/*

 * jchuff.c

 *

 * 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 Huffman entropy encoding routines.

 *

 * Much of the complexity here has to do with supporting output suspension.

 * If the data destination module demands suspension, we want to be able to

 * back up to the start of the current MCU.  To do this, we copy state

 * variables into local working storage, and update them back to the

 * permanent JPEG objects only upon successful completion of an MCU.

 */

#define JPEG_INTERNALS

#include "jinclude.h"

#include "jpeglib.h"

#include "jchuff.h"		/* Declarations shared with jcphuff.c */

/* Expanded entropy encoder object for Huffman encoding.

 *

 * The savable_state subrecord contains fields that change within an MCU,

 * but must not be updated permanently until we complete the MCU.

 */

typedef struct {

  INT32 put_buffer;		/* current bit-accumulation buffer */

  int put_bits;			/* # of bits now in it */

  int last_dc_val[MAX_COMPS_IN_SCAN]; /* last DC coef for each component */

} savable_state;

/* This macro is to work around compilers with missing or broken

 * structure assignment.  You'll need to fix this code if you have

 * such a compiler and you change MAX_COMPS_IN_SCAN.

 */

#ifndef NO_STRUCT_ASSIGN

#define ASSIGN_STATE(dest,src)  ((dest) = (src))

#else

#if MAX_COMPS_IN_SCAN == 4

#define ASSIGN_STATE(dest,src)  \

	((dest).put_buffer = (src).put_buffer, \

	 (dest).put_bits = (src).put_bits, \

	 (dest).last_dc_val[0] = (src).last_dc_val[0], \

	 (dest).last_dc_val[1] = (src).last_dc_val[1], \

	 (dest).last_dc_val[2] = (src).last_dc_val[2], \

	 (dest).last_dc_val[3] = (src).last_dc_val[3])

#endif

#endif

typedef struct {

  struct jpeg_entropy_encoder pub; /* public fields */

  savable_state saved;		/* Bit buffer & DC state at start of MCU */

  /* These fields are NOT loaded into local working state. */

  unsigned int restarts_to_go;	/* MCUs left in this restart interval */

  int next_restart_num;		/* next restart number to write (0-7) */

  /* Pointers to derived tables (these workspaces have image lifespan) */

  c_derived_tbl * dc_derived_tbls[NUM_HUFF_TBLS];

  c_derived_tbl * ac_derived_tbls[NUM_HUFF_TBLS];

#ifdef ENTROPY_OPT_SUPPORTED	/* Statistics tables for optimization */

  long * dc_count_ptrs[NUM_HUFF_TBLS];

  long * ac_count_ptrs[NUM_HUFF_TBLS];

#endif

} huff_entropy_encoder;

typedef huff_entropy_encoder * huff_entropy_ptr;

/* Working state while writing an MCU.

 * This struct contains all the fields that are needed by subroutines.

 */

typedef struct {

  JOCTET * next_output_byte;	/* => next byte to write in buffer */

  size_t free_in_buffer;	/* # of byte spaces remaining in buffer */

  savable_state cur;		/* Current bit buffer & DC state */

  j_compress_ptr cinfo;		/* dump_buffer needs access to this */

} working_state;

/* Forward declarations */

METHODDEF(boolean) encode_mcu_huff JPP((j_compress_ptr cinfo,

					JBLOCKROW *MCU_data));

METHODDEF(void) finish_pass_huff JPP((j_compress_ptr cinfo));

#ifdef ENTROPY_OPT_SUPPORTED

METHODDEF(boolean) encode_mcu_gather JPP((j_compress_ptr cinfo,

					  JBLOCKROW *MCU_data));

METHODDEF(void) finish_pass_gather JPP((j_compress_ptr cinfo));

#endif

/*

 * Initialize for a Huffman-compressed scan.

 * If gather_statistics is TRUE, we do not output anything during the scan,

 * just count the Huffman symbols used and generate Huffman code tables.

