Deflater.cs source code in C# .NET

Source code for the .NET framework in C#

                        
Code:

                         / 4.0 / 4.0 / untmp / DEVDIV_TFS / Dev10 / Releases / RTMRel / ndp / fx / src / Sys / System / IO / compression / Deflater.cs / 1305376 / Deflater.cs
                        

                        
                            // ==++== 
//
//  Copyright (c) Microsoft Corporation.  All rights reserved.
//
//  zlib.h -- interface of the 'zlib' general purpose compression library 
//  version 1.2.1, November 17th, 2003
// 
//  Copyright (C) 1995-2003 Jean-loup Gailly and Mark Adler 
//
//  This software is provided 'as-is', without any express or implied 
//  warranty.  In no event will the authors be held liable for any damages
//  arising from the use of this software.
//
//  Permission is granted to anyone to use this software for any purpose, 
//  including commercial applications, and to alter it and redistribute it
//  freely, subject to the following restrictions: 
// 
//  1. The origin of this software must not be misrepresented; you must not
//     claim that you wrote the original software. If you use this software 
//     in a product, an acknowledgment in the product documentation would be
//     appreciated but is not required.
//  2. Altered source versions must be plainly marked as such, and must not be
//     misrepresented as being the original software. 
//  3. This notice may not be removed or altered from any source distribution.
// 
// 
// ==--==
// Compression engine 


namespace System.IO.Compression {
    using System; 
    using System.Diagnostics;
 
    internal class Deflater { 
        private const int MinBlockSize = 256;
        private const int MaxHeaderFooterGoo = 120; 
        private const int CleanCopySize = DeflateStream.DefaultBufferSize - MaxHeaderFooterGoo;
        private const double BadCompressionThreshold = 1.0;

        private FastEncoder deflateEncoder; 
        private CopyEncoder copyEncoder;
 
        private DeflateInput input; 
        private OutputBuffer output;
        private DeflaterState processingState; 
        private DeflateInput inputFromHistory;

        public Deflater() {
            deflateEncoder = new FastEncoder(); 
            copyEncoder = new CopyEncoder();
            input = new DeflateInput(); 
            output = new OutputBuffer(); 

            processingState = DeflaterState.NotStarted; 
        }

        public bool NeedsInput() {
            return input.Count == 0 && deflateEncoder.BytesInHistory == 0; 
        }
 
        // Sets the input to compress. The only buffer copy occurs when the input is copied 
        // to the FastEncoderWindow
        public void SetInput(byte[] inputBuffer, int startIndex, int count) { 
            Debug.Assert(input.Count == 0, "We have something left in previous input!");

            input.Buffer = inputBuffer;
            input.Count = count; 
            input.StartIndex = startIndex;
 
            if (count > 0 && count < MinBlockSize) { 
                // user is writing small buffers. If buffer size is below MinBlockSize, we
                // need to switch to a small data mode, to avoid block headers and footers 
                // dominating the output.
                switch (processingState) {
                    case DeflaterState.NotStarted :
                    case DeflaterState.CheckingForIncompressible: 
                        // clean states, needs a block header first
                        processingState = DeflaterState.StartingSmallData; 
                        break; 
                    case DeflaterState.CompressThenCheck:
                        // already has correct block header 
                        processingState = DeflaterState.HandlingSmallData;
                        break;
                }
            } 
        }
 
        public int GetDeflateOutput(byte[] outputBuffer) { 
            Debug.Assert(outputBuffer != null, "Can't pass in a null output buffer!");
            Debug.Assert(!NeedsInput(), "GetDeflateOutput should only be called after providing input"); 

            output.UpdateBuffer(outputBuffer);

            switch(processingState) { 

                case DeflaterState.NotStarted: { 
                        // first call. Try to compress but if we get bad compression ratio, switch to uncompressed blocks. 
                        Debug.Assert(deflateEncoder.BytesInHistory == 0, "have leftover bytes in window");
 
                        // save these in case we need to switch to uncompressed format
                        DeflateInput.InputState initialInputState = input.DumpState();
                        OutputBuffer.BufferState initialOutputState = output.DumpState();
 
                        deflateEncoder.GetBlockHeader(output);
                        deflateEncoder.GetCompressedData(input, output); 
 
                        if (!UseCompressed(deflateEncoder.LastCompressionRatio)) {
                            // we're expanding; restore state and switch to uncompressed 
                            input.RestoreState(initialInputState);
                            output.RestoreState(initialOutputState);
                            copyEncoder.GetBlock(input, output, false);
                            FlushInputWindows(); 
                            processingState = DeflaterState.CheckingForIncompressible;
                        } 
                        else { 
                            processingState = DeflaterState.CompressThenCheck;
                        } 

                        break;
                    }
                case DeflaterState.CompressThenCheck: { 
                        // continue assuming data is compressible. If we reach data that indicates otherwise
                        // finish off remaining data in history and decide whether to compress on a 
                        // block-by-block basis 
                        deflateEncoder.GetCompressedData(input, output);
 
                        if (!UseCompressed(deflateEncoder.LastCompressionRatio)) {
                            processingState = DeflaterState.SlowDownForIncompressible1;
                            inputFromHistory = deflateEncoder.UnprocessedInput;
                        } 
                        break;
                    } 
                case DeflaterState.SlowDownForIncompressible1: { 
                        // finish off previous compressed block
                        deflateEncoder.GetBlockFooter(output); 

