553N/A * reserved comment block 0N/A * DO NOT REMOVE OR ALTER! 0N/A * Copyright (C) 1991-1997, Thomas G. Lane. 553N/A * This file is part of the Independent JPEG Group's software. 0N/A * For conditions of distribution and use, see the accompanying README file. 0N/A * This file contains declarations for Huffman entropy decoding routines 0N/A * that are shared between the sequential decoder (jdhuff.c) and the 0N/A * progressive decoder (jdphuff.c). No other modules need to see these. 0N/A/* Short forms of external names for systems with brain-damaged linkers. */ 553N/A#
endif /* NEED_SHORT_EXTERNAL_NAMES */ 0N/A/* Derived data constructed for each Huffman table */ 0N/A /* Basic tables: (element [0] of each array is unused) */ 0N/A /* (maxcode[17] is a sentinel to ensure jpeg_huff_decode terminates) */ 0N/A /* valoffset[k] = huffval[] index of 1st symbol of code length k, less 0N/A * the smallest code of length k; so given a code of length k, the 0N/A * corresponding symbol is huffval[code + valoffset[k]] 0N/A /* Link to public Huffman table (needed only in jpeg_huff_decode) */ 0N/A /* Lookahead tables: indexed by the next HUFF_LOOKAHEAD bits of 0N/A * the input data stream. If the next Huffman code is no more 0N/A * than HUFF_LOOKAHEAD bits long, we can obtain its length and 0N/A * the corresponding symbol directly from these tables. 0N/A/* Expand a Huffman table definition into the derived format */ 0N/A * Fetching the next N bits from the input stream is a time-critical operation 0N/A * for the Huffman decoders. We implement it with a combination of inline 0N/A * macros and out-of-line subroutines. Note that N (the number of bits 0N/A * demanded at one time) never exceeds 15 for JPEG use. 0N/A * We read source bytes into get_buffer and dole out bits as needed. 0N/A * If get_buffer already contains enough bits, they are fetched in-line 0N/A * by the macros CHECK_BIT_BUFFER and GET_BITS. When there aren't enough 0N/A * bits, jpeg_fill_bit_buffer is called; it will attempt to fill get_buffer 0N/A * as full as possible (not just to the number of bits needed; this 0N/A * prefetching reduces the overhead cost of calling jpeg_fill_bit_buffer). 0N/A * Note that jpeg_fill_bit_buffer may return FALSE to indicate suspension. 0N/A * On TRUE return, jpeg_fill_bit_buffer guarantees that get_buffer contains 0N/A * at least the requested number of bits --- dummy zeroes are inserted if 0N/A * reasonably fast, making bit_buf_type be long and setting BIT_BUF_SIZE 0N/A * appropriately should be a win. Unfortunately we can't define the size 0N/A * with something like #define BIT_BUF_SIZE (sizeof(bit_buf_type)*8) 0N/A * because not all machines measure sizeof in 8-bit bytes. 0N/Atypedef struct {
/* Bitreading state saved across MCUs */ 0N/Atypedef struct {
/* Bitreading working state within an MCU */ 0N/A /* Current data source location */ 0N/A /* We need a copy, rather than munging the original, in case of suspension */ 0N/A /* Bit input buffer --- note these values are kept in register variables, 0N/A * not in this struct, inside the inner loops. 0N/A /* Pointer needed by jpeg_fill_bit_buffer. */ 0N/A/* Macros to declare and load/save bitread local variables. */ 0N/A * These macros provide the in-line portion of bit fetching. 0N/A * Use CHECK_BIT_BUFFER to ensure there are N bits in get_buffer 0N/A * before using GET_BITS, PEEK_BITS, or DROP_BITS. 0N/A * The variables get_buffer and bits_left are assumed to be locals, 0N/A * but the state struct might not be (jpeg_huff_decode needs this). 0N/A * CHECK_BIT_BUFFER(state,n,action); 0N/A * Ensure there are N bits in get_buffer; if suspend, take action. 0N/A * val = GET_BITS(n); 0N/A * Fetch next N bits. 0N/A * val = PEEK_BITS(n); 0N/A * Fetch next N bits without removing them from the buffer. 0N/A * Discard next N bits. 0N/A * The value N should be a simple variable, not an expression, because it 0N/A * is evaluated multiple times. /* Load up the bit buffer to a depth of at least 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. /* Out-of-line case for Huffman code fetching */