 */

METHODDEF(void)

start_pass_huff (j_compress_ptr cinfo, boolean gather_statistics)

{

  huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;

  int ci, dctbl, actbl;

  jpeg_component_info * compptr;

  if (gather_statistics) {

#ifdef ENTROPY_OPT_SUPPORTED

    entropy->pub.encode_mcu = encode_mcu_gather;

    entropy->pub.finish_pass = finish_pass_gather;

#else

    ERREXIT(cinfo, JERR_NOT_COMPILED);

#endif

  } else {

    entropy->pub.encode_mcu = encode_mcu_huff;

    entropy->pub.finish_pass = finish_pass_huff;

  }

  for (ci = 0; ci < cinfo->comps_in_scan; ci++) {

    compptr = cinfo->cur_comp_info[ci];

    dctbl = compptr->dc_tbl_no;

    actbl = compptr->ac_tbl_no;

    if (gather_statistics) {

#ifdef ENTROPY_OPT_SUPPORTED

      /* Check for invalid table indexes */

      /* (make_c_derived_tbl does this in the other path) */

      if (dctbl < 0 || dctbl >= NUM_HUFF_TBLS)

	ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, dctbl);

      if (actbl < 0 || actbl >= NUM_HUFF_TBLS)

	ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, actbl);

      /* Allocate and zero the statistics tables */

      /* Note that jpeg_gen_optimal_table expects 257 entries in each table! */

      if (entropy->dc_count_ptrs[dctbl] == NULL)

	entropy->dc_count_ptrs[dctbl] = (long *)

	  (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,

				      257 * SIZEOF(long));

      MEMZERO(entropy->dc_count_ptrs[dctbl], 257 * SIZEOF(long));

      if (entropy->ac_count_ptrs[actbl] == NULL)

	entropy->ac_count_ptrs[actbl] = (long *)

	  (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,

				      257 * SIZEOF(long));

      MEMZERO(entropy->ac_count_ptrs[actbl], 257 * SIZEOF(long));

#endif

    } else {

      /* Compute derived values for Huffman tables */

      /* We may do this more than once for a table, but it's not expensive */

      jpeg_make_c_derived_tbl(cinfo, TRUE, dctbl,

			      & entropy->dc_derived_tbls[dctbl]);

      jpeg_make_c_derived_tbl(cinfo, FALSE, actbl,

			      & entropy->ac_derived_tbls[actbl]);

    }

    /* Initialize DC predictions to 0 */

    entropy->saved.last_dc_val[ci] = 0;

  }

  /* Initialize bit buffer to empty */

  entropy->saved.put_buffer = 0;

  entropy->saved.put_bits = 0;

  /* Initialize restart stuff */

  entropy->restarts_to_go = cinfo->restart_interval;

  entropy->next_restart_num = 0;

}

/*

 * Compute the derived values for a Huffman table.

 * This routine also performs some validation checks on the table.

 *

 * Note this is also used by jcphuff.c.

 */

GLOBAL(void)

jpeg_make_c_derived_tbl (j_compress_ptr cinfo, boolean isDC, int tblno,

			 c_derived_tbl ** pdtbl)

{

  JHUFF_TBL *htbl;

  c_derived_tbl *dtbl;

  int p, i, l, lastp, si, maxsymbol;

  char huffsize[257];

  unsigned int huffcode[257];

  unsigned int code;

  /* Note that huffsize[] and huffcode[] are filled in code-length order,

   * paralleling the order of the symbols themselves in htbl->huffval[].

   */

  /* Find the input Huffman table */

  if (tblno < 0 || tblno >= NUM_HUFF_TBLS)

    ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, tblno);

  htbl =

    isDC ? cinfo->dc_huff_tbl_ptrs[tblno] : cinfo->ac_huff_tbl_ptrs[tblno];

  if (htbl == NULL)

    ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, tblno);

  /* Allocate a workspace if we haven't already done so. */

  if (*pdtbl == NULL)

    *pdtbl = (c_derived_tbl *)

      (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,

				  SIZEOF(c_derived_tbl));

  dtbl = *pdtbl;

  /* Figure C.1: make table of Huffman code length for each symbol */

  p = 0;

  for (l = 1; l <= 16; l++) {

    i = (int) htbl->bits[l];

    if (i < 0 || p + i > 256)	/* protect against table overrun */

      ERREXIT(cinfo, JERR_BAD_HUFF_TABLE);

    while (i--)

      huffsize[p++] = (char) l;

  }

  huffsize[p] = 0;

  lastp = p;

  /* Figure C.2: generate the codes themselves */

  /* We also validate that the counts represent a legal Huffman code tree. */

  code = 0;

  si = huffsize[0];

  p = 0;

  while (huffsize[p]) {

    while (((int) huffsize[p]) == si) {

      huffcode[p++] = code;

      code++;

    }

    /* code is now 1 more than the last code used for codelength si; but

     * it must still fit in si bits, since no code is allowed to be all ones.

