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---
jdhuff.h (8138B)
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1 /*
2 * jdhuff.h
3 *
4 * Copyright (C) 1991-1997, Thomas G. Lane.
5 * This file is part of the Independent JPEG Group's software.
6 * For conditions of distribution and use, see the accompanying README file.
7 *
8 * This file contains declarations for Huffman entropy decoding routines
9 * that are shared between the sequential decoder (jdhuff.c) and the
10 * progressive decoder (jdphuff.c). No other modules need to see these.
11 */
12
13 /* Short forms of external names for systems with brain-damaged linkers. */
14
15 #ifdef NEED_SHORT_EXTERNAL_NAMES
16 #define jpeg_make_d_derived_tbl jMkDDerived
17 #define jpeg_fill_bit_buffer jFilBitBuf
18 #define jpeg_huff_decode jHufDecode
19 #endif /* NEED_SHORT_EXTERNAL_NAMES */
20
21
22 /* Derived data constructed for each Huffman table */
23
24 #define HUFF_LOOKAHEAD 8 /* # of bits of lookahead */
25
26 typedef struct {
27 /* Basic tables: (element [0] of each array is unused) */
28 INT32 maxcode[18]; /* largest code of length k (-1 if none) */
29 /* (maxcode[17] is a sentinel to ensure jpeg_huff_decode terminates) */
30 INT32 valoffset[17]; /* huffval[] offset for codes of length k */
31 /* valoffset[k] = huffval[] index of 1st symbol of code length k, less
32 * the smallest code of length k; so given a code of length k, the
33 * corresponding symbol is huffval[code + valoffset[k]]
34 */
35
36 /* Link to public Huffman table (needed only in jpeg_huff_decode) */
37 JHUFF_TBL *pub;
38
39 /* Lookahead tables: indexed by the next HUFF_LOOKAHEAD bits of
40 * the input data stream. If the next Huffman code is no more
41 * than HUFF_LOOKAHEAD bits long, we can obtain its length and
42 * the corresponding symbol directly from these tables.
43 */
44 int look_nbits[1<<HUFF_LOOKAHEAD]; /* # bits, or 0 if too long */
45 UINT8 look_sym[1<<HUFF_LOOKAHEAD]; /* symbol, or unused */
46 } d_derived_tbl;
47
48 /* Expand a Huffman table definition into the derived format */
49 EXTERN(void) jpeg_make_d_derived_tbl
50 JPP((j_decompress_ptr cinfo, boolean isDC, int tblno,
51 d_derived_tbl ** pdtbl));
52
53
54 /*
55 * Fetching the next N bits from the input stream is a time-critical operation
56 * for the Huffman decoders. We implement it with a combination of inline
57 * macros and out-of-line subroutines. Note that N (the number of bits
58 * demanded at one time) never exceeds 15 for JPEG use.
59 *
60 * We read source bytes into get_buffer and dole out bits as needed.
61 * If get_buffer already contains enough bits, they are fetched in-line
62 * by the macros CHECK_BIT_BUFFER and GET_BITS. When there aren't enough
63 * bits, jpeg_fill_bit_buffer is called; it will attempt to fill get_buffer
64 * as full as possible (not just to the number of bits needed; this
65 * prefetching reduces the overhead cost of calling jpeg_fill_bit_buffer).
66 * Note that jpeg_fill_bit_buffer may return FALSE to indicate suspension.
67 * On TRUE return, jpeg_fill_bit_buffer guarantees that get_buffer contains
68 * at least the requested number of bits --- dummy zeroes are inserted if
69 * necessary.
70 */
71
72 typedef INT32 bit_buf_type; /* type of bit-extraction buffer */
73 #define BIT_BUF_SIZE 32 /* size of buffer in bits */
74
75 /* If long is > 32 bits on your machine, and shifting/masking longs is
76 * reasonably fast, making bit_buf_type be long and setting BIT_BUF_SIZE
77 * appropriately should be a win. Unfortunately we can't define the size
78 * with something like #define BIT_BUF_SIZE (sizeof(bit_buf_type)*8)
79 * because not all machines measure sizeof in 8-bit bytes.
80 */
81
82 typedef struct { /* Bitreading state saved across MCUs */
83 bit_buf_type get_buffer; /* current bit-extraction buffer */
84 int bits_left; /* # of unused bits in it */
85 } bitread_perm_state;
86
87 typedef struct { /* Bitreading working state within an MCU */
88 /* Current data source location */
89 /* We need a copy, rather than munging the original, in case of suspension */
90 const JOCTET * next_input_byte; /* => next byte to read from source */
91 size_t bytes_in_buffer; /* # of bytes remaining in source buffer */
92 /* Bit input buffer --- note these values are kept in register variables,
93 * not in this struct, inside the inner loops.
