00001 /* 00002 * jdhuff.h 00003 * 00004 * Copyright (C) 1991-1997, 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) and the 00010 * progressive decoder (jdphuff.c). No other modules need to see these. 00011 */ 00012 00013 /* Short forms of external names for systems with brain-damaged linkers. */ 00014 00015 #ifdef NEED_SHORT_EXTERNAL_NAMES 00016 #define jpeg_make_d_derived_tbl jMkDDerived 00017 #define jpeg_fill_bit_buffer jFilBitBuf 00018 #define jpeg_huff_decode jHufDecode 00019 #endif /* NEED_SHORT_EXTERNAL_NAMES */ 00020 00021 00022 /* Derived data constructed for each Huffman table */ 00023 00024 #define HUFF_LOOKAHEAD 8 /* # of bits of lookahead */ 00025 00026 typedef struct { 00027 /* Basic tables: (element [0] of each array is unused) */ 00028 INT32 maxcode[18]; /* largest code of length k (-1 if none) */ 00029 /* (maxcode[17] is a sentinel to ensure jpeg_huff_decode terminates) */ 00030 INT32 valoffset[17]; /* huffval[] offset for codes of length k */ 00031 /* valoffset[k] = huffval[] index of 1st symbol of code length k, less 00032 * the smallest code of length k; so given a code of length k, the 00033 * corresponding symbol is huffval[code + valoffset[k]] 00034 */ 00035 00036 /* Link to public Huffman table (needed only in jpeg_huff_decode) */ 00037 JHUFF_TBL *pub; 00038 00039 /* Lookahead tables: indexed by the next HUFF_LOOKAHEAD bits of 00040 * the input data stream. If the next Huffman code is no more 00041 * than HUFF_LOOKAHEAD bits long, we can obtain its length and 00042 * the corresponding symbol directly from these tables. 00043 */ 00044 int look_nbits[1<<HUFF_LOOKAHEAD]; /* # bits, or 0 if too long */ 00045 UINT8 look_sym[1<<HUFF_LOOKAHEAD]; /* symbol, or unused */ 00046 } d_derived_tbl; 00047 00048 /* Expand a Huffman table definition into the derived format */ 00049 EXTERN(void) jpeg_make_d_derived_tbl 00050 JPP((j_decompress_ptr cinfo, boolean isDC, int tblno, 00051 d_derived_tbl ** pdtbl)); 00052 00053 00054 /* 00055 * Fetching the next N bits from the input stream is a time-critical operation 00056 * for the Huffman decoders. We implement it with a combination of inline 00057 * macros and out-of-line subroutines. Note that N (the number of bits 00058 * demanded at one time) never exceeds 15 for JPEG use. 00059 * 00060 * We read source bytes into get_buffer and dole out bits as needed. 00061 * If get_buffer already contains enough bits, they are fetched in-line 00062 * by the macros CHECK_BIT_BUFFER and GET_BITS. When there aren't enough 00063 * bits, jpeg_fill_bit_buffer is called; it will attempt to fill get_buffer 00064 * as full as possible (not just to the number of bits needed; this 00065 * prefetching reduces the overhead cost of calling jpeg_fill_bit_buffer). 00066 * Note that jpeg_fill_bit_buffer may return FALSE to indicate suspension. 00067 * On TRUE return, jpeg_fill_bit_buffer guarantees that get_buffer contains 00068 * at least the requested number of bits --- dummy zeroes are inserted if 00069 * necessary. 00070 */ 00071 00072 typedef INT32 bit_buf_type; /* type of bit-extraction buffer */ 00073 #define BIT_BUF_SIZE 32 /* size of buffer in bits */ 00074 00075 /* If long is > 32 bits on your machine, and shifting/masking longs is 00076 * reasonably fast, making bit_buf_type be long and setting BIT_BUF_SIZE 00077 * appropriately should be a win. Unfortunately we can't define the size 00078 * with something like #define BIT_BUF_SIZE (sizeof(bit_buf_type)*8) 00079 * because not all machines measure sizeof in 8-bit bytes. 00080 */ 00081 00082 typedef struct { /* Bitreading state saved across MCUs */ 00083 bit_buf_type get_buffer; /* current bit-extraction buffer */ 00084 int bits_left; /* # of unused bits in it */ 00085 } bitread_perm_state; 00086 00087 typedef struct { /* Bitreading working state within an MCU */ 00088 /* Current data source location */ 00089 /* We need a copy, rather than munging the original, in case of suspension */ 00090 const JOCTET * next_input_byte; /* => next byte to read from source */ 00091 size_t bytes_in_buffer; /* # of bytes remaining in source buffer */ 00092 /* Bit input buffer --- note these values are kept in register variables, 00093 * not in this struct, inside the inner loops. 