     */

    if (((INT32) code) >= (((INT32) 1) << si))

      ERREXIT(cinfo, JERR_BAD_HUFF_TABLE);

    code <<= 1;

    si++;

  }

  /* Figure C.3: generate encoding tables */

  /* These are code and size indexed by symbol value */

  /* Set all codeless symbols to have code length 0;

   * this lets us detect duplicate VAL entries here, and later

   * allows emit_bits to detect any attempt to emit such symbols.

   */

  MEMZERO(dtbl->ehufsi, SIZEOF(dtbl->ehufsi));

  /* This is also a convenient place to check for out-of-range

   * and duplicated VAL entries.  We allow 0..255 for AC symbols

   * but only 0..15 for DC.  (We could constrain them further

   * based on data depth and mode, but this seems enough.)

   */

  maxsymbol = isDC ? 15 : 255;

  for (p = 0; p < lastp; p++) {

    i = htbl->huffval[p];

    if (i < 0 || i > maxsymbol || dtbl->ehufsi[i])

      ERREXIT(cinfo, JERR_BAD_HUFF_TABLE);

    dtbl->ehufco[i] = huffcode[p];

    dtbl->ehufsi[i] = huffsize[p];

  }

}

/* Outputting bytes to the file */

/* Emit a byte, taking 'action' if must suspend. */

#define emit_byte(state,val,action)  \

	{ *(state)->next_output_byte++ = (JOCTET) (val);  \

	  if (--(state)->free_in_buffer == 0)  \

	    if (! dump_buffer(state))  \

	      { action; } }

LOCAL(boolean)

dump_buffer (working_state * state)

/* Empty the output buffer; return TRUE if successful, FALSE if must suspend */

{

  struct jpeg_destination_mgr * dest = state->cinfo->dest;

  if (! (*dest->empty_output_buffer) (state->cinfo))

    return FALSE;

  /* After a successful buffer dump, must reset buffer pointers */

  state->next_output_byte = dest->next_output_byte;

  state->free_in_buffer = dest->free_in_buffer;

  return TRUE;

}

/* Outputting bits to the file */

/* Only the right 24 bits of 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 put_buffer

 * between calls, so 24 bits are sufficient.

 */

INLINE

LOCAL(boolean)

emit_bits (working_state * state, unsigned int code, int size)

/* Emit some bits; return TRUE if successful, FALSE if must suspend */

{

  /* This routine is heavily used, so it's worth coding tightly. */

  register INT32 put_buffer = (INT32) code;

  register int put_bits = state->cur.put_bits;

  /* if size is 0, caller used an invalid Huffman table entry */

  if (size == 0)

    ERREXIT(state->cinfo, JERR_HUFF_MISSING_CODE);

  put_buffer &= (((INT32) 1)<

  put_bits += size;		/* new number of bits in buffer */

  put_buffer <<= 24 - put_bits; /* align incoming bits */

  put_buffer |= state->cur.put_buffer; /* and merge with old buffer contents */

  while (put_bits >= 8) {

    int c = (int) ((put_buffer >> 16) & 0xFF);

    emit_byte(state, c, return FALSE);

    if (c == 0xFF) {		/* need to stuff a zero byte? */

      emit_byte(state, 0, return FALSE);

    }

    put_buffer <<= 8;

    put_bits -= 8;

  }

  state->cur.put_buffer = put_buffer; /* update state variables */

  state->cur.put_bits = put_bits;

  return TRUE;

}

LOCAL(boolean)

flush_bits (working_state * state)

{

  if (! emit_bits(state, 0x7F, 7)) /* fill any partial byte with ones */

    return FALSE;

  state->cur.put_buffer = 0;	/* and reset bit-buffer to empty */

  state->cur.put_bits = 0;

  return TRUE;

}

/* Encode a single block's worth of coefficients */

LOCAL(boolean)

encode_one_block (working_state * state, JCOEFPTR block, int last_dc_val,

		  c_derived_tbl *dctbl, c_derived_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] - last_dc_val;

  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;

  }

  /* Check for out-of-range coefficient values.

   * Since we're encoding a difference, the range limit is twice as much.

