jchuff.c

/*
 * jchuff.c
 *
 * Copyright (C) 1991, 1992, 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 Huffman entropy encoding routines.
 * These routines are invoked via the methods entropy_encode,
 * entropy_encoder_init/term, and entropy_optimize.
 */

#include "jinclude.h"


/* Static variables to avoid passing 'round extra parameters */

static compress_info_ptr cinfo;

static INT32 huff_put_buffer;	/* current bit-accumulation buffer */
static int huff_put_bits;	/* # of bits now in it */

static char * output_buffer;	/* output buffer */
static int bytes_in_buffer;



LOCAL void
fix_huff_tbl (HUFF_TBL * htbl)
/* Compute derived values for a Huffman table */
{
  int p, i, l, lastp, si;
  char huffsize[257];
  UINT16 huffcode[257];
  UINT16 code;
  
  /* Figure C.1: make table of Huffman code length for each symbol */
  /* Note that this is in code-length order. */

  p = 0;
  for (l = 1; l <= 16; l++) {
    for (i = 1; i <= (int) htbl->bits[l]; i++)
      huffsize[p++] = (char) l;
  }
  huffsize[p] = 0;
  lastp = p;
  
  /* Figure C.2: generate the codes themselves */
  /* Note that this is in code-length order. */
  
  code = 0;
  si = huffsize[0];
  p = 0;
  while (huffsize[p]) {
    while (((int) huffsize[p]) == si) {
      huffcode[p++] = code;
      code++;
    }
    code <<= 1;
    si++;
  }
  
  /* Figure C.3: generate encoding tables */
  /* These are code and size indexed by symbol value */

  /* Set any codeless symbols to have code length 0;
   * this allows emit_bits to detect any attempt to emit such symbols.
   */
  MEMZERO((void *) htbl->ehufsi, SIZEOF(htbl->ehufsi));

  for (p = 0; p < lastp; p++) {
    htbl->ehufco[htbl->huffval[p]] = huffcode[p];
    htbl->ehufsi[htbl->huffval[p]] = huffsize[p];
  }
  
  /* We don't bother to fill in the decoding tables mincode[], maxcode[], */
  /* and valptr[], since they are not used for encoding. */
}


/* Outputting bytes to the file */

LOCAL void
flush_bytes (void)
{
  if (bytes_in_buffer)
    (*cinfo->methods->entropy_output) (cinfo, output_buffer, bytes_in_buffer);
  bytes_in_buffer = 0;
}


#define emit_byte(val)  \
  MAKESTMT( if (bytes_in_buffer >= JPEG_BUF_SIZE) \
	      flush_bytes(); \
	    output_buffer[bytes_in_buffer++] = (char) (val); )



/* Outputting bits to the file */

/* Only the right 24 bits of huff_put_buffer are used; the valid bits are
 * left-justified in this part.  At most 16 bits can be passed to emit_bits
 * in one call, and we never retain more than 7 bits in huff_put_buffer
 * between calls, so 24 bits are sufficient.
 */

LOCAL void
emit_bits (UINT16 code, int size)
{
  /* This routine is heavily used, so it's worth coding tightly. */
  register INT32 put_buffer = code;
  register int put_bits = huff_put_bits;

  /* if size is 0, caller used an invalid Huffman table entry */
  if (size == 0)
    ERREXIT(cinfo->emethods, "Missing Huffman code table entry");

  put_buffer &= (((INT32) 1) << size) - 1; /* Mask off any excess bits in code */
  
  put_bits += size;		/* new number of bits in buffer */
  
  put_buffer <<= 24 - put_bits; /* align incoming bits */

  put_buffer |= huff_put_buffer; /* and merge with old buffer contents */
  
  while (put_bits >= 8) {
    int c = (int) ((put_buffer >> 16) & 0xFF);
    
    emit_byte(c);
    if (c == 0xFF) {		/* need to stuff a zero byte? */
      emit_byte(0);
    }
    put_buffer <<= 8;
    put_bits -= 8;
  }

  huff_put_buffer = put_buffer;	/* Update global variables */
  huff_put_bits = put_bits;
}


