1/*
2 * reserved comment block
3 * DO NOT REMOVE OR ALTER!
4 */
5/*
6 * jdhuff.h
7 *
8 * Copyright (C) 1991-1997, Thomas G. Lane.
9 * This file is part of the Independent JPEG Group's software.
10 * For conditions of distribution and use, see the accompanying README file.
11 *
12 * This file contains declarations for Huffman entropy decoding routines
13 * that are shared between the sequential decoder (jdhuff.c) and the
14 * progressive decoder (jdphuff.c).  No other modules need to see these.
15 */
16
17/* Short forms of external names for systems with brain-damaged linkers. */
18
19#ifdef NEED_SHORT_EXTERNAL_NAMES
20#define jpeg_make_d_derived_tbl jMkDDerived
21#define jpeg_fill_bit_buffer    jFilBitBuf
22#define jpeg_huff_decode        jHufDecode
23#endif /* NEED_SHORT_EXTERNAL_NAMES */
24
25
26/* Derived data constructed for each Huffman table */
27
28#define HUFF_LOOKAHEAD  8       /* # of bits of lookahead */
29
30typedef struct {
31  /* Basic tables: (element [0] of each array is unused) */
32  INT32 maxcode[18];            /* largest code of length k (-1 if none) */
33  /* (maxcode[17] is a sentinel to ensure jpeg_huff_decode terminates) */
34  INT32 valoffset[17];          /* huffval[] offset for codes of length k */
35  /* valoffset[k] = huffval[] index of 1st symbol of code length k, less
36   * the smallest code of length k; so given a code of length k, the
37   * corresponding symbol is huffval[code + valoffset[k]]
38   */
39
40  /* Link to public Huffman table (needed only in jpeg_huff_decode) */
41  JHUFF_TBL *pub;
42
43  /* Lookahead tables: indexed by the next HUFF_LOOKAHEAD bits of
44   * the input data stream.  If the next Huffman code is no more
45   * than HUFF_LOOKAHEAD bits long, we can obtain its length and
46   * the corresponding symbol directly from these tables.
47   */
48  int look_nbits[1<<HUFF_LOOKAHEAD]; /* # bits, or 0 if too long */
49  UINT8 look_sym[1<<HUFF_LOOKAHEAD]; /* symbol, or unused */
50} d_derived_tbl;
51
52/* Expand a Huffman table definition into the derived format */
53EXTERN(void) jpeg_make_d_derived_tbl
54        JPP((j_decompress_ptr cinfo, boolean isDC, int tblno,
55             d_derived_tbl ** pdtbl));
56
57
58/*
59 * Fetching the next N bits from the input stream is a time-critical operation
60 * for the Huffman decoders.  We implement it with a combination of inline
61 * macros and out-of-line subroutines.  Note that N (the number of bits
62 * demanded at one time) never exceeds 15 for JPEG use.
63 *
64 * We read source bytes into get_buffer and dole out bits as needed.
65 * If get_buffer already contains enough bits, they are fetched in-line
66 * by the macros CHECK_BIT_BUFFER and GET_BITS.  When there aren't enough
67 * bits, jpeg_fill_bit_buffer is called; it will attempt to fill get_buffer
68 * as full as possible (not just to the number of bits needed; this
69 * prefetching reduces the overhead cost of calling jpeg_fill_bit_buffer).
70 * Note that jpeg_fill_bit_buffer may return FALSE to indicate suspension.
71 * On TRUE return, jpeg_fill_bit_buffer guarantees that get_buffer contains
72 * at least the requested number of bits --- dummy zeroes are inserted if
73 * necessary.
74 */
75
76typedef INT32 bit_buf_type;     /* type of bit-extraction buffer */
77#define BIT_BUF_SIZE  32        /* size of buffer in bits */
78
79/* If long is > 32 bits on your machine, and shifting/masking longs is
80 * reasonably fast, making bit_buf_type be long and setting BIT_BUF_SIZE
81 * appropriately should be a win.  Unfortunately we can't define the size
82 * with something like  #define BIT_BUF_SIZE (sizeof(bit_buf_type)*8)
83 * because not all machines measure sizeof in 8-bit bytes.
84 */
85
86typedef struct {                /* Bitreading state saved across MCUs */
87  bit_buf_type get_buffer;      /* current bit-extraction buffer */
88  int bits_left;                /* # of unused bits in it */
89} bitread_perm_state;
90
91typedef struct {                /* Bitreading working state within an MCU */
92  /* Current data source location */
93  /* We need a copy, rather than munging the original, in case of suspension */
94  const JOCTET * next_input_byte; /* => next byte to read from source */
95  size_t bytes_in_buffer;       /* # of bytes remaining in source buffer */
96  /* Bit input buffer --- note these values are kept in register variables,
97   * not in this struct, inside the inner loops.
