//  * jdhuff.h
//  *
//  * Copyright (C) 1991-1995, 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 mincode[17];		// smallest code of length k 
  INT32 maxcode[18];		// largest code of length k (-1 if none) 
  // (maxcode[17] is a sentinel to ensure jpeg_huff_decode terminates) 
  int valptr[17];		// huffval[] index of 1st symbol of length 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,
				JHUFF_TBL * htbl, 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 do this 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 
  boolean printed_eod;		// flag to suppress multiple warning msgs 
} bitread_perm_state;

typedef struct {		// Bitreading working state within an MCU 
  // current data source state 
  const JOCTET * next_input_byte; // => next byte to read from source 
  size_t bytes_in_buffer;	// # of bytes remaining in source buffer 
  int unread_marker;		// nonzero if we have hit a marker 
//  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 
  // pointers needed by jpeg_fill_bit_buffer 
  j_decompress_ptr cinfo;	// back link to decompress master record 
  boolean * printed_eod_ptr;	// => flag in permanent state 
} 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; \
	br_state.unread_marker = cinfop->unread_marker; \
	get_buffer = permstate.get_buffer; \
	bits_left = permstate.bits_left; \
	br_state.printed_eod_ptr = & permstate.printed_eod

#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; \
	cinfop->unread_marker = br_state.unread_marker; \
	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));