LOCAL void
flush_bits (void)
{
  emit_bits((UINT16) 0x7F, 7);	/* fill any partial byte with ones */
  huff_put_buffer = 0;		/* and reset bit-buffer to empty */
  huff_put_bits = 0;
}



/* Encode a single block's worth of coefficients */
/* Note that the DC coefficient has already been converted to a difference */

LOCAL void
encode_one_block (JBLOCK block, HUFF_TBL *dctbl, HUFF_TBL *actbl)
{
  register int temp, temp2;
  register int nbits;
  register int k, r, i;
  
  /* Encode the DC coefficient difference per section F.1.2.1 */
  
  temp = temp2 = block[0];

  if (temp < 0) {
    temp = -temp;		/* temp is abs value of input */
    /* For a negative input, want temp2 = bitwise complement of abs(input) */
    /* This code assumes we are on a two's complement machine */
    temp2--;
  }
  
  /* Find the number of bits needed for the magnitude of the coefficient */
  nbits = 0;
  while (temp) {
    nbits++;
    temp >>= 1;
  }
  
  /* Emit the Huffman-coded symbol for the number of bits */
  emit_bits(dctbl->ehufco[nbits], dctbl->ehufsi[nbits]);

  /* Emit that number of bits of the value, if positive, */
  /* or the complement of its magnitude, if negative. */
  if (nbits)			/* emit_bits rejects calls with size 0 */
    emit_bits((UINT16) temp2, nbits);
  
  /* Encode the AC coefficients per section F.1.2.2 */
  
  r = 0;			/* r = run length of zeros */
  
  for (k = 1; k < DCTSIZE2; k++) {
    if ((temp = block[k]) == 0) {
      r++;
    } else {
      /* if run length > 15, must emit special run-length-16 codes (0xF0) */
      while (r > 15) {
	emit_bits(actbl->ehufco[0xF0], actbl->ehufsi[0xF0]);
	r -= 16;
      }

      temp2 = temp;
      if (temp < 0) {
	temp = -temp;		/* temp is abs value of input */
	/* This code assumes we are on a two's complement machine */
	temp2--;
      }
      
      /* Find the number of bits needed for the magnitude of the coefficient */
      nbits = 1;		/* there must be at least one 1 bit */
      while (temp >>= 1)
	nbits++;
      
      /* Emit Huffman symbol for run length / number of bits */
      i = (r << 4) + nbits;
      emit_bits(actbl->ehufco[i], actbl->ehufsi[i]);
      
      /* Emit that number of bits of the value, if positive, */
      /* or the complement of its magnitude, if negative. */
      emit_bits((UINT16) temp2, nbits);
      
      r = 0;
    }
  }

  /* If the last coef(s) were zero, emit an end-of-block code */
  if (r > 0)
    emit_bits(actbl->ehufco[0], actbl->ehufsi[0]);
}



/*
 * Initialize for a Huffman-compressed scan.
 * This is invoked after writing the SOS marker.
 * The pipeline controller must establish the entropy_output method pointer
 * before calling this routine.
 */

METHODDEF void
huff_init (compress_info_ptr xinfo)
{
  short ci;
  jpeg_component_info * compptr;

  /* Initialize static variables */
  cinfo = xinfo;
  huff_put_buffer = 0;
  huff_put_bits = 0;