                        processingState = DeflaterState.SlowDownForIncompressible2;
                        goto case DeflaterState.SlowDownForIncompressible2; // yeah I know, but there's no fallthrough
                    } 

                case DeflaterState.SlowDownForIncompressible2: { 
                        // clear out data from history, but add them as uncompressed blocks 
                        if (inputFromHistory.Count > 0) {
                            copyEncoder.GetBlock(inputFromHistory, output, false); 
                        }

                        if (inputFromHistory.Count == 0) {
                            // now we're clean 
                            deflateEncoder.FlushInput();
                            processingState = DeflaterState.CheckingForIncompressible; 
                        } 
                        break;
                    } 

                case DeflaterState.CheckingForIncompressible: {
                        // decide whether to compress on a block-by-block basis
                        Debug.Assert(deflateEncoder.BytesInHistory == 0, "have leftover bytes in window"); 

                        // save these in case we need to store as uncompressed 
                        DeflateInput.InputState initialInputState = input.DumpState(); 
                        OutputBuffer.BufferState initialOutputState = output.DumpState();
 
                        // enforce max so we can ensure state between calls
                        deflateEncoder.GetBlock(input, output, CleanCopySize);

                        if (!UseCompressed(deflateEncoder.LastCompressionRatio)) { 
                            // we're expanding; restore state and switch to uncompressed
                            input.RestoreState(initialInputState); 
                            output.RestoreState(initialOutputState); 
                            copyEncoder.GetBlock(input, output, false);
                            FlushInputWindows(); 
                        }

                        break;
                    } 

                case DeflaterState.StartingSmallData: { 
                        // add compressed header and data, but not footer. Subsequent calls will keep 
                        // adding compressed data (no header and no footer). We're doing this to
                        // avoid overhead of header and footer size relative to compressed payload. 
                        deflateEncoder.GetBlockHeader(output);

                        processingState = DeflaterState.HandlingSmallData;
                        goto case DeflaterState.HandlingSmallData; // yeah I know, but there's no fallthrough 
                    }
 
                case DeflaterState.HandlingSmallData: { 
                        // continue adding compressed data
                        deflateEncoder.GetCompressedData(input, output); 
                        break;
                    }
            }
 
            return output.BytesWritten;
        } 
 
        public int Finish(byte[] outputBuffer) {
            Debug.Assert(outputBuffer != null, "Can't pass in a null output buffer!"); 
            Debug.Assert(processingState == DeflaterState.NotStarted ||
                            processingState == DeflaterState.CheckingForIncompressible ||
                            processingState == DeflaterState.HandlingSmallData ||
                            processingState == DeflaterState.CompressThenCheck || 
                            processingState == DeflaterState.SlowDownForIncompressible1,
                            "got unexpected processing state = " + processingState); 
 
            Debug.Assert(NeedsInput());
 
            // no need to add end of block info if we didn't write anything
            if (processingState == DeflaterState.NotStarted) {
                return 0;
            } 

            output.UpdateBuffer(outputBuffer); 
 
            if (processingState == DeflaterState.CompressThenCheck ||
                        processingState == DeflaterState.HandlingSmallData || 
                        processingState == DeflaterState.SlowDownForIncompressible1) {

                // need to finish off block
                deflateEncoder.GetBlockFooter(output); 
            }
 
            // write final block 
            WriteFinal();
            return output.BytesWritten; 
        }

        // Is compression ratio under threshold?
        private bool UseCompressed(double ratio) { 
            return (ratio <= BadCompressionThreshold);
        } 
 
        private void FlushInputWindows() {
            deflateEncoder.FlushInput(); 
        }

        private void WriteFinal() {
            copyEncoder.GetBlock(null, output, true); 
        }
 
        // These states allow us to assume that data is compressible and keep compression ratios at least 
        // as good as historical values, but switch to different handling if that approach may increase the
        // data. If we detect we're getting a bad compression ratio, we switch to CheckingForIncompressible 
        // state and decide to compress on a block by block basis.
        //
        // If we're getting small data buffers, we want to avoid overhead of excessive header and footer
        // info, so we add one header and keep adding blocks as compressed. This means that if the user uses 
        // small buffers, they won't get the "don't increase size" improvements.
        // 
        // An earlier iteration of this fix handled that data separately by buffering this data until it 
        // reached a reasonable size, but given that Flush is not implemented on DeflateStream, this meant
        // data could be flushed only on Dispose. In the future, it would be reasonable to revisit this, in 
        // case this isn't breaking.
        //
        // NotStarted                 -> CheckingForIncompressible, CompressThenCheck, StartingSmallData
        // CompressThenCheck          -> SlowDownForIncompressible1 
        // SlowDownForIncompressible1 -> SlowDownForIncompressible2
        // SlowDownForIncompressible2 -> CheckingForIncompressible 
        // StartingSmallData          -> HandlingSmallData 
        internal enum DeflaterState {
 
            // no bytes to write yet
            NotStarted,

            // transient states 
            SlowDownForIncompressible1,
            SlowDownForIncompressible2, 
            StartingSmallData, 

            // stable state: may transition to CheckingForIncompressible (via transient states) if it 
            // appears we're expanding data
            CompressThenCheck,

            // sink states 
            CheckingForIncompressible,
            HandlingSmallData 
        } 

    } 
}

// File provided for Reference Use Only by Microsoft Corporation (c) 2007.

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