linux-packaging-mono/mcs/class/referencesource/System/sys/system/IO/compression/OutputWindow.cs

namespace System.IO.Compression
{
    using System;
    using System.Diagnostics;
    using System.Globalization;

    // This class maintains a window for decompressed output.
    // We need to keep this because the decompressed information can be 
    // a literal or a length/distance pair. For length/distance pair,
    // we need to look back in the output window and copy bytes from there.
    // We use a byte array of WindowSize circularly.
    //
    internal class OutputWindow {
        
        private const int WindowSize = 32768;
        private const int WindowMask = 32767;

        private byte[] window = new byte[WindowSize]; //The window is 2^15 bytes
        private int end;       // this is the position to where we should write next byte 
        private int bytesUsed; // The number of bytes in the output window which is not consumed.

        // Add a byte to output window
        public void Write(byte b) {
            Debug.WriteLineIf(CompressionTracingSwitch.Verbose, String.Format(CultureInfo.InvariantCulture, "Literal: {0}", b), "Compression");
            Debug.Assert(bytesUsed < WindowSize, "Can't add byte when window is full!");
            window[end++] = b;
            end &= WindowMask;
            ++bytesUsed;
        }

        public void WriteLengthDistance(int length, int distance) {
            Debug.WriteLineIf(CompressionTracingSwitch.Verbose, String.Format(CultureInfo.InvariantCulture, "Length/Distance: {0}:{1}", length, distance), "Compression");
            Debug.Assert((bytesUsed + length) <= WindowSize, "Not enough space");

            // move backwards distance bytes in the output stream, 
            // and copy length bytes from this position to the output stream.
            bytesUsed += length;
            int copyStart = (end - distance) & WindowMask;  // start position for coping.

            int border = WindowSize - length;
            if (copyStart <= border && end < border) {  
                if (length <= distance) {
                    System.Array.Copy(window, copyStart, window, end, length);
                    end += length;
                } else {
                    // The referenced string may overlap the current
                    // position; for example, if the last 2 bytes decoded have values
                    // X and Y, a string reference with <length = 5, distance = 2>
                    // adds X,Y,X,Y,X to the output stream.
                    while (length-- > 0) {
                        window[end++] = window[copyStart++];
                    }
                }
            } 
            else { // copy byte by byte
                while (length-- > 0) {
                    window[end++] = window[copyStart++];
                    end &= WindowMask;                    
                    copyStart &= WindowMask;
                }
            }
        }

        // Copy up to length of bytes from input directly.
        // This is used for uncompressed block.
        public int CopyFrom(InputBuffer input, int length) {
            length = Math.Min(Math.Min(length, WindowSize - bytesUsed), input.AvailableBytes);
            int copied;

            // We might need wrap around to copy all bytes.
            int tailLen = WindowSize - end;
            if (length > tailLen) { 
                // copy the first part     
                copied = input.CopyTo(window, end, tailLen);
                if (copied == tailLen) {  
                    // only try to copy the second part if we have enough bytes in input
                    copied += input.CopyTo(window, 0, length - tailLen);
                }
            } 
            else {  
                // only one copy is needed if there is no wrap around.
                copied = input.CopyTo(window, end, length);
            }

            end = (end + copied) & WindowMask;
            bytesUsed += copied;
            return copied;
        }

        // Free space in output window
        public int FreeBytes {
            get {             
                return WindowSize - bytesUsed; 
            }
        }

        // bytes not consumed in output window
        public int AvailableBytes {
            get {
                return bytesUsed;
            }
        }

        // copy the decompressed bytes to output array.        
        public int CopyTo(byte[] output, int offset, int length) {
            int copy_end;

            if (length > bytesUsed) {   // we can copy all the decompressed bytes out
                copy_end = end;
                length = bytesUsed;
            } else {
                copy_end = (end - bytesUsed + length) & WindowMask;  // copy length of bytes
            }

            int copied = length;

            int tailLen = length - copy_end;   
            if ( tailLen > 0) {    // this means we need to copy two parts seperately
                // copy tailLen bytes from the end of output window
                System.Array.Copy(window, WindowSize - tailLen,
                                  output, offset, tailLen);
                offset += tailLen;
                length = copy_end;
            }
            System.Array.Copy(window, copy_end - length, output, offset, length);
            bytesUsed -= copied;
            Debug.Assert(bytesUsed >= 0, "check this function and find why we copied more bytes than we have"); 
            return copied;
        }
    }
}