  /* Initialize the output buffer */
  output_buffer = (char *) (*cinfo->emethods->alloc_small)
				((size_t) JPEG_BUF_SIZE);
  bytes_in_buffer = 0;

  for (ci = 0; ci < cinfo->comps_in_scan; ci++) {
    compptr = cinfo->cur_comp_info[ci];
    /* Make sure requested tables are present */
    if (cinfo->dc_huff_tbl_ptrs[compptr->dc_tbl_no] == NULL ||
	cinfo->ac_huff_tbl_ptrs[compptr->ac_tbl_no] == NULL)
      ERREXIT(cinfo->emethods, "Use of undefined Huffman table");
    /* Compute derived values for Huffman tables */
    /* We may do this more than once for same table, but it's not a big deal */
    fix_huff_tbl(cinfo->dc_huff_tbl_ptrs[compptr->dc_tbl_no]);
    fix_huff_tbl(cinfo->ac_huff_tbl_ptrs[compptr->ac_tbl_no]);
    /* Initialize DC predictions to 0 */
    cinfo->last_dc_val[ci] = 0;
  }

  /* Initialize restart stuff */
  cinfo->restarts_to_go = cinfo->restart_interval;
  cinfo->next_restart_num = 0;
}


/*
 * Emit a restart marker & resynchronize predictions.
 */

LOCAL void
emit_restart (compress_info_ptr cinfo)
{
  short ci;

  flush_bits();

  emit_byte(0xFF);
  emit_byte(RST0 + cinfo->next_restart_num);

  /* Re-initialize DC predictions to 0 */
  for (ci = 0; ci < cinfo->comps_in_scan; ci++)
    cinfo->last_dc_val[ci] = 0;

  /* Update restart state */
  cinfo->restarts_to_go = cinfo->restart_interval;
  cinfo->next_restart_num++;
  cinfo->next_restart_num &= 7;
}


/*
 * Encode and output one MCU's worth of Huffman-compressed coefficients.
 */

METHODDEF void
huff_encode (compress_info_ptr cinfo, JBLOCK *MCU_data)
{
  short blkn, ci;
  jpeg_component_info * compptr;
  JCOEF temp;

  /* Account for restart interval, emit restart marker if needed */
  if (cinfo->restart_interval) {
    if (cinfo->restarts_to_go == 0)
      emit_restart(cinfo);
    cinfo->restarts_to_go--;
  }

  for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {
    ci = cinfo->MCU_membership[blkn];
    compptr = cinfo->cur_comp_info[ci];
    /* Convert DC value to difference, update last_dc_val */
    temp = MCU_data[blkn][0];
    MCU_data[blkn][0] -= cinfo->last_dc_val[ci];
    cinfo->last_dc_val[ci] = temp;
    encode_one_block(MCU_data[blkn],
		     cinfo->dc_huff_tbl_ptrs[compptr->dc_tbl_no],
		     cinfo->ac_huff_tbl_ptrs[compptr->ac_tbl_no]);
  }
}


/*
 * Finish up at the end of a Huffman-compressed scan.
 */

METHODDEF void
huff_term (compress_info_ptr cinfo)
{
  /* Flush out the last data */
  flush_bits();
  flush_bytes();
  /* Release the I/O buffer */
  (*cinfo->emethods->free_small) ((void *) output_buffer);
}




/*
 * Huffman coding optimization.
 *
 * This actually is optimization, in the sense that we find the best possible
 * Huffman table(s) for the given data.  We first scan the supplied data and
 * count the number of uses of each symbol that is to be Huffman-coded.
 * (This process must agree with the code above.)  Then we build an
 * optimal Huffman coding tree for the observed counts.
 */

#ifdef ENTROPY_OPT_SUPPORTED


/* These are static so htest_one_block can find 'em */
static long * dc_count_ptrs[NUM_HUFF_TBLS];
static long * ac_count_ptrs[NUM_HUFF_TBLS];


LOCAL void
gen_huff_coding (compress_info_ptr cinfo, HUFF_TBL *htbl, long freq[])
/* Generate the optimal coding for the given counts */
{
#define MAX_CLEN 32		/* assumed maximum initial code length */
  UINT8 bits[MAX_CLEN+1];	/* bits[k] = # of symbols with code length k */
  short codesize[257];		/* codesize[k] = code length of symbol k */
  short others[257];		/* next symbol in current branch of tree */
  int c1, c2;
  int p, i, j;
  long v;

