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