ref: d9d2925fa024c2eceeea18c9b3afda30af96301e
dir: /sys/src/cmd/gs/jpeg/jdhuff.h/
/* * jdhuff.h * * Copyright (C) 1991-1997, Thomas G. Lane. * This file is part of the Independent JPEG Group's software. * For conditions of distribution and use, see the accompanying README file. * * This file contains declarations for Huffman entropy decoding routines * that are shared between the sequential decoder (jdhuff.c) and the * progressive decoder (jdphuff.c). No other modules need to see these. */ /* Short forms of external names for systems with brain-damaged linkers. */ #ifdef NEED_SHORT_EXTERNAL_NAMES #define jpeg_make_d_derived_tbl jMkDDerived #define jpeg_fill_bit_buffer jFilBitBuf #define jpeg_huff_decode jHufDecode #endif /* NEED_SHORT_EXTERNAL_NAMES */ /* Derived data constructed for each Huffman table */ #define HUFF_LOOKAHEAD 8 /* # of bits of lookahead */ typedef struct { /* Basic tables: (element [0] of each array is unused) */ INT32 maxcode[18]; /* largest code of length k (-1 if none) */ /* (maxcode[17] is a sentinel to ensure jpeg_huff_decode terminates) */ INT32 valoffset[17]; /* huffval[] offset for codes of length k */ /* valoffset[k] = huffval[] index of 1st symbol of code length k, less * the smallest code of length k; so given a code of length k, the * corresponding symbol is huffval[code + valoffset[k]] */ /* Link to public Huffman table (needed only in jpeg_huff_decode) */ JHUFF_TBL *pub; /* Lookahead tables: indexed by the next HUFF_LOOKAHEAD bits of * the input data stream. If the next Huffman code is no more * than HUFF_LOOKAHEAD bits long, we can obtain its length and * the corresponding symbol directly from these tables. */ int look_nbits[1<<HUFF_LOOKAHEAD]; /* # bits, or 0 if too long */ UINT8 look_sym[1<<HUFF_LOOKAHEAD]; /* symbol, or unused */ } d_derived_tbl; /* Expand a Huffman table definition into the derived format */ EXTERN(void) jpeg_make_d_derived_tbl JPP((j_decompress_ptr cinfo, boolean isDC, int tblno, d_derived_tbl ** pdtbl)); /* * Fetching the next N bits from the input stream is a time-critical operation * for the Huffman decoders. We implement it with a combination of inline * macros and out-of-line subroutines. Note that N (the number of bits * demanded at one time) never exceeds 15 for JPEG use. * * We read source bytes into get_buffer and dole out bits as needed. * If get_buffer already contains enough bits, they are fetched in-line * by the macros CHECK_BIT_BUFFER and GET_BITS. When there aren't enough * bits, jpeg_fill_bit_buffer is called; it will attempt to fill get_buffer * as full as possible (not just to the number of bits needed; this * prefetching reduces the overhead cost of calling jpeg_fill_bit_buffer). * Note that jpeg_fill_bit_buffer may return FALSE to indicate suspension. * On TRUE return, jpeg_fill_bit_buffer guarantees that get_buffer contains * at least the requested number of bits --- dummy zeroes are inserted if * necessary. */ typedef INT32 bit_buf_type; /* type of bit-extraction buffer */ #define BIT_BUF_SIZE 32 /* size of buffer in bits */ /* If long is > 32 bits on your machine, and shifting/masking longs is * reasonably fast, making bit_buf_type be long and setting BIT_BUF_SIZE * appropriately should be a win. Unfortunately we can't define the size * with something like #define BIT_BUF_SIZE (sizeof(bit_buf_type)*8) * because not all machines measure sizeof in 8-bit bytes. */ typedef struct { /* Bitreading state saved across MCUs */ bit_buf_type get_buffer; /* current bit-extraction buffer */ int bits_left; /* # of unused bits in it */ } bitread_perm_state; typedef struct { /* Bitreading working state within an MCU */ /* Current data source location */ /* We need a copy, rather than munging the original, in case of suspension */ const JOCTET * next_input_byte; /* => next byte to read from source */ size_t bytes_in_buffer; /* # of bytes remaining in source buffer */ /* Bit input buffer --- note these values are kept