00001 /* 00002 * jdhuff.h 00003 * 00004 * Copyright (C) 1991-1998, Thomas G. Lane. 00005 * This file is part of the Independent JPEG Group's software. 00006 * For conditions of distribution and use, see the accompanying README file. 00007 * 00008 * This file contains declarations for Huffman entropy decoding routines 00009 * that are shared between the sequential decoder (jdhuff.c), the 00010 * progressive decoder (jdphuff.c) and the lossless decoder (jdlhuff.c). 00011 * No other modules need to see these. 00012 */ 00013 00014 /* Short forms of external names for systems with brain-damaged linkers. */ 00015 00016 #ifdef NEED_SHORT_EXTERNAL_NAMES 00017 #define jpeg_make_d_derived_tbl jpeg12_make_d_derived_tbl 00018 #define jpeg_fill_bit_buffer jpeg12_fill_bit_buffer 00019 #define jpeg_huff_decode jpeg12_huff_decode 00020 #endif /* NEED_SHORT_EXTERNAL_NAMES */ 00021 00022 00023 /* Derived data constructed for each Huffman table */ 00024 00025 #define HUFF_LOOKAHEAD 8 /* # of bits of lookahead */ 00026 00027 typedef struct { 00028 /* Basic tables: (element [0] of each array is unused) */ 00029 IJG_INT32 maxcode[18]; /* largest code of length k (-1 if none) */ 00030 /* (maxcode[17] is a sentinel to ensure jpeg_huff_decode terminates) */ 00031 IJG_INT32 valoffset[17]; /* huffval[] offset for codes of length k */ 00032 /* valoffset[k] = huffval[] index of 1st symbol of code length k, less 00033 * the smallest code of length k; so given a code of length k, the 00034 * corresponding symbol is huffval[code + valoffset[k]] 00035 */ 00036 00037 /* Link to public Huffman table (needed only in jpeg_huff_decode) */ 00038 JHUFF_TBL *pub; 00039 00040 /* Lookahead tables: indexed by the next HUFF_LOOKAHEAD bits of 00041 * the input data stream. If the next Huffman code is no more 00042 * than HUFF_LOOKAHEAD bits long, we can obtain its length and 00043 * the corresponding symbol directly from these tables. 00044 */ 00045 int look_nbits[1<<HUFF_LOOKAHEAD]; /* # bits, or 0 if too long */ 00046 UINT8 look_sym[1<<HUFF_LOOKAHEAD]; /* symbol, or unused */ 00047 } d_derived_tbl; 00048 00049 /* Expand a Huffman table definition into the derived format */ 00050 EXTERN(void) jpeg_make_d_derived_tbl 00051 JPP((j_decompress_ptr cinfo, boolean isDC, int tblno, 00052 d_derived_tbl ** pdtbl)); 00053 00054 00055 /* 00056 * Fetching the next N bits from the input stream is a time-critical operation 00057 * for the Huffman decoders. We implement it with a combination of inline 00058 * macros and out-of-line subroutines. Note that N (the number of bits 00059 * demanded at one time) never exceeds 15 for JPEG use. 00060 * 00061 * We read source bytes into get_buffer and dole out bits as needed. 00062 * If get_buffer already contains enough bits, they are fetched in-line 00063 * by the macros CHECK_BIT_BUFFER and GET_BITS. When there aren't enough 00064 * bits, jpeg_fill_bit_buffer is called; it will attempt to fill get_buffer 00065 * as full as possible (not just to the number of bits needed; this 00066 * prefetching reduces the overhead cost of calling jpeg_fill_bit_buffer). 00067 * Note that jpeg_fill_bit_buffer may return FALSE to indicate suspension. 00068 * On TRUE return, jpeg_fill_bit_buffer guarantees that get_buffer contains 00069 * at least the requested number of bits --- dummy zeroes are inserted if 00070 * necessary. 