94 */
95 bit_buf_type get_buffer; /* current bit-extraction buffer */
96 int bits_left; /* # of unused bits in it */
97 /* Pointer needed by jpeg_fill_bit_buffer. */
98 j_decompress_ptr cinfo; /* back link to decompress master record */
99 } bitread_working_state;
100
101 /* Macros to declare and load/save bitread local variables. */
102 #define BITREAD_STATE_VARS \
103 register bit_buf_type get_buffer; \
104 register int bits_left; \
105 bitread_working_state br_state
106
107 #define BITREAD_LOAD_STATE(cinfop,permstate) \
108 br_state.cinfo = cinfop; \
109 br_state.next_input_byte = cinfop->src->next_input_byte; \
110 br_state.bytes_in_buffer = cinfop->src->bytes_in_buffer; \
111 get_buffer = permstate.get_buffer; \
112 bits_left = permstate.bits_left;
113
114 #define BITREAD_SAVE_STATE(cinfop,permstate) \
115 cinfop->src->next_input_byte = br_state.next_input_byte; \
116 cinfop->src->bytes_in_buffer = br_state.bytes_in_buffer; \
117 permstate.get_buffer = get_buffer; \
118 permstate.bits_left = bits_left
119
120 /*
121 * These macros provide the in-line portion of bit fetching.
122 * Use CHECK_BIT_BUFFER to ensure there are N bits in get_buffer
123 * before using GET_BITS, PEEK_BITS, or DROP_BITS.
124 * The variables get_buffer and bits_left are assumed to be locals,
125 * but the state struct might not be (jpeg_huff_decode needs this).
126 * CHECK_BIT_BUFFER(state,n,action);
127 * Ensure there are N bits in get_buffer; if suspend, take action.
128 * val = GET_BITS(n);
129 * Fetch next N bits.
130 * val = PEEK_BITS(n);
131 * Fetch next N bits without removing them from the buffer.
132 * DROP_BITS(n);
133 * Discard next N bits.
134 * The value N should be a simple variable, not an expression, because it
135 * is evaluated multiple times.
136 */
137
138 #define CHECK_BIT_BUFFER(state,nbits,action) \
139 { if (bits_left < (nbits)) { \
140 if (! jpeg_fill_bit_buffer(&(state),get_buffer,bits_left,nbits)) \
141 { action; } \
142 get_buffer = (state).get_buffer; bits_left = (state).bits_left; } }
143
144 #define GET_BITS(nbits) \
145 (((int) (get_buffer >> (bits_left -= (nbits)))) & ((1<<(nbits))-1))
146
147 #define PEEK_BITS(nbits) \
148 (((int) (get_buffer >> (bits_left - (nbits)))) & ((1<<(nbits))-1))
149
150 #define DROP_BITS(nbits) \
151 (bits_left -= (nbits))
152
153 /* Load up the bit buffer to a depth of at least nbits */
154 EXTERN(boolean) jpeg_fill_bit_buffer
155 JPP((bitread_working_state * state, register bit_buf_type get_buffer,
156 register int bits_left, int nbits));
157
158
159 /*
160 * Code for extracting next Huffman-coded symbol from input bit stream.
161 * Again, this is time-critical and we make the main paths be macros.
162 *
163 * We use a lookahead table to process codes of up to HUFF_LOOKAHEAD bits
164 * without looping. Usually, more than 95% of the Huffman codes will be 8
165 * or fewer bits long. The few overlength codes are handled with a loop,
166 * which need not be inline code.
167 *
168 * Notes about the HUFF_DECODE macro:
169 * 1. Near the end of the data segment, we may fail to get enough bits
170 * for a lookahead. In that case, we do it the hard way.
171 * 2. If the lookahead table contains no entry, the next code must be
172 * more than HUFF_LOOKAHEAD bits long.
173 * 3. jpeg_huff_decode returns -1 if forced to suspend.
174 */
175
176 #define HUFF_DECODE(result,state,htbl,failaction,slowlabel) \
177 { register int nb, look; \
178 if (bits_left < HUFF_LOOKAHEAD) { \
179 if (! jpeg_fill_bit_buffer(&state,get_buffer,bits_left, 0)) {failaction;} \
180 get_buffer = state.get_buffer; bits_left = state.bits_left; \
181 if (bits_left < HUFF_LOOKAHEAD) { \
182 nb = 1; goto slowlabel; \
183 } \
184 } \
185 look = PEEK_BITS(HUFF_LOOKAHEAD); \
186 if ((nb = htbl->look_nbits[look]) != 0) { \
187 DROP_BITS(nb); \
188 result = htbl->look_sym[look]; \
189 } else { \
190 nb = HUFF_LOOKAHEAD+1; \
191 slowlabel: \
192 if ((result=jpeg_huff_decode(&state,get_buffer,bits_left,htbl,nb)) < 0) \
193 { failaction; } \
194 get_buffer = state.get_buffer; bits_left = state.bits_left; \
195 } \
196 }
197
198 /* Out-of-line case for Huffman code fetching */
199 EXTERN(int) jpeg_huff_decode
200 JPP((bitread_working_state * state, register bit_buf_type get_buffer,
201 register int bits_left, d_derived_tbl * htbl, int min_bits));