  /* This algorithm is explained in section K.2 of the JPEG standard */

  MEMZERO((void *) bits, SIZEOF(bits));
  MEMZERO((void *) codesize, SIZEOF(codesize));
  for (i = 0; i < 257; i++)
    others[i] = -1;		/* init links to empty */
  
  freq[256] = 1;		/* make sure there is a nonzero count */
  /* including the pseudo-symbol 256 in the Huffman procedure guarantees
   * that no real symbol is given code-value of all ones, because 256
   * will be placed in the largest codeword category.
   */

  /* Huffman's basic algorithm to assign optimal code lengths to symbols */

  for (;;) {
    /* Find the smallest nonzero frequency, set c1 = its symbol */
    /* In case of ties, take the larger symbol number */
    c1 = -1;
    v = 1000000000L;
    for (i = 0; i <= 256; i++) {
      if (freq[i] && freq[i] <= v) {
	v = freq[i];
	c1 = i;
      }
    }

    /* Find the next smallest nonzero frequency, set c2 = its symbol */
    /* In case of ties, take the larger symbol number */
    c2 = -1;
    v = 1000000000L;
    for (i = 0; i <= 256; i++) {
      if (freq[i] && freq[i] <= v && i != c1) {
	v = freq[i];
	c2 = i;
      }
    }

    /* Done if we've merged everything into one frequency */
    if (c2 < 0)
      break;
    
    /* Else merge the two counts/trees */
    freq[c1] += freq[c2];
    freq[c2] = 0;

    /* Increment the codesize of everything in c1's tree branch */
    codesize[c1]++;
    while (others[c1] >= 0) {
      c1 = others[c1];
      codesize[c1]++;
    }
    
    others[c1] = c2;		/* chain c2 onto c1's tree branch */
    
    /* Increment the codesize of everything in c2's tree branch */
    codesize[c2]++;
    while (others[c2] >= 0) {
      c2 = others[c2];
      codesize[c2]++;
    }
  }

  /* Now count the number of symbols of each code length */
  for (i = 0; i <= 256; i++) {
    if (codesize[i]) {
      /* The JPEG standard seems to think that this can't happen, */
      /* but I'm paranoid... */
      if (codesize[i] > MAX_CLEN)
	ERREXIT(cinfo->emethods, "Huffman code size table overflow");

      bits[codesize[i]]++;
    }
  }

  /* JPEG doesn't allow symbols with code lengths over 16 bits, so if the pure
   * Huffman procedure assigned any such lengths, we must adjust the coding.
   * Here is what the JPEG spec says about how this next bit works:
   * Since symbols are paired for the longest Huffman code, the symbols are
   * removed from this length category two at a time.  The prefix for the pair
   * (which is one bit shorter) is allocated to one of the pair; then,
   * skipping the BITS entry for that prefix length, a code word from the next
   * shortest nonzero BITS entry is converted into a prefix for two code words
   * one bit longer.
   */
  
  for (i = MAX_CLEN; i > 16; i--) {
    while (bits[i] > 0) {
      j = i - 2;		/* find length of new prefix to be used */
      while (bits[j] == 0)
	j--;
      
      bits[i] -= 2;		/* remove two symbols */
      bits[i-1]++;		/* one goes in this length */
      bits[j+1] += 2;		/* two new symbols in this length */
      bits[j]--;		/* symbol of this length is now a prefix */
    }
  }

  /* Remove the count for the pseudo-symbol 256 from the largest codelength */
  while (bits[i] == 0)		/* find largest codelength still in use */
    i--;
  bits[i]--;
  
  /* Return final symbol counts (only for lengths 0..16) */
  memcpy((void *) htbl->bits, (void *) bits, SIZEOF(htbl->bits));
  