00071 */ 00072 00073 typedef IJG_INT32 bit_buf_type; /* type of bit-extraction buffer */ 00074 #define BIT_BUF_SIZE 32 /* size of buffer in bits */ 00075 00076 /* If long is > 32 bits on your machine, and shifting/masking longs is 00077 * reasonably fast, making bit_buf_type be long and setting BIT_BUF_SIZE 00078 * appropriately should be a win. Unfortunately we can't define the size 00079 * with something like #define BIT_BUF_SIZE (sizeof(bit_buf_type)*8) 00080 * because not all machines measure sizeof in 8-bit bytes. 00081 */ 00082 00083 typedef struct { /* Bitreading state saved across MCUs */ 00084 bit_buf_type get_buffer; /* current bit-extraction buffer */ 00085 int bits_left; /* # of unused bits in it */ 00086 } bitread_perm_state; 00087 00088 typedef struct { /* Bitreading working state within an MCU */ 00089 /* Current data source location */ 00090 /* We need a copy, rather than munging the original, in case of suspension */ 00091 const JOCTET * next_input_byte; /* => next byte to read from source */ 00092 size_t bytes_in_buffer; /* # of bytes remaining in source buffer */ 00093 /* Bit input buffer --- note these values are kept in register variables, 00094 * not in this struct, inside the inner loops. 00095 */ 00096 bit_buf_type get_buffer; /* current bit-extraction buffer */ 00097 int bits_left; /* # of unused bits in it */ 00098 /* Pointer needed by jpeg_fill_bit_buffer. */ 00099 j_decompress_ptr cinfo; /* back link to decompress master record */ 00100 } bitread_working_state; 00101 00102 /* Macros to declare and load/save bitread local variables. */ 00103 #define BITREAD_STATE_VARS \ 00104 register bit_buf_type get_buffer; \ 00105 register int bits_left; \ 00106 bitread_working_state br_state 00107 00108 #define BITREAD_LOAD_STATE(cinfop,permstate) \ 00109 br_state.cinfo = cinfop; \ 00110 br_state.next_input_byte = cinfop->src->next_input_byte; \ 00111 br_state.bytes_in_buffer = cinfop->src->bytes_in_buffer; \ 00112 get_buffer = permstate.get_buffer; \ 00113 bits_left = permstate.bits_left; 00114 00115 #define BITREAD_SAVE_STATE(cinfop,permstate) \ 00116 cinfop->src->next_input_byte = br_state.next_input_byte; \ 00117 cinfop->src->bytes_in_buffer = br_state.bytes_in_buffer; \ 00118 permstate.get_buffer = get_buffer; \ 00119 permstate.bits_left = bits_left 00120 00121 /* 00122 * These macros provide the in-line portion of bit fetching. 00123 * Use CHECK_BIT_BUFFER to ensure there are N bits in get_buffer 00124 * before using GET_BITS, PEEK_BITS, or DROP_BITS. 00125 * The variables get_buffer and bits_left are assumed to be locals, 00126 * but the state struct might not be (jpeg_huff_decode needs this). 00127 * CHECK_BIT_BUFFER(state,n,action); 00128 * Ensure there are N bits in get_buffer; if suspend, take action. 00129 * val = GET_BITS(n); 00130 * Fetch next N bits. 00131 * val = PEEK_BITS(n); 00132 * Fetch next N bits without removing them from the buffer. 00133 * DROP_BITS(n); 00134 * Discard next N bits. 00135 * The value N should be a simple variable, not an expression, because it 00136 * is evaluated multiple times. 