/* * 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));