00094 */ 00095 bit_buf_type get_buffer; /* current bit-extraction buffer */ 00096 int bits_left; /* # of unused bits in it */ 00097 /* Pointer needed by jpeg_fill_bit_buffer. */ 00098 j_decompress_ptr cinfo; /* back link to decompress master record */ 00099 } bitread_working_state; 00100 00101 /* Macros to declare and load/save bitread local variables. */ 00102 #define BITREAD_STATE_VARS \ 00103 register bit_buf_type get_buffer; \ 00104 register int bits_left; \ 00105 bitread_working_state br_state 00106 00107 #define BITREAD_LOAD_STATE(cinfop,permstate) \ 00108 br_state.cinfo = cinfop; \ 00109 br_state.next_input_byte = cinfop->src->next_input_byte; \ 00110 br_state.bytes_in_buffer = cinfop->src->bytes_in_buffer; \ 00111 get_buffer = permstate.get_buffer; \ 00112 bits_left = permstate.bits_left; 00113 00114 #define BITREAD_SAVE_STATE(cinfop,permstate) \ 00115 cinfop->src->next_input_byte = br_state.next_input_byte; \ 00116 cinfop->src->bytes_in_buffer = br_state.bytes_in_buffer; \ 00117 permstate.get_buffer = get_buffer; \ 00118 permstate.bits_left = bits_left 00119 00120 /* 00121 * These macros provide the in-line portion of bit fetching. 00122 * Use CHECK_BIT_BUFFER to ensure there are N bits in get_buffer 00123 * before using GET_BITS, PEEK_BITS, or DROP_BITS. 00124 * The variables get_buffer and bits_left are assumed to be locals, 00125 * but the state struct might not be (jpeg_huff_decode needs this). 00126 * CHECK_BIT_BUFFER(state,n,action); 00127 * Ensure there are N bits in get_buffer; if suspend, take action. 00128 * val = GET_BITS(n); 00129 * Fetch next N bits. 00130 * val = PEEK_BITS(n); 00131 * Fetch next N bits without removing them from the buffer. 00132 * DROP_BITS(n); 00133 * Discard next N bits. 00134 * The value N should be a simple variable, not an expression, because it 00135 * is evaluated multiple times. 00136 */ 00137 00138 #define CHECK_BIT_BUFFER(state,nbits,action) \ 00139 { if (bits_left < (nbits)) { \ 00140 if (! jpeg_fill_bit_buffer(&(state),get_buffer,bits_left,nbits)) \ 00141 { action; } \ 00142 get_buffer = (state).get_buffer; bits_left = (state).bits_left; } } 00143 00144 #define GET_BITS(nbits) \ 00145 (((int) (get_buffer >> (bits_left -= (nbits)))) & ((1<<(nbits))-1)) 00146 00147 #define PEEK_BITS(nbits) \ 00148 (((int) (get_buffer >> (bits_left - (nbits)))) & ((1<<(nbits))-1)) 00149 00150 #define DROP_BITS(nbits) \ 00151 (bits_left -= (nbits)) 00152 00153 /* Load up the bit buffer to a depth of at least nbits */ 00154 EXTERN(boolean) jpeg_fill_bit_buffer 00155 JPP((bitread_working_state * state, register bit_buf_type get_buffer, 00156 register int bits_left, int nbits)); 00157 00158 00159 /* 00160 * Code for extracting next Huffman-coded symbol from input bit stream. 00161 * Again, this is time-critical and we make the main paths be macros. 00162 * 00163 * We use a lookahead table to process codes of up to HUFF_LOOKAHEAD bits 00164 * without looping. Usually, more than 95% of the Huffman codes will be 8 00165 * or fewer bits long. The few overlength codes are handled with a loop, 00166 * which need not be inline code. 00167 * 00168 * Notes about the HUFF_DECODE macro: 00169 * 1. Near the end of the data segment, we may fail to get enough bits 00170 * for a lookahead. In that case, we do it the hard way. 00171 * 2. If the lookahead table contains no entry, the next code must be 00172 * more than HUFF_LOOKAHEAD bits long. 00173 * 3. jpeg_huff_decode returns -1 if forced to suspend. 00174 */ 00175 00176 #define HUFF_DECODE(result,state,htbl,failaction,slowlabel) \ 00177 { register int nb, look; \ 00178 if (bits_left < HUFF_LOOKAHEAD) { \ 00179 if (! jpeg_fill_bit_buffer(&state,get_buffer,bits_left, 0)) {failaction;} \ 00180 get_buffer = state.get_buffer; bits_left = state.bits_left; \ 00181 if (bits_left < HUFF_LOOKAHEAD) { \ 00182 nb = 1; goto slowlabel; \ 00183 } \ 00184 } \ 00185 look = PEEK_BITS(HUFF_LOOKAHEAD); \ 00186 if ((nb = htbl->look_nbits[look]) != 0) { \ 00187 DROP_BITS(nb); \ 00188 result = htbl->look_sym[look]; \ 00189 } else { \ 00190 nb = HUFF_LOOKAHEAD+1; \ 00191 slowlabel: \ 00192 if ((result=jpeg_huff_decode(&state,get_buffer,bits_left,htbl,nb)) < 0) \ 00193 { failaction; } \ 00194 get_buffer = state.get_buffer; bits_left = state.bits_left; \ 00195 } \ 00196 } 00197 00198 /* Out-of-line case for Huffman code fetching */ 00199 EXTERN(int) jpeg_huff_decode 00200 JPP((bitread_working_state * state, register bit_buf_type get_buffer, 00201 register int bits_left, d_derived_tbl * htbl, int min_bits));