huffmancodes.cs

PDF文件格式解析库源代码

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

namespace QiHe.CodeLib.Compress.DeflateFormat
{
    public partial class HuffmanCodes
    {
        public int ReadLiteralOrLength(BitStream input)
        {
            return AlphabetHuffmanCode.ReadSymbol(input);
        }

        public int ReadDistance(BitStream input)
        {
            return DistanceHuffmanCode.ReadSymbol(input);
        }

        static public HuffmanCodes Decode(BitStream input)
        {
            int NumberOfLiteralLengthCodes = input.ReadBits(5) + 257;
            int NumberOfDistanceCodes = input.ReadBits(5) + 1;
            int NumberOfCodeLengthCodes = input.ReadBits(4) + 4;

            int[] CodeLengths = new int[CodeLengthOrder.Length];
            for (int i = 0; i < NumberOfCodeLengthCodes; i++)
            {
                int len = input.ReadBits(3);
                CodeLengths[CodeLengthOrder[i]] = len;
            }

            HuffmanTree CodeLengthHuffmanCode = HuffmanTree.FromCodeLengths(CodeLengths);
            int[] alphabetCodeLengths = ReadCodeLengths(input, NumberOfLiteralLengthCodes, CodeLengthHuffmanCode);
            int[] distanceCodeLengths = ReadCodeLengths(input, NumberOfDistanceCodes, CodeLengthHuffmanCode);

            return new HuffmanCodes(alphabetCodeLengths, distanceCodeLengths);
        }

        private static int[] ReadCodeLengths(BitStream input, int NumberOfLengthCodes, HuffmanTree CodeLengthHuffmanCode)
        {
            List<int> codeLengths = new List<int>();
            int length = -1;
            while (codeLengths.Count < NumberOfLengthCodes)
            {
                int symbol = CodeLengthHuffmanCode.ReadSymbol(input);
                if (symbol >= 0 && symbol <= 15)
                {
                    length = symbol;
                    codeLengths.Add(length);
                }
                else if (symbol == 16)
                {
                    int times = input.ReadBits(2) + 3;
                    for (int t = 0; t < times; t++)
                    {
                        codeLengths.Add(length);
                    }
                }
                else if (symbol == 17)
                {
                    int times = input.ReadBits(3) + 3;
                    for (int t = 0; t < times; t++)
                    {
                        codeLengths.Add(0);
                    }
                }
                else if (symbol == 18)
                {
                    int times = input.ReadBits(7) + 11;
                    for (int t = 0; t < times; t++)
                    {
                        codeLengths.Add(0);
                    }
                }
                else
                {
                    throw new Exception("Invalid code length symbol.");
                }
            }
            return codeLengths.ToArray();
        }

    }
}