4 * This file was part of the Independent JPEG Group's software:
5 * Copyright (C) 1991-1997, Thomas G. Lane.
6 * libjpeg-turbo Modifications:
7 * Copyright (C) 2010-2011, D. R. Commander.
8 * For conditions of distribution and use, see the accompanying README file.
10 * This file contains declarations for Huffman entropy decoding routines
11 * that are shared between the sequential decoder (jdhuff.c) and the
12 * progressive decoder (jdphuff.c). No other modules need to see these.
16 /* Derived data constructed for each Huffman table */
18 #define HUFF_LOOKAHEAD 8 /* # of bits of lookahead */
21 /* Basic tables: (element [0] of each array is unused) */
22 INT32 maxcode[18]; /* largest code of length k (-1 if none) */
23 /* (maxcode[17] is a sentinel to ensure jpeg_huff_decode terminates) */
24 INT32 valoffset[18]; /* huffval[] offset for codes of length k */
25 /* valoffset[k] = huffval[] index of 1st symbol of code length k, less
26 * the smallest code of length k; so given a code of length k, the
27 * corresponding symbol is huffval[code + valoffset[k]]
30 /* Link to public Huffman table (needed only in jpeg_huff_decode) */
33 /* Lookahead table: indexed by the next HUFF_LOOKAHEAD bits of
34 * the input data stream. If the next Huffman code is no more
35 * than HUFF_LOOKAHEAD bits long, we can obtain its length and
36 * the corresponding symbol directly from this tables.
38 * The lower 8 bits of each table entry contain the number of
39 * bits in the corresponding Huffman code, or HUFF_LOOKAHEAD + 1
40 * if too long. The next 8 bits of each entry contain the
43 int lookup[1<<HUFF_LOOKAHEAD];
46 /* Expand a Huffman table definition into the derived format */
47 EXTERN(void) jpeg_make_d_derived_tbl
48 (j_decompress_ptr cinfo, boolean isDC, int tblno,
49 d_derived_tbl ** pdtbl);
53 * Fetching the next N bits from the input stream is a time-critical operation
54 * for the Huffman decoders. We implement it with a combination of inline
55 * macros and out-of-line subroutines. Note that N (the number of bits
56 * demanded at one time) never exceeds 15 for JPEG use.
58 * We read source bytes into get_buffer and dole out bits as needed.
59 * If get_buffer already contains enough bits, they are fetched in-line
60 * by the macros CHECK_BIT_BUFFER and GET_BITS. When there aren't enough
61 * bits, jpeg_fill_bit_buffer is called; it will attempt to fill get_buffer
62 * as full as possible (not just to the number of bits needed; this
63 * prefetching reduces the overhead cost of calling jpeg_fill_bit_buffer).
64 * Note that jpeg_fill_bit_buffer may return FALSE to indicate suspension.
65 * On TRUE return, jpeg_fill_bit_buffer guarantees that get_buffer contains
66 * at least the requested number of bits --- dummy zeroes are inserted if
70 #if !defined(_WIN32) && !defined(SIZEOF_SIZE_T)
71 #error Cannot determine word size
74 #if SIZEOF_SIZE_T==8 || defined(_WIN64)
76 typedef size_t bit_buf_type; /* type of bit-extraction buffer */
77 #define BIT_BUF_SIZE 64 /* size of buffer in bits */
81 typedef INT32 bit_buf_type; /* type of bit-extraction buffer */
82 #define BIT_BUF_SIZE 32 /* size of buffer in bits */
86 /* If long is > 32 bits on your machine, and shifting/masking longs is
87 * reasonably fast, making bit_buf_type be long and setting BIT_BUF_SIZE
88 * appropriately should be a win. Unfortunately we can't define the size
89 * with something like #define BIT_BUF_SIZE (sizeof(bit_buf_type)*8)
90 * because not all machines measure sizeof in 8-bit bytes.
93 typedef struct { /* Bitreading state saved across MCUs */
94 bit_buf_type get_buffer; /* current bit-extraction buffer */
95 int bits_left; /* # of unused bits in it */
98 typedef struct { /* Bitreading working state within an MCU */
99 /* Current data source location */
100 /* We need a copy, rather than munging the original, in case of suspension */
101 const JOCTET * next_input_byte; /* => next byte to read from source */
102 size_t bytes_in_buffer; /* # of bytes remaining in source buffer */
103 /* Bit input buffer --- note these values are kept in register variables,
104 * not in this struct, inside the inner loops.
106 bit_buf_type get_buffer; /* current bit-extraction buffer */
107 int bits_left; /* # of unused bits in it */
108 /* Pointer needed by jpeg_fill_bit_buffer. */
109 j_decompress_ptr cinfo; /* back link to decompress master record */
110 } bitread_working_state;
112 /* Macros to declare and load/save bitread local variables. */
113 #define BITREAD_STATE_VARS \
114 register bit_buf_type get_buffer; \
115 register int bits_left; \
116 bitread_working_state br_state
118 #define BITREAD_LOAD_STATE(cinfop,permstate) \
119 br_state.cinfo = cinfop; \
120 br_state.next_input_byte = cinfop->src->next_input_byte; \
121 br_state.bytes_in_buffer = cinfop->src->bytes_in_buffer; \
122 get_buffer = permstate.get_buffer; \
123 bits_left = permstate.bits_left;
125 #define BITREAD_SAVE_STATE(cinfop,permstate) \
126 cinfop->src->next_input_byte = br_state.next_input_byte; \
127 cinfop->src->bytes_in_buffer = br_state.bytes_in_buffer; \
128 permstate.get_buffer = get_buffer; \
129 permstate.bits_left = bits_left
132 * These macros provide the in-line portion of bit fetching.
