89 lines
3.4 KiB
C++
89 lines
3.4 KiB
C++
// Copyright 2012 Google Inc. All Rights Reserved.
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//
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// Use of this source code is governed by a BSD-style license
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// that can be found in the COPYING file in the root of the source
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// tree. An additional intellectual property rights grant can be found
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// in the file PATENTS. All contributing project authors may
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// be found in the AUTHORS file in the root of the source tree.
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// -----------------------------------------------------------------------------
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//
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// Utilities for building and looking up Huffman trees.
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//
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// Author: Urvang Joshi (urvang@google.com)
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#ifndef WEBP_UTILS_HUFFMAN_H_
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#define WEBP_UTILS_HUFFMAN_H_
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#include <assert.h>
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#include "webp/format_constants.h"
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#include "webp/types.h"
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#ifdef __cplusplus
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extern "C" {
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#endif
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#define HUFFMAN_TABLE_BITS 8
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#define HUFFMAN_TABLE_MASK ((1 << HUFFMAN_TABLE_BITS) - 1)
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#define LENGTHS_TABLE_BITS 7
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#define LENGTHS_TABLE_MASK ((1 << LENGTHS_TABLE_BITS) - 1)
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// Huffman lookup table entry
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typedef struct {
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uint8_t bits; // number of bits used for this symbol
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uint16_t value; // symbol value or table offset
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} HuffmanCode;
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// long version for holding 32b values
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typedef struct {
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int bits; // number of bits used for this symbol,
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// or an impossible value if not a literal code.
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uint32_t value; // 32b packed ARGB value if literal,
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// or non-literal symbol otherwise
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} HuffmanCode32;
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#define HUFFMAN_PACKED_BITS 6
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#define HUFFMAN_PACKED_TABLE_SIZE (1u << HUFFMAN_PACKED_BITS)
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// Huffman table group.
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// Includes special handling for the following cases:
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// - is_trivial_literal: one common literal base for RED/BLUE/ALPHA (not GREEN)
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// - is_trivial_code: only 1 code (no bit is read from bitstream)
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// - use_packed_table: few enough literal symbols, so all the bit codes
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// can fit into a small look-up table packed_table[]
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// The common literal base, if applicable, is stored in 'literal_arb'.
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typedef struct HTreeGroup HTreeGroup;
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struct HTreeGroup {
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HuffmanCode* htrees[HUFFMAN_CODES_PER_META_CODE];
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int is_trivial_literal; // True, if huffman trees for Red, Blue & Alpha
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// Symbols are trivial (have a single code).
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uint32_t literal_arb; // If is_trivial_literal is true, this is the
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// ARGB value of the pixel, with Green channel
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// being set to zero.
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int is_trivial_code; // true if is_trivial_literal with only one code
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int use_packed_table; // use packed table below for short literal code
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// table mapping input bits to a packed values, or escape case to literal code
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HuffmanCode32 packed_table[HUFFMAN_PACKED_TABLE_SIZE];
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};
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// Creates the instance of HTreeGroup with specified number of tree-groups.
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HTreeGroup* VP8LHtreeGroupsNew(int num_htree_groups);
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// Releases the memory allocated for HTreeGroup.
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void VP8LHtreeGroupsFree(HTreeGroup* const htree_groups);
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// Builds Huffman lookup table assuming code lengths are in symbol order.
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// The 'code_lengths' is pre-allocated temporary memory buffer used for creating
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// the huffman table.
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// Returns built table size or 0 in case of error (invalid tree or
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// memory error).
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int VP8LBuildHuffmanTable(HuffmanCode* const root_table, int root_bits,
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const int code_lengths[], int code_lengths_size);
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#ifdef __cplusplus
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} // extern "C"
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#endif
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#endif // WEBP_UTILS_HUFFMAN_H_
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