compressor.cs

PDF文件格式解析库源代码

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

namespace QiHe.CodeLib.Compress
{
    public class Compressor
    {
        const int MaximumLength = 258;
        const int MaximumDistance = 32768;

        public static IEnumerable<CompressedItem> AnalyzeItems(Stream input)
        {
            DoubleBufferedStreamReader reader = new DoubleBufferedStreamReader(input, MaximumLength, MaximumDistance);
            Dictionary<int, List<long>> chain = new Dictionary<int, List<long>>();
            foreach (CompressedItem item in AnalyzeItems(reader, chain))
            {
                yield return item;
            }
        }

        public static IEnumerable<CompressedItem> AnalyzeItems(Stream input, CompressionOption option)
        {
            DoubleBufferedStreamReader reader = new DoubleBufferedStreamReader(input, MaximumLength, MaximumDistance);
            Dictionary<int, List<long>> chain = new Dictionary<int, List<long>>();
            switch (option)
            {
                case CompressionOption.Faster:
                    foreach (CompressedItem item in AnalyzeItemsFastMode(reader, chain))
                    {
                        yield return item;
                    }
                    break;
                case CompressionOption.Normal:
                    foreach (CompressedItem item in AnalyzeItems(reader, chain))
                    {
                        yield return item;
                    }
                    break;
                case CompressionOption.Smaller:
                    foreach (CompressedItem item in AnalyzeItemsLazyMode(reader, chain))
                    {
                        yield return item;
                    }
                    break;
            }
        }

        //similar to LZSS algorithm with hashing
        static IEnumerable<CompressedItem> AnalyzeItems(DoubleBufferedStreamReader input, Dictionary<int, List<long>> chain)
        {
            while (true)
            {
                int key = Examine3Bytes(input);
                if (key == -1) break;

                List<long> positions = GetPositions(chain, key);
                if (positions.Count > 0)
                {
                    SlideWindow(positions, input.Position);
                }

                bool foundmatch = false;
                if (positions.Count > 0)
                {
                    LengthDistancePair match = FindLongestMatch(input, positions);
                    if (match.Length > 0)
                    {
                        yield return match;
                        positions.Add(input.Position);
                        for (int i = 0; i < match.Length - 1; i++)
                        {
                            input.Position++;
                            key = Examine3Bytes(input);
                            GetPositions(chain, key).Add(input.Position);
                        }
                        input.Position++;
                        foundmatch = true;
                    }
                }
                if (!foundmatch)
                {
                    positions.Add(input.Position);
                    byte value = input.ReadByte();
                    yield return new LiteralByte(value);
                }
            }

            byte[] data = input.ReadToEnd();
            if (data.Length > 0)
            {
                foreach (byte value in data)
                {
                    yield return new LiteralByte(value);
                }
            }
        }


        static IEnumerable<CompressedItem> AnalyzeItemsFastMode(DoubleBufferedStreamReader input, Dictionary<int, List<long>> chain)
        {
            while (true)
            {
                int key = Examine3Bytes(input);
                if (key == -1) break;

                List<long> positions = GetPositions(chain, key);
                if (positions.Count > 0)
                {
                    SlideWindow(positions, input.Position);
                }

                bool foundmatch = false;
                if (positions.Count > 0)
                {
                    LengthDistancePair match = FindLongestMatch(input, positions);
                    if (match.Length > 0)
                    {
                        yield return match;
                        positions.Add(input.Position);
                        input.Position += match.Length;
                        foundmatch = true;
                    }
                }
                if (!foundmatch)
                {
                    positions.Add(input.Position);
                    byte value = input.ReadByte();
                    yield return new LiteralByte(value);
                }
            }

            byte[] data = input.ReadToEnd();
            if (data.Length > 0)
            {
                foreach (byte value in data)
                {
                    yield return new LiteralByte(value);
                }
            }
        }

        static IEnumerable<CompressedItem> AnalyzeItemsLazyMode(DoubleBufferedStreamReader input, Dictionary<int, List<long>> chain)
        {
            LengthDistancePair zeroMatch = new LengthDistancePair(0, 0);
            LengthDistancePair lastMatch = zeroMatch;
            byte lastByte = 0;
            bool isTrying = false;
            while (true)
            {
                LengthDistancePair match = FindLongestMatch(input, chain);
                if (match == null) break;
                ChainPosition(input, chain);

