source: trunk/libjpeg/jdhuff.h @ 15

Last change on this file since 15 was 15, checked in by Eugene Romanenko, 16 years ago

needed libs update

File size: 8.1 KB
RevLine 
[15]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
26typedef 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 */
49EXTERN(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
72typedef 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
82typedef 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
87typedef 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 */
154EXTERN(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; \
191slowlabel: \
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 */
199EXTERN(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));
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