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1 /*
2  * jdhuff.h
3  *
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, 2015-2016, D. R. Commander.
8  * For conditions of distribution and use, see the accompanying README.ijg
9  * file.
10  *
11  * This file contains declarations for Huffman entropy decoding routines
12  * that are shared between the sequential decoder (jdhuff.c) and the
13  * progressive decoder (jdphuff.c).  No other modules need to see these.
14  */
15 
16 #include "jconfigint.h"
17 
18 
19 /* Derived data constructed for each Huffman table */
20 
21 #define HUFF_LOOKAHEAD  8       /* # of bits of lookahead */
22 
23 typedef struct {
24   /* Basic tables: (element [0] of each array is unused) */
25   JLONG maxcode[18];            /* largest code of length k (-1 if none) */
26   /* (maxcode[17] is a sentinel to ensure jpeg_huff_decode terminates) */
27   JLONG valoffset[18];          /* huffval[] offset for codes of length k */
28   /* valoffset[k] = huffval[] index of 1st symbol of code length k, less
29    * the smallest code of length k; so given a code of length k, the
30    * corresponding symbol is huffval[code + valoffset[k]]
31    */
32 
33   /* Link to public Huffman table (needed only in jpeg_huff_decode) */
34   JHUFF_TBL *pub;
35 
36   /* Lookahead table: indexed by the next HUFF_LOOKAHEAD bits of
37    * the input data stream.  If the next Huffman code is no more
38    * than HUFF_LOOKAHEAD bits long, we can obtain its length and
39    * the corresponding symbol directly from this tables.
40    *
41    * The lower 8 bits of each table entry contain the number of
42    * bits in the corresponding Huffman code, or HUFF_LOOKAHEAD + 1
43    * if too long.  The next 8 bits of each entry contain the
44    * symbol.
45    */
46   int lookup[1 << HUFF_LOOKAHEAD];
47 } d_derived_tbl;
48 
49 /* Expand a Huffman table definition into the derived format */
50 EXTERN(void) jpeg_make_d_derived_tbl(j_decompress_ptr cinfo, boolean isDC,
51                                      int tblno, 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 #if !defined(_WIN32) && !defined(SIZEOF_SIZE_T)
73 #error Cannot determine word size
74 #endif
75 
76 #if SIZEOF_SIZE_T == 8 || defined(_WIN64)
77 
78 typedef size_t bit_buf_type;            /* type of bit-extraction buffer */
79 #define BIT_BUF_SIZE  64                /* size of buffer in bits */
80 
81 #else
82 
83 typedef unsigned long bit_buf_type;     /* type of bit-extraction buffer */
84 #define BIT_BUF_SIZE  32                /* size of buffer in bits */
85 
86 #endif
87 
88 /* If long is > 32 bits on your machine, and shifting/masking longs is
89  * reasonably fast, making bit_buf_type be long and setting BIT_BUF_SIZE
90  * appropriately should be a win.  Unfortunately we can't define the size
91  * with something like  #define BIT_BUF_SIZE (sizeof(bit_buf_type)*8)
92  * because not all machines measure sizeof in 8-bit bytes.
93  */
94 
95 typedef struct {                /* Bitreading state saved across MCUs */
96   bit_buf_type get_buffer;      /* current bit-extraction buffer */
97   int bits_left;                /* # of unused bits in it */
98 } bitread_perm_state;
99 
100 typedef struct {                /* Bitreading working state within an MCU */
101   /* Current data source location */
102   /* We need a copy, rather than munging the original, in case of suspension */
103   const JOCTET *next_input_byte; /* => next byte to read from source */
104   size_t bytes_in_buffer;       /* # of bytes remaining in source buffer */
105   /* Bit input buffer --- note these values are kept in register variables,
106    * not in this struct, inside the inner loops.
107    */
108   bit_buf_type get_buffer;      /* current bit-extraction buffer */
109   int bits_left;                /* # of unused bits in it */
110   /* Pointer needed by jpeg_fill_bit_buffer. */
111   j_decompress_ptr cinfo;       /* back link to decompress master record */
112 } bitread_working_state;
113 
114 /* Macros to declare and load/save bitread local variables. */
115 #define BITREAD_STATE_VARS \
116   register bit_buf_type get_buffer; \
117   register int bits_left; \
118   bitread_working_state br_state
119 
120 #define BITREAD_LOAD_STATE(cinfop, permstate) \
121   br_state.cinfo = cinfop; \
122   br_state.next_input_byte = cinfop->src->next_input_byte; \
123   br_state.bytes_in_buffer = cinfop->src->bytes_in_buffer; \
124   get_buffer = permstate.get_buffer; \
125   bits_left = permstate.bits_left;
126 
127 #define BITREAD_SAVE_STATE(cinfop, permstate) \
128   cinfop->src->next_input_byte = br_state.next_input_byte; \
129   cinfop->src->bytes_in_buffer = br_state.bytes_in_buffer; \
130   permstate.get_buffer = get_buffer; \
131   permstate.bits_left = bits_left
132 
133 /*
134  * These macros provide the in-line portion of bit fetching.