98   */
99  bit_buf_type get_buffer;      /* current bit-extraction buffer */
100  int bits_left;                /* # of unused bits in it */
101  /* Pointer needed by jpeg_fill_bit_buffer. */
102  j_decompress_ptr cinfo;       /* back link to decompress master record */
103} bitread_working_state;
104
105/* Macros to declare and load/save bitread local variables. */
106#define BITREAD_STATE_VARS  \
107        register bit_buf_type get_buffer;  \
108        register int bits_left;  \
109        bitread_working_state br_state
110
111#define BITREAD_LOAD_STATE(cinfop,permstate)  \
112        br_state.cinfo = cinfop; \
113        br_state.next_input_byte = cinfop->src->next_input_byte; \
114        br_state.bytes_in_buffer = cinfop->src->bytes_in_buffer; \
115        get_buffer = permstate.get_buffer; \
116        bits_left = permstate.bits_left;
117
118#define BITREAD_SAVE_STATE(cinfop,permstate)  \
119        cinfop->src->next_input_byte = br_state.next_input_byte; \
120        cinfop->src->bytes_in_buffer = br_state.bytes_in_buffer; \
121        permstate.get_buffer = get_buffer; \
122        permstate.bits_left = bits_left
123
124/*
125 * These macros provide the in-line portion of bit fetching.
126 * Use CHECK_BIT_BUFFER to ensure there are N bits in get_buffer
127 * before using GET_BITS, PEEK_BITS, or DROP_BITS.
128 * The variables get_buffer and bits_left are assumed to be locals,
129 * but the state struct might not be (jpeg_huff_decode needs this).
130 *      CHECK_BIT_BUFFER(state,n,action);
131 *              Ensure there are N bits in get_buffer; if suspend, take action.
132 *      val = GET_BITS(n);
133 *              Fetch next N bits.
134 *      val = PEEK_BITS(n);
135 *              Fetch next N bits without removing them from the buffer.
136 *      DROP_BITS(n);
137 *              Discard next N bits.
138 * The value N should be a simple variable, not an expression, because it
139 * is evaluated multiple times.
140 */
141
142#define CHECK_BIT_BUFFER(state,nbits,action) \
143        { if (bits_left < (nbits)) {  \
144            if (! jpeg_fill_bit_buffer(&(state),get_buffer,bits_left,nbits))  \
145              { action; }  \
146            get_buffer = (state).get_buffer; bits_left = (state).bits_left; } }
147
148#define GET_BITS(nbits) \
149        (((int) (get_buffer >> (bits_left -= (nbits)))) & ((1<<(nbits))-1))
150
151#define PEEK_BITS(nbits) \
152        (((int) (get_buffer >> (bits_left -  (nbits)))) & ((1<<(nbits))-1))
153
154#define DROP_BITS(nbits) \
155        (bits_left -= (nbits))
156
157/* Load up the bit buffer to a depth of at least nbits */
158EXTERN(boolean) jpeg_fill_bit_buffer
159        JPP((bitread_working_state * state, register bit_buf_type get_buffer,
160             register int bits_left, int nbits));
161
162
163/*
164 * Code for extracting next Huffman-coded symbol from input bit stream.
165 * Again, this is time-critical and we make the main paths be macros.
166 *
167 * We use a lookahead table to process codes of up to HUFF_LOOKAHEAD bits
168 * without looping.  Usually, more than 95% of the Huffman codes will be 8
169 * or fewer bits long.  The few overlength codes are handled with a loop,
170 * which need not be inline code.
171 *
172 * Notes about the HUFF_DECODE macro:
173 * 1. Near the end of the data segment, we may fail to get enough bits
174 *    for a lookahead.  In that case, we do it the hard way.
175 * 2. If the lookahead table contains no entry, the next code must be
176 *    more than HUFF_LOOKAHEAD bits long.
177 * 3. jpeg_huff_decode returns -1 if forced to suspend.
178 */
179
180#define HUFF_DECODE(result,state,htbl,failaction,slowlabel) \
181{ register int nb, look; \
182  if (bits_left < HUFF_LOOKAHEAD) { \
183    if (! jpeg_fill_bit_buffer(&state,get_buffer,bits_left, 0)) {failaction;} \
184    get_buffer = state.get_buffer; bits_left = state.bits_left; \
185    if (bits_left < HUFF_LOOKAHEAD) { \
186      nb = 1; goto slowlabel; \
187    } \
188  } \
189  look = PEEK_BITS(HUFF_LOOKAHEAD); \
190  if ((nb = htbl->look_nbits[look]) != 0) { \
191    DROP_BITS(nb); \
192    result = htbl->look_sym[look]; \
193  } else { \
194    nb = HUFF_LOOKAHEAD+1; \
195slowlabel: \
196    if ((result=jpeg_huff_decode(&state,get_buffer,bits_left,htbl,nb)) < 0) \
197        { failaction; } \
198    get_buffer = state.get_buffer; bits_left = state.bits_left; \
199  } \
200}
201
202/* Out-of-line case for Huffman code fetching */
203EXTERN(int) jpeg_huff_decode
204        JPP((bitread_working_state * state, register bit_buf_type get_buffer,
205             register int bits_left, d_derived_tbl * htbl, int min_bits));
206