in register variables, * not in this struct, inside the inner loops. */ bit_buf_type get_buffer; /* current bit-extraction buffer */ int bits_left; /* # of unused bits in it */ /* Pointer needed by jpeg_fill_bit_buffer. */ j_decompress_ptr cinfo; /* back link to decompress master record */ } bitread_working_state; /* Macros to declare and load/save bitread local variables. */ #define BITREAD_STATE_VARS \ register bit_buf_type get_buffer; \ register int bits_left; \ bitread_working_state br_state #define BITREAD_LOAD_STATE(cinfop,permstate) \ br_state.cinfo = cinfop; \ br_state.next_input_byte = cinfop->src->next_input_byte; \ br_state.bytes_in_buffer = cinfop->src->bytes_in_buffer; \ get_buffer = permstate.get_buffer; \ bits_left = permstate.bits_left; #define BITREAD_SAVE_STATE(cinfop,permstate) \ cinfop->src->next_input_byte = br_state.next_input_byte; \ cinfop->src->bytes_in_buffer = br_state.bytes_in_buffer; \ permstate.get_buffer = get_buffer; \ permstate.bits_left = bits_left /* * These macros provide the in-line portion of bit fetching. * Use CHECK_BIT_BUFFER to ensure there are N bits in get_buffer * before using GET_BITS, PEEK_BITS, or DROP_BITS. * The variables get_buffer and bits_left are assumed to be locals, * but the state struct might not be (jpeg_huff_decode needs this). * CHECK_BIT_BUFFER(state,n,action); * Ensure there are N bits in get_buffer; if suspend, take action. * val = GET_BITS(n); * Fetch next N bits. * val = PEEK_BITS(n); * Fetch next N bits without removing them from the buffer. * DROP_BITS(n); * Discard next N bits. * The value N should be a simple variable, not an expression, because it * is evaluated multiple times. */ #define CHECK_BIT_BUFFER(state,nbits,action) \ { if (bits_left < (nbits)) { \ if (! jpeg_fill_bit_buffer(&(state),get_buffer,bits_left,nbits)) \ { action; } \ get_buffer = (state).get_buffer; bits_left = (state).bits_left; } } #define GET_BITS(nbits) \ (((int) (get_buffer >> (bits_left -= (nbits)))) & ((1<<(nbits))-1)) #define PEEK_BITS(nbits) \ (((int) (get_buffer >> (bits_left - (nbits)))) & ((1<<(nbits))-1)) #define DROP_BITS(nbits) \ (bits_left -= (nbits)) /* Load up the bit buffer to a depth of at least nbits */ EXTERN(boolean) jpeg_fill_bit_buffer JPP((bitread_working_state * state, register bit_buf_type get_buffer, register int bits_left, int nbits)); /* * Code for extracting next Huffman-coded symbol from input bit stream. * Again, this is time-critical and we make the main paths be macros. * * We use a lookahead table to process codes of up to HUFF_LOOKAHEAD bits * without looping. Usually, more than 95% of the Huffman codes will be 8 * or fewer bits long. The few overlength codes are handled with a loop, * which need not be inline code. * * Notes about the HUFF_DECODE macro: * 1. Near the end of the data segment, we may fail to get enough bits * for a lookahead. In that case, we do it the hard way. * 2. If the lookahead table contains no entry, the next code must be * more than HUFF_LOOKAHEAD bits long. * 3. jpeg_huff_decode returns -1 if forced to suspend. */ #define HUFF_DECODE(result,state,htbl,failaction,slowlabel) \ { register int nb, look; \ if (bits_left < HUFF_LOOKAHEAD) { \ if (! jpeg_fill_bit_buffer(&state,get_buffer,bits_left, 0)) {failaction;} \ get_buffer = state.get_buffer; bits_left = state.bits_left; \ if (bits_left < HUFF_LOOKAHEAD) { \ nb = 1; goto slowlabel; \ } \ } \ look = PEEK_BITS(HUFF_LOOKAHEAD); \ if ((nb = htbl->look_nbits[look]) != 0) { \ DROP_BITS(nb); \ result = htbl->look_sym[look]; \ } else { \ nb = HUFF_LOOKAHEAD+1; \ slowlabel: \ if ((result=jpeg_huff_decode(&state,get_buffer,bits_left,htbl,nb)) < 0) \ { failaction; } \ get_buffer = state.get_buffer; bits_left = state.bits_left; \ } \ } /* Out-of-line case for Huffman code fetching */ EXTERN(int) jpeg_huff_decode JPP((bitread_working_state * state, register bit_buf_type get_buffer, register int bits_left, d_derived_tbl * htbl, int min_bits));