135  * Use CHECK_BIT_BUFFER to ensure there are N bits in get_buffer
136  * before using GET_BITS, PEEK_BITS, or DROP_BITS.
137  * The variables get_buffer and bits_left are assumed to be locals,
138  * but the state struct might not be (jpeg_huff_decode needs this).
139  *      CHECK_BIT_BUFFER(state, n, action);
140  *              Ensure there are N bits in get_buffer; if suspend, take action.
141  *      val = GET_BITS(n);
142  *              Fetch next N bits.
143  *      val = PEEK_BITS(n);
144  *              Fetch next N bits without removing them from the buffer.
145  *      DROP_BITS(n);
146  *              Discard next N bits.
147  * The value N should be a simple variable, not an expression, because it
148  * is evaluated multiple times.
149  */
150 
151 #define CHECK_BIT_BUFFER(state, nbits, action) { \
152   if (bits_left < (nbits)) { \
153     if (!jpeg_fill_bit_buffer(&(state), get_buffer, bits_left, nbits)) \
154       { action; } \
155     get_buffer = (state).get_buffer;  bits_left = (state).bits_left; \
156   } \
157 }
158 
159 #define GET_BITS(nbits) \
160   (((int)(get_buffer >> (bits_left -= (nbits)))) & ((1 << (nbits)) - 1))
161 
162 #define PEEK_BITS(nbits) \
163   (((int)(get_buffer >> (bits_left -  (nbits)))) & ((1 << (nbits)) - 1))
164 
165 #define DROP_BITS(nbits) \
166   (bits_left -= (nbits))
167 
168 /* Load up the bit buffer to a depth of at least nbits */
169 EXTERN(boolean) jpeg_fill_bit_buffer(bitread_working_state *state,
170                                      register bit_buf_type get_buffer,
171                                      register int bits_left, int nbits);
172 
173 
174 /*
175  * Code for extracting next Huffman-coded symbol from input bit stream.
176  * Again, this is time-critical and we make the main paths be macros.
177  *
178  * We use a lookahead table to process codes of up to HUFF_LOOKAHEAD bits
179  * without looping.  Usually, more than 95% of the Huffman codes will be 8
180  * or fewer bits long.  The few overlength codes are handled with a loop,
181  * which need not be inline code.
182  *
183  * Notes about the HUFF_DECODE macro:
184  * 1. Near the end of the data segment, we may fail to get enough bits
185  *    for a lookahead.  In that case, we do it the hard way.
186  * 2. If the lookahead table contains no entry, the next code must be
187  *    more than HUFF_LOOKAHEAD bits long.
188  * 3. jpeg_huff_decode returns -1 if forced to suspend.
189  */
190 
191 #define HUFF_DECODE(result, state, htbl, failaction, slowlabel) { \
192   register int nb, look; \
193   if (bits_left < HUFF_LOOKAHEAD) { \
194     if (!jpeg_fill_bit_buffer(&state, get_buffer, bits_left, 0)) \
195       { failaction; } \
196     get_buffer = state.get_buffer;  bits_left = state.bits_left; \
197     if (bits_left < HUFF_LOOKAHEAD) { \
198       nb = 1;  goto slowlabel; \
199     } \
200   } \
201   look = PEEK_BITS(HUFF_LOOKAHEAD); \
202   if ((nb = (htbl->lookup[look] >> HUFF_LOOKAHEAD)) <= HUFF_LOOKAHEAD) { \
203     DROP_BITS(nb); \
204     result = htbl->lookup[look] & ((1 << HUFF_LOOKAHEAD) - 1); \
205   } else { \
206 slowlabel: \
207     if ((result = \
208          jpeg_huff_decode(&state, get_buffer, bits_left, htbl, nb)) < 0) \
209       { failaction; } \
210     get_buffer = state.get_buffer;  bits_left = state.bits_left; \
211   } \
212 }
213 
214 #define HUFF_DECODE_FAST(s, nb, htbl) \
215   FILL_BIT_BUFFER_FAST; \
216   s = PEEK_BITS(HUFF_LOOKAHEAD); \
217   s = htbl->lookup[s]; \
218   nb = s >> HUFF_LOOKAHEAD; \
219   /* Pre-execute the common case of nb <= HUFF_LOOKAHEAD */ \
220   DROP_BITS(nb); \
221   s = s & ((1 << HUFF_LOOKAHEAD) - 1); \
222   if (nb > HUFF_LOOKAHEAD) { \
223     /* Equivalent of jpeg_huff_decode() */ \
224     /* Don't use GET_BITS() here because we don't want to modify bits_left */ \
225     s = (get_buffer >> bits_left) & ((1 << (nb)) - 1); \
226     while (s > htbl->maxcode[nb]) { \
227       s <<= 1; \
228       s |= GET_BITS(1); \
229       nb++; \
230     } \
231     s = htbl->pub->huffval[(int)(s + htbl->valoffset[nb]) & 0xFF]; \
232   }
233 
234 /* Out-of-line case for Huffman code fetching */
235 EXTERN(int) jpeg_huff_decode(bitread_working_state *state,
236                              register bit_buf_type get_buffer,
237                              register int bits_left, d_derived_tbl *htbl,
238                              int min_bits);
239