer = new BinaryWriter(ms);
            for (int i = 0; i < CurrentState.Length; i++)
            {
                if (inChangeSequence == false)
                {
                    if (CurrentState[i] == LastState[i])
                        continue;

                    inChangeSequence = true;
                    beginChangeSequence = i;
                    continue;
                }

                if (i - beginChangeSequence == 254 || i == CurrentState.Length - 1)
                {
                    writer.Write((byte)(i - beginChangeSequence + 1));
                    writer.Write(beginChangeSequence);
                    writer.Write(LastState, beginChangeSequence, i - beginChangeSequence + 1);
                    inChangeSequence = false;
                    continue;
                }

                if (CurrentState[i] == LastState[i])
                {
                    writer.Write((byte)(i - beginChangeSequence));
                    writer.Write(beginChangeSequence);
                    writer.Write(LastState, beginChangeSequence, i - beginChangeSequence);
                    inChangeSequence = false;
                }
            }
            LastState = CurrentState;
            ms.Position = 0;
            RewindBuf.Push(ms);
        }

        private void Rewind64K()
        {
            var ms = RewindBuf.Pop();
            var reader = new BinaryReader(ms);
            var output = new MemoryStream(LastState);
            while (ms.Position < ms.Length - 1)
            {
                byte len = reader.ReadByte();
                ushort offset = reader.ReadUInt16();
                output.Position = offset;
                output.Write(ms.GetBuffer(), (int)ms.Position, len);
                ms.Position += len;
            }
            reader.Close();
            output.Position = 0;
            Global.Emulator.LoadStateBinary(new BinaryReader(output));
        }

        private void RewindLarge()
        {
            var ms = RewindBuf.Pop();
            var reader = new BinaryReader(ms);
            var output = new MemoryStream(LastState);
            while (ms.Position < ms.Length - 1)
            {
                byte len = reader.ReadByte();
                int offset = reader.ReadInt32();
                output.Position = offset;
                output.Write(ms.GetBuffer(), (int)ms.Position, len);
                ms.Position += len;
            }
            reader.Close();
            output.Position = 0;
            Global.Emulator.LoadStateBinary(new BinaryReader(output));
        }

        private void Rewind(int frames)
        {
            for (int i = 0; i < frames; i++)
            {
                if (RewindBuf.Count == 0)
                    return;
                if (LastState.Length < 0x10000)
                    Rewind64K();
                else
                    RewindLarge();
            }

            Global.Emulator.FrameAdvance(true);
            Global.Sound.UpdateSound(Global.Emulator.SoundProvider);
            Render();
        }

        public void ResetRewindBuffer()
        {
            RewindBuf.Clear();
            LastState = null;
        }
    }
}