00137 */ 00138 00139 #define CHECK_BIT_BUFFER(state,nbits,action) \ 00140 { if (bits_left < (nbits)) { \ 00141 if (! jpeg_fill_bit_buffer(&(state),get_buffer,bits_left,nbits)) \ 00142 { action; } \ 00143 get_buffer = (state).get_buffer; bits_left = (state).bits_left; } } 00144 00145 #define GET_BITS(nbits) \ 00146 (((int) (get_buffer >> (bits_left -= (nbits)))) & ((1<<(nbits))-1)) 00147 00148 #define PEEK_BITS(nbits) \ 00149 (((int) (get_buffer >> (bits_left - (nbits)))) & ((1<<(nbits))-1)) 00150 00151 #define DROP_BITS(nbits) \ 00152 (bits_left -= (nbits)) 00153 00154 /* Load up the bit buffer to a depth of at least nbits */ 00155 EXTERN(boolean) jpeg_fill_bit_buffer 00156 JPP((bitread_working_state * state, register bit_buf_type get_buffer, 00157 register int bits_left, int nbits)); 00158 00159 00160 /* 00161 * Code for extracting next Huffman-coded symbol from input bit stream. 00162 * Again, this is time-critical and we make the main paths be macros. 00163 * 00164 * We use a lookahead table to process codes of up to HUFF_LOOKAHEAD bits 00165 * without looping. Usually, more than 950f the Huffman codes will be 8 00166 * or fewer bits long. The few overlength codes are handled with a loop, 00167 * which need not be inline code. 00168 * 00169 * Notes about the HUFF_DECODE macro: 00170 * 1. Near the end of the data segment, we may fail to get enough bits 00171 * for a lookahead. In that case, we do it the hard way. 00172 * 2. If the lookahead table contains no entry, the next code must be 00173 * more than HUFF_LOOKAHEAD bits long. 00174 * 3. jpeg_huff_decode returns -1 if forced to suspend. 00175 */ 00176 00177 #define HUFF_DECODE(result,state,htbl,failaction,slowlabel) \ 00178 { register int nb, look; \ 00179 if (bits_left < HUFF_LOOKAHEAD) { \ 00180 if (! jpeg_fill_bit_buffer(&state,get_buffer,bits_left, 0)) {failaction;} \ 00181 get_buffer = state.get_buffer; bits_left = state.bits_left; \ 00182 if (bits_left < HUFF_LOOKAHEAD) { \ 00183 nb = 1; goto slowlabel; \ 00184 } \ 00185 } \ 00186 look = PEEK_BITS(HUFF_LOOKAHEAD); \ 00187 if ((nb = htbl->look_nbits[look]) != 0) { \ 00188 DROP_BITS(nb); \ 00189 result = htbl->look_sym[look]; \ 00190 } else { \ 00191 nb = HUFF_LOOKAHEAD+1; \ 00192 slowlabel: \ 00193 if ((result=jpeg_huff_decode(&state,get_buffer,bits_left,htbl,nb)) < 0) \ 00194 { failaction; } \ 00195 get_buffer = state.get_buffer; bits_left = state.bits_left; \ 00196 } \ 00197 } 00198 00199 /* Out-of-line case for Huffman code fetching */ 00200 EXTERN(int) jpeg_huff_decode 00201 JPP((bitread_working_state * state, register bit_buf_type get_buffer, 00202 register int bits_left, d_derived_tbl * htbl, int min_bits)); 00203 00204 00205 /* Common fields between sequential, progressive and lossless Huffman entropy 00206 * decoder master structs. 00207 */ 00208 00209 #define huffd_common_fields \ 00210 boolean insufficient_data; /* set TRUE after emmitting warning */ \ 00211 /* These fields are loaded into local variables at start of each MCU. \ 00212 * In case of suspension, we exit WITHOUT updating them. \ 00213 */ \ 00214 bitread_perm_state bitstate /* Bit buffer at start of MCU */ 00215 00216 /* Routines that are to be used by any or all of the entropy decoders are 00217 * declared to receive a pointer to this structure. There are no actual 00218 * instances of huffd_common_struct, only of shuff_entropy_decoder, 00219 * phuff_entropy_decoder and lhuff_entropy_decoder. 00220 */ 00221 struct huffd_common_struct { 00222 huffd_common_fields; /* Fields common to all decoder struct types */ 00223 /* Additional fields follow in an actual shuff_entropy_decoder, 00224 * phuff_entropy_decoder or lhuff_entropy_decoder struct. All four structs 00225 * must agree on these initial fields! (This would be a lot cleaner in C++.) 00226 */ 00227 }; 00228 00229 typedef struct huffd_common_struct * huffd_common_ptr;