MISR Toolkit  1.5.1
jdhuff.h
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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) */
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 */
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 */
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 */
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));
bit_buf_type get_buffer
Definition: jdhuff.h:95
boolean int d_derived_tbl ** pdtbl
Definition: jdhuff.h:50
const JOCTET * next_input_byte
Definition: jdhuff.h:90
register bit_buf_type register int d_derived_tbl * htbl
Definition: jdhuff.h:200
size_t bytes_in_buffer
Definition: jdhuff.h:91
j_decompress_ptr cinfo
Definition: jdhuff.h:98
EXTERN(void) jpeg_make_d_derived_tbl JPP((j_decompress_ptr cinfo
boolean isDC
Definition: jdhuff.h:50
register bit_buf_type register int d_derived_tbl int min_bits
Definition: jdhuff.h:200
bit_buf_type get_buffer
Definition: jdhuff.h:83
long INT32
Definition: jmorecfg.h:161
boolean int tblno
Definition: jdhuff.h:50
register bit_buf_type register int int nbits
Definition: jdhuff.h:155
INT32 bit_buf_type
Definition: jdhuff.h:72
#define HUFF_LOOKAHEAD
Definition: jdhuff.h:24
short UINT8
Definition: jmorecfg.h:140
char JOCTET
Definition: jmorecfg.h:115
register bit_buf_type get_buffer
Definition: jdhuff.h:155
#define JPP(arglist)
Definition: jpeglib.h:818
JHUFF_TBL * pub
Definition: jdhuff.h:37
register bit_buf_type register int bits_left
Definition: jdhuff.h:155

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