  /* Return a list of the symbols sorted by code length */
  /* It's not real clear to me why we don't need to consider the codelength
   * changes made above, but the JPEG spec seems to think this works.
   */
  p = 0;
  for (i = 1; i <= MAX_CLEN; i++) {
    for (j = 0; j <= 255; j++) {
      if (codesize[j] == i) {
	htbl->huffval[p] = (UINT8) j;
	p++;
      }
    }
  }
}


/* Process a single block's worth of coefficients */
/* Note that the DC coefficient has already been converted to a difference */

LOCAL void
htest_one_block (JBLOCK block, JCOEF block0,
		 long dc_counts[], long ac_counts[])
{
  register INT32 temp;
  register int nbits;
  register int k, r;
  
  /* Encode the DC coefficient difference per section F.1.2.1 */
  
  /* Find the number of bits needed for the magnitude of the coefficient */
  temp = block0;
  if (temp < 0) temp = -temp;
  
  for (nbits = 0; temp; nbits++)
    temp >>= 1;
  
  /* Count the Huffman symbol for the number of bits */
  dc_counts[nbits]++;
  
  /* Encode the AC coefficients per section F.1.2.2 */
  
  r = 0;			/* r = run length of zeros */
  
  for (k = 1; k < DCTSIZE2; k++) {
    if ((temp = block[k]) == 0) {
      r++;
    } else {
      /* if run length > 15, must emit special run-length-16 codes (0xF0) */
      while (r > 15) {
	ac_counts[0xF0]++;
	r -= 16;
      }
      
      /* Find the number of bits needed for the magnitude of the coefficient */
      if (temp < 0) temp = -temp;
      
      for (nbits = 0; temp; nbits++)
	temp >>= 1;
      
      /* Count Huffman symbol for run length / number of bits */
      ac_counts[(r << 4) + nbits]++;
      
      r = 0;
    }
  }

  /* If the last coef(s) were zero, emit an end-of-block code */
  if (r > 0)
    ac_counts[0]++;
}



/*
 * Trial-encode one MCU's worth of Huffman-compressed coefficients.
 */

LOCAL void
htest_encode (compress_info_ptr cinfo, JBLOCK *MCU_data)
{
  short blkn, ci;
  jpeg_component_info * compptr;

  /* Take care of restart intervals if needed */
  if (cinfo->restart_interval) {
    if (cinfo->restarts_to_go == 0) {
      /* Re-initialize DC predictions to 0 */
      for (ci = 0; ci < cinfo->comps_in_scan; ci++)
	cinfo->last_dc_val[ci] = 0;
      /* Update restart state */
      cinfo->restarts_to_go = cinfo->restart_interval;
    }
    cinfo->restarts_to_go--;
  }

  for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {
    ci = cinfo->MCU_membership[blkn];
    compptr = cinfo->cur_comp_info[ci];
    /* NB: unlike the real entropy encoder, we may not change the input data */
    htest_one_block(MCU_data[blkn],
		    (JCOEF) (MCU_data[blkn][0] - cinfo->last_dc_val[ci]),
		    dc_count_ptrs[compptr->dc_tbl_no],
		    ac_count_ptrs[compptr->ac_tbl_no]);
    cinfo->last_dc_val[ci] = MCU_data[blkn][0];
  }
}



/*
 * Find the best coding parameters for a Huffman-coded scan.
 * When called, the scan data has already been converted to a sequence of
 * MCU groups of quantized coefficients, which are stored in a "big" array.
 * The source_method knows how to iterate through that array.
 * On return, the MCU data is unmodified, but the Huffman tables referenced
 * by the scan components may have been altered.
 */

METHODDEF void
huff_optimize (compress_info_ptr cinfo, MCU_output_caller_ptr source_method)
/* Optimize Huffman-coding parameters (Huffman symbol table) */
{
  int i, tbl;
  HUFF_TBL **htblptr;

