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93 lines
3.9 KiB
C
93 lines
3.9 KiB
C
/********************************************************************
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* *
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* THIS FILE IS PART OF THE OggTheora SOFTWARE CODEC SOURCE CODE. *
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* USE, DISTRIBUTION AND REPRODUCTION OF THIS LIBRARY SOURCE IS *
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* GOVERNED BY A BSD-STYLE SOURCE LICENSE INCLUDED WITH THIS SOURCE *
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* IN 'COPYING'. PLEASE READ THESE TERMS BEFORE DISTRIBUTING. *
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* *
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* THE Theora SOURCE CODE IS COPYRIGHT (C) 2002-2009 *
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* by the Xiph.Org Foundation and contributors http://www.xiph.org/ *
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* *
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********************************************************************
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function:
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last mod: $Id: huffdec.h 16503 2009-08-22 18:14:02Z giles $
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********************************************************************/
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#if !defined(_huffdec_H)
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# define _huffdec_H (1)
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# include "huffman.h"
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# include "bitpack.h"
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typedef struct oc_huff_node oc_huff_node;
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/*A node in the Huffman tree.
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Instead of storing every branching in the tree, subtrees can be collapsed
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into one node, with a table of size 1<<nbits pointing directly to its
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descedents nbits levels down.
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This allows more than one bit to be read at a time, and avoids following all
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the intermediate branches with next to no increased code complexity once
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the collapsed tree has been built.
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We do _not_ require that a subtree be complete to be collapsed, but instead
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store duplicate pointers in the table, and record the actual depth of the
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node below its parent.
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This tells us the number of bits to advance the stream after reaching it.
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This turns out to be equivalent to the method described in \cite{Hash95},
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without the requirement that codewords be sorted by length.
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If the codewords were sorted by length (so-called ``canonical-codes''), they
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could be decoded much faster via either Lindell and Moffat's approach or
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Hashemian's Condensed Huffman Code approach, the latter of which has an
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extremely small memory footprint.
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We can't use Choueka et al.'s finite state machine approach, which is
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extremely fast, because we can't allow multiple symbols to be output at a
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time; the codebook can and does change between symbols.
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It also has very large memory requirements, which impairs cache coherency.
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@ARTICLE{Hash95,
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author="Reza Hashemian",
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title="Memory Efficient and High-Speed Search {Huffman} Coding",
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journal="{IEEE} Transactions on Communications",
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volume=43,
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number=10,
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pages="2576--2581",
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month=Oct,
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year=1995
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}*/
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struct oc_huff_node{
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/*The number of bits of the code needed to descend through this node.
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0 indicates a leaf node.
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Otherwise there are 1<<nbits nodes in the nodes table, which can be
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indexed by reading nbits bits from the stream.*/
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unsigned char nbits;
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/*The value of a token stored in a leaf node.
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The value in non-leaf nodes is undefined.*/
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unsigned char token;
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/*The depth of the current node, relative to its parent in the collapsed
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tree.
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This can be less than its parent's nbits value, in which case there are
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1<<nbits-depth copies of this node in the table, and the bitstream should
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only be advanced depth bits after reaching this node.*/
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unsigned char depth;
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/*The table of child nodes.
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The ACTUAL size of this array is 1<<nbits, despite what the declaration
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below claims.
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The exception is that for leaf nodes the size is 0.*/
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oc_huff_node *nodes[2];
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};
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int oc_huff_trees_unpack(oc_pack_buf *_opb,
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oc_huff_node *_nodes[TH_NHUFFMAN_TABLES]);
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int oc_huff_trees_copy(oc_huff_node *_dst[TH_NHUFFMAN_TABLES],
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const oc_huff_node *const _src[TH_NHUFFMAN_TABLES]);
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void oc_huff_trees_clear(oc_huff_node *_nodes[TH_NHUFFMAN_TABLES]);
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int oc_huff_token_decode(oc_pack_buf *_opb,const oc_huff_node *_node);
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#endif
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