133 * Use CHECK_BIT_BUFFER to ensure there are N bits in get_buffer
134 * before using GET_BITS, PEEK_BITS, or DROP_BITS.
135 * The variables get_buffer and bits_left are assumed to be locals,
136 * but the state struct might not be (jpeg_huff_decode needs this).
137 * CHECK_BIT_BUFFER(state,n,action);
138 * Ensure there are N bits in get_buffer; if suspend, take action.
141 * val = PEEK_BITS(n);
142 * Fetch next N bits without removing them from the buffer.
144 * Discard next N bits.
145 * The value N should be a simple variable, not an expression, because it
146 * is evaluated multiple times.
149 #define CHECK_BIT_BUFFER(state,nbits,action) \
150 { if (bits_left < (nbits)) { \
151 if (! jpeg_fill_bit_buffer(&(state),get_buffer,bits_left,nbits)) \
153 get_buffer = (state).get_buffer; bits_left = (state).bits_left; } }
155 #define GET_BITS(nbits) \
156 (((int) (get_buffer >> (bits_left -= (nbits)))) & ((1<<(nbits))-1))
158 #define PEEK_BITS(nbits) \
159 (((int) (get_buffer >> (bits_left - (nbits)))) & ((1<<(nbits))-1))
161 #define DROP_BITS(nbits) \
162 (bits_left -= (nbits))
164 /* Load up the bit buffer to a depth of at least nbits */
165 EXTERN(boolean) jpeg_fill_bit_buffer
166 (bitread_working_state * state, register bit_buf_type get_buffer,
167 register int bits_left, int nbits);
171 * Code for extracting next Huffman-coded symbol from input bit stream.
172 * Again, this is time-critical and we make the main paths be macros.
174 * We use a lookahead table to process codes of up to HUFF_LOOKAHEAD bits
175 * without looping. Usually, more than 95% of the Huffman codes will be 8
176 * or fewer bits long. The few overlength codes are handled with a loop,
177 * which need not be inline code.
179 * Notes about the HUFF_DECODE macro:
180 * 1. Near the end of the data segment, we may fail to get enough bits
181 * for a lookahead. In that case, we do it the hard way.
182 * 2. If the lookahead table contains no entry, the next code must be
183 * more than HUFF_LOOKAHEAD bits long.
184 * 3. jpeg_huff_decode returns -1 if forced to suspend.
187 #define HUFF_DECODE(result,state,htbl,failaction,slowlabel) \
188 { register int nb, look; \
189 if (bits_left < HUFF_LOOKAHEAD) { \
190 if (! jpeg_fill_bit_buffer(&state,get_buffer,bits_left, 0)) {failaction;} \
191 get_buffer = state.get_buffer; bits_left = state.bits_left; \
192 if (bits_left < HUFF_LOOKAHEAD) { \
193 nb = 1; goto slowlabel; \
196 look = PEEK_BITS(HUFF_LOOKAHEAD); \
197 if ((nb = (htbl->lookup[look] >> HUFF_LOOKAHEAD)) <= HUFF_LOOKAHEAD) { \
199 result = htbl->lookup[look] & ((1 << HUFF_LOOKAHEAD) - 1); \
202 if ((result=jpeg_huff_decode(&state,get_buffer,bits_left,htbl,nb)) < 0) \
204 get_buffer = state.get_buffer; bits_left = state.bits_left; \
208 #define HUFF_DECODE_FAST(s,nb,htbl) \
209 FILL_BIT_BUFFER_FAST; \
210 s = PEEK_BITS(HUFF_LOOKAHEAD); \
211 s = htbl->lookup[s]; \
212 nb = s >> HUFF_LOOKAHEAD; \
213 /* Pre-execute the common case of nb <= HUFF_LOOKAHEAD */ \
215 s = s & ((1 << HUFF_LOOKAHEAD) - 1); \
216 if (nb > HUFF_LOOKAHEAD) { \
217 /* Equivalent of jpeg_huff_decode() */ \
218 /* Don't use GET_BITS() here because we don't want to modify bits_left */ \
219 s = (get_buffer >> bits_left) & ((1 << (nb)) - 1); \
220 while (s > htbl->maxcode[nb]) { \
225 s = htbl->pub->huffval[ (int) (s + htbl->valoffset[nb]) & 0xFF ]; \
228 /* Out-of-line case for Huffman code fetching */
229 EXTERN(int) jpeg_huff_decode
230 (bitread_working_state * state, register bit_buf_type get_buffer,
231 register int bits_left, d_derived_tbl * htbl, int min_bits);