  /* Allocate and zero the count tables */
  /* Note that gen_huff_coding expects 257 entries in each table! */

  for (i = 0; i < NUM_HUFF_TBLS; i++) {
    dc_count_ptrs[i] = NULL;
    ac_count_ptrs[i] = NULL;
  }

  for (i = 0; i < cinfo->comps_in_scan; i++) {
    /* Create DC table */
    tbl = cinfo->cur_comp_info[i]->dc_tbl_no;
    if (dc_count_ptrs[tbl] == NULL) {
      dc_count_ptrs[tbl] = (long *) (*cinfo->emethods->alloc_small)
					(257 * SIZEOF(long));
      MEMZERO((void *) dc_count_ptrs[tbl], 257 * SIZEOF(long));
    }
    /* Create AC table */
    tbl = cinfo->cur_comp_info[i]->ac_tbl_no;
    if (ac_count_ptrs[tbl] == NULL) {
      ac_count_ptrs[tbl] = (long *) (*cinfo->emethods->alloc_small)
					(257 * SIZEOF(long));
      MEMZERO((void *) ac_count_ptrs[tbl], 257 * SIZEOF(long));
    }
  }

  /* Initialize DC predictions to 0 */
  for (i = 0; i < cinfo->comps_in_scan; i++) {
    cinfo->last_dc_val[i] = 0;
  }
  /* Initialize restart stuff */
  cinfo->restarts_to_go = cinfo->restart_interval;

  /* Scan the MCU data, count symbol uses */
  (*source_method) (cinfo, htest_encode);

  /* Now generate optimal Huffman tables */
  for (tbl = 0; tbl < NUM_HUFF_TBLS; tbl++) {
    if (dc_count_ptrs[tbl] != NULL) {
      htblptr = & cinfo->dc_huff_tbl_ptrs[tbl];
      if (*htblptr == NULL)
	*htblptr = (HUFF_TBL *) (*cinfo->emethods->alloc_small) (SIZEOF(HUFF_TBL));
      /* Set sent_table FALSE so updated table will be written to JPEG file. */
      (*htblptr)->sent_table = FALSE;
      /* Compute the optimal Huffman encoding */
      gen_huff_coding(cinfo, *htblptr, dc_count_ptrs[tbl]);
      /* Release the count table */
      (*cinfo->emethods->free_small) ((void *) dc_count_ptrs[tbl]);
    }
    if (ac_count_ptrs[tbl] != NULL) {
      htblptr = & cinfo->ac_huff_tbl_ptrs[tbl];
      if (*htblptr == NULL)
	*htblptr = (HUFF_TBL *) (*cinfo->emethods->alloc_small) (SIZEOF(HUFF_TBL));
      /* Set sent_table FALSE so updated table will be written to JPEG file. */
      (*htblptr)->sent_table = FALSE;
      /* Compute the optimal Huffman encoding */
      gen_huff_coding(cinfo, *htblptr, ac_count_ptrs[tbl]);
      /* Release the count table */
      (*cinfo->emethods->free_small) ((void *) ac_count_ptrs[tbl]);
    }
  }
}


#endif /* ENTROPY_OPT_SUPPORTED */


/*
 * The method selection routine for Huffman entropy encoding.
 */

GLOBAL void
jselchuffman (compress_info_ptr cinfo)
{
  if (! cinfo->arith_code) {
    cinfo->methods->entropy_encoder_init = huff_init;
    cinfo->methods->entropy_encode = huff_encode;
    cinfo->methods->entropy_encoder_term = huff_term;
#ifdef ENTROPY_OPT_SUPPORTED
    cinfo->methods->entropy_optimize = huff_optimize;
    /* The standard Huffman tables are only valid for 8-bit data precision.
     * If the precision is higher, force optimization on so that usable
     * tables will be computed.  This test can be removed if default tables
     * are supplied that are valid for the desired precision.
     */
    if (cinfo->data_precision > 8)
      cinfo->optimize_coding = TRUE;
    if (cinfo->optimize_coding)
      cinfo->total_passes++;	/* one pass needed for entropy optimization */
#endif
  }
}