                if (match.Length > lastMatch.Length || lastMatch.Length == 0)
                {
                    if (isTrying)
                    {
                        yield return new LiteralByte(lastByte);
                    }

                    lastByte = input.ReadByte();
                    lastMatch = match;
                    isTrying = true;
                }
                else
                {
                    yield return lastMatch;

                    for (int i = 2; i < lastMatch.Length; i++)
                    {
                        input.Position++;
                        ChainPosition(input, chain);
                    }
                    input.Position++;
                    lastMatch = zeroMatch;
                    isTrying = false;
                }
            }
            if (isTrying)
            {
                if (lastMatch.Length > 0)
                {
                    yield return lastMatch;
                    input.Position += lastMatch.Length - 1;
                }
                else
                {
                    yield return new LiteralByte(lastByte);
                }
            }

            byte[] data = input.ReadToEnd();
            if (data.Length > 0)
            {
                if (data.Length > 3)
                {
                    throw new Exception(data.Length + " bytes uncompressed.");
                }
                foreach (byte value in data)
                {
                    yield return new LiteralByte(value);
                }
            }
        }

        private static void ChainPosition(DoubleBufferedStreamReader input, Dictionary<int, List<long>> chain)
        {
            int key = Examine3Bytes(input);
            GetPositions(chain, key).Add(input.Position);
        }

        private static LengthDistancePair FindLongestMatch(DoubleBufferedStreamReader input, Dictionary<int, List<long>> chain)
        {
            int key = Examine3Bytes(input);
            if (key == -1) return null;

            List<long> positions = GetPositions(chain, key);
            if (positions.Count > 0)
            {
                SlideWindow(positions, input.Position);
            }

            return FindLongestMatch(input, positions);
        }

        private static List<long> GetPositions(Dictionary<int, List<long>> chain, int key)
        {
            if (!chain.ContainsKey(key))
            {
                chain[key] = new List<long>();
            }
            return chain[key];
        }

        private static LengthDistancePair FindLongestMatch(DoubleBufferedStreamReader input, List<long> positions)
        {
            input.Mode = DoubleBufferedStreamReader.WorkMode.Seeking;
            long pos0 = input.Position;
            int maxlen = 0;
            int maxdis = 0;
            byte[] nextBytes = input.PeekBytes(MaximumLength);
            for (int i = positions.Count - 1; i >= 0; i--)
            {
                int len = Match(input, positions[i], nextBytes);
                int dis = (int)(pos0 - positions[i]);
                if (len > maxlen
                    //trade off between length and distance
                    //&& len > DeflateFormat.DeflateCoding.Distance[dis].ExtraBits - 2
                    )
                {
                    maxlen = len;
                    maxdis = dis;
                }
            }
            input.Position = pos0;
            input.Mode = DoubleBufferedStreamReader.WorkMode.Reading;
            return new LengthDistancePair(maxlen, maxdis);
        }

        private static void SlideWindow(List<long> positions, long currentPos)
        {
            List<long> tobeRemoved = new List<long>();
            foreach (long pos in positions)
            {
                if (currentPos - pos > MaximumDistance)
                {
                    tobeRemoved.Add(pos);
                }
                else
                {
                    break;
                }
            }
            foreach (long pos in tobeRemoved)
            {
                positions.Remove(pos);
            }
        }

        private static int Match(DoubleBufferedStreamReader input, long pos, byte[] nextBytes)
        {
            int length = 3;
            input.Position = pos + length;
            while (length < nextBytes.Length)
            {
                if (input.ReadByte() == nextBytes[length])
                {
                    length++;
                }
                else
                {
                    break;
                }
            }
            return length;
        }

        private static int Examine3Bytes(DoubleBufferedStreamReader input)
        {
            byte[] bytes = input.PeekBytes(3);
            if (bytes.Length == 3)
            {
                //int value = bytes[0] + bytes[1] * 256 + bytes[2] * 65536;
                int value = (((bytes[0] << 8) + bytes[1]) << 8) + bytes[2];
                return value;
            }
            else
            {
                return -1;
            }
        }
    }
}