102 lines
3.4 KiB
C#
102 lines
3.4 KiB
C#
//
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// Copyright (c) 2008-2011, Kenneth Bell
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//
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// Permission is hereby granted, free of charge, to any person obtaining a
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// copy of this software and associated documentation files (the "Software"),
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// to deal in the Software without restriction, including without limitation
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// the rights to use, copy, modify, merge, publish, distribute, sublicense,
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// and/or sell copies of the Software, and to permit persons to whom the
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// Software is furnished to do so, subject to the following conditions:
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//
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// The above copyright notice and this permission notice shall be included in
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// all copies or substantial portions of the Software.
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//
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// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
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// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
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// DEALINGS IN THE SOFTWARE.
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//
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namespace DiscUtils.Compression
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{
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/// <summary>
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/// A canonical Huffman tree implementation.
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/// </summary>
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/// <remarks>
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/// A lookup table is created that will take any bit sequence (max tree depth in length),
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/// indicating the output symbol. In WIM files, in practice, no chunk exceeds 32768 bytes
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/// in length, so we often end up generating a bigger lookup table than the data it's
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/// encoding. This makes for exceptionally fast symbol lookups O(1), but is inefficient
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/// overall.
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/// </remarks>
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internal sealed class HuffmanTree
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{
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private readonly uint[] _buffer;
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private readonly int _numBits; // Max bits per symbol
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private readonly int _numSymbols; // Max symbols
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public HuffmanTree(uint[] lengths)
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{
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Lengths = lengths;
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_numSymbols = lengths.Length;
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uint maxLength = 0;
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for (int i = 0; i < Lengths.Length; ++i)
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{
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if (Lengths[i] > maxLength)
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{
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maxLength = Lengths[i];
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}
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}
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_numBits = (int)maxLength;
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_buffer = new uint[1 << _numBits];
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Build();
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}
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public uint[] Lengths { get; }
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public uint NextSymbol(BitStream bitStream)
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{
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uint symbol = _buffer[bitStream.Peek(_numBits)];
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// We may have over-read, reset bitstream position
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bitStream.Consume((int)Lengths[symbol]);
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return symbol;
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}
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private void Build()
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{
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int position = 0;
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// For each bit-length...
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for (int i = 1; i <= _numBits; ++i)
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{
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// Check each symbol
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for (uint symbol = 0; symbol < _numSymbols; ++symbol)
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{
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if (Lengths[symbol] == i)
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{
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int numToFill = 1 << (_numBits - i);
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for (int n = 0; n < numToFill; ++n)
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{
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_buffer[position + n] = symbol;
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}
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position += numToFill;
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}
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}
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}
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for (int i = position; i < _buffer.Length; ++i)
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{
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_buffer[i] = uint.MaxValue;
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}
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}
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}
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} |