using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;

namespace SentinelKernel.System.FileSystem.FAT
{
    public class FatStream : Stream
    {
        protected readonly Listing.FatFile mFile;
        protected readonly FatFileSystem mFS = null;
        protected byte[] mReadBuffer;
        //TODO: In future we might read this in as needed rather than
        // all at once. This structure will also consume 2% of file size in RAM 
        // (for default cluster size of 2kb, ie 4 bytes per cluster)
        // so we might consider a way to flush it and only keep parts.
        // Example, a 100 MB file will require 2MB for this structure. That is
        // probably acceptable for the mid term future.
        protected List<UInt64> mFatTable;
        protected UInt64? mReadBufferPosition;

        public FatStream(Listing.FatFile aFile)
        {
            mFile = aFile;
            mFS = mFile.FileSystem;
            mReadBuffer = mFile.FileSystem.NewClusterArray();

            if (mFile.Size > 0)
            {
                mFatTable = mFile.GetFatTable();
            }
        }

        public override bool CanSeek
        {
            get { return true; }
        }

        public override bool CanRead
        {
            get { return true; }
        }

        public override bool CanWrite
        {
            get { return false; }
        }

        public override long Length
        {
            get { return (long)mFile.Size; }
        }

        protected UInt64 mPosition;
        public override long Position
        {
            get
            {
                return (long)mPosition;
            }
            set
            {
                if (value < 0L)
                {
                    throw new ArgumentOutOfRangeException("value");
                }
                mPosition = (ulong)value;
            }
        }

        public override int Read(byte[] aBuffer, int aOffset, int aCount)
        {
            return Read(aBuffer, (Int64)aOffset, (Int64)aCount);
        }

        public int Read(byte[] aBuffer, Int64 aOffset, Int64 aCount)
        {
            if (aCount < 0)
            {
                throw new ArgumentOutOfRangeException("aCount");
            }
            if (aOffset < 0)
            {
                throw new ArgumentOutOfRangeException("aOffset");
            }
            if (aBuffer == null || aBuffer.Length - aOffset < aCount)
            {
                throw new ArgumentException("Invalid offset length!");
            }
            if (mFile.FirstClusterNum == 0)
            {
                // FirstSector can be 0 for 0 length files
                return 0;
            }
            if (mPosition == mFile.Size)
            {
                // EOF
                return 0;
            }

            // reduce count, so that no out of bound exception occurs if not existing
            // entry is used in line mFS.ReadCluster(mFatTable[(int)xClusterIdx], xCluster);
            ulong xMaxReadableBytes = mFile.Size - mPosition;
            ulong xCount = (ulong)aCount;
            if (xCount > xMaxReadableBytes)
            {
                xCount = xMaxReadableBytes;
            }

            var xCluster = mFS.NewClusterArray();
            UInt32 xClusterSize = mFS.BytesPerCluster;

            while (xCount > 0)
            {
                UInt64 xClusterIdx = mPosition / xClusterSize;
                UInt64 xPosInCluster = mPosition % xClusterSize;
                mFS.ReadCluster((ulong)mFatTable[(int)xClusterIdx], xCluster);
                long xReadSize;
                if (xPosInCluster + xCount > xClusterSize)
                {
                    xReadSize = (long)(xClusterSize - xPosInCluster - 1);
                }
                else
                {
                    xReadSize = (long)xCount;
                }
                // no need for a long version, because internal Array.Copy() does a cast down to int, and a range check,
                // or we do a semantic change here
                Array.Copy(xCluster, (long)xPosInCluster, aBuffer, aOffset, xReadSize);

                aOffset += xReadSize;
                xCount -= (ulong)xReadSize;
                xCount = 0;
            }

            mPosition += (ulong)aOffset;
            return (int)aOffset;
        }

        public override void Flush()
        {
            throw new NotImplementedException();
        }

        public override long Seek(long offset, SeekOrigin origin)
        {
            throw new NotImplementedException();
        }

        public override void SetLength(long value)
        {
            throw new NotImplementedException();
        }

        public override void Write(byte[] aBuffer, int aOffset, int aCount)
        {
            Write(aBuffer, (long)aOffset, (long)aCount);
        }

        public void Write(byte[] buffer, long offset, long count)
        {
            throw new NotImplementedException();
        }
    }
}