   */

  if (nbits > MAX_COEF_BITS+1)

    ERREXIT(state->cinfo, JERR_BAD_DCT_COEF);

  /* Emit the Huffman-coded symbol for the number of bits */

  if (! emit_bits(state, dctbl->ehufco[nbits], dctbl->ehufsi[nbits]))

    return FALSE;

  /* 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 */

    if (! emit_bits(state, (unsigned int) temp2, nbits))

      return FALSE;

  /* 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[jpeg_natural_order[k]]) == 0) {

      r++;

    } else {

      /* if run length > 15, must emit special run-length-16 codes (0xF0) */

      while (r > 15) {

	if (! emit_bits(state, actbl->ehufco[0xF0], actbl->ehufsi[0xF0]))

	  return FALSE;

	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++;

      /* Check for out-of-range coefficient values */

      if (nbits > MAX_COEF_BITS)

	ERREXIT(state->cinfo, JERR_BAD_DCT_COEF);

      /* Emit Huffman symbol for run length / number of bits */

      i = (r << 4) + nbits;

      if (! emit_bits(state, actbl->ehufco[i], actbl->ehufsi[i]))

	return FALSE;

      /* Emit that number of bits of the value, if positive, */

      /* or the complement of its magnitude, if negative. */

      if (! emit_bits(state, (unsigned int) temp2, nbits))

	return FALSE;

      r = 0;

    }

  }

  /* If the last coef(s) were zero, emit an end-of-block code */

  if (r > 0)

    if (! emit_bits(state, actbl->ehufco[0], actbl->ehufsi[0]))

      return FALSE;

  return TRUE;

}

/*

 * Emit a restart marker & resynchronize predictions.

 */

LOCAL(boolean)

emit_restart (working_state * state, int restart_num)

{

  int ci;

  if (! flush_bits(state))

    return FALSE;

  emit_byte(state, 0xFF, return FALSE);

  emit_byte(state, JPEG_RST0 + restart_num, return FALSE);

  /* Re-initialize DC predictions to 0 */

  for (ci = 0; ci < state->cinfo->comps_in_scan; ci++)

    state->cur.last_dc_val[ci] = 0;

  /* The restart counter is not updated until we successfully write the MCU. */

  return TRUE;

}

/*

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

 */

METHODDEF(boolean)

encode_mcu_huff (j_compress_ptr cinfo, JBLOCKROW *MCU_data)

{

  huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;

  working_state state;

  int blkn, ci;

  jpeg_component_info * compptr;

  /* Load up working state */

  state.next_output_byte = cinfo->dest->next_output_byte;

  state.free_in_buffer = cinfo->dest->free_in_buffer;

  ASSIGN_STATE(state.cur, entropy->saved);

  state.cinfo = cinfo;

  /* Emit restart marker if needed */

  if (cinfo->restart_interval) {

    if (entropy->restarts_to_go == 0)

      if (! emit_restart(&state, entropy->next_restart_num))

	return FALSE;

  }

  /* Encode the MCU data blocks */

  for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {

    ci = cinfo->MCU_membership[blkn];

    compptr = cinfo->cur_comp_info[ci];

    if (! encode_one_block(&state,

			   MCU_data[blkn][0], state.cur.last_dc_val[ci],

			   entropy->dc_derived_tbls[compptr->dc_tbl_no],

			   entropy->ac_derived_tbls[compptr->ac_tbl_no]))

      return FALSE;

    /* Update last_dc_val */

    state.cur.last_dc_val[ci] = MCU_data[blkn][0][0];

  }

  /* Completed MCU, so update state */

  cinfo->dest->next_output_byte = state.next_output_byte;

  cinfo->dest->free_in_buffer = state.free_in_buffer;

  ASSIGN_STATE(entropy->saved, state.cur);

  /* Update restart-interval state too */

  if (cinfo->restart_interval) {

    if (entropy->restarts_to_go == 0) {

      entropy->restarts_to_go = cinfo->restart_interval;

      entropy->next_restart_num++;

      entropy->next_restart_num &= 7;

    }

    entropy->restarts_to_go--;

  }

  return TRUE;

}

/*

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

 */

METHODDEF(void)

finish_pass_huff (j_compress_ptr cinfo)

{

  huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;

  working_state state;

  /* Load up working state ... flush_bits needs it */

  state.next_output_byte = cinfo->dest->next_output_byte;

  state.free_in_buffer = cinfo->dest->free_in_buffer;

  ASSIGN_STATE(state.cur, entropy->saved);

  state.cinfo = cinfo;

  /* Flush out the last data */

  if (! flush_bits(&state))

    ERREXIT(cinfo, JERR_CANT_SUSPEND);

  /* Update state */

  cinfo->dest->next_output_byte = state.next_output_byte;

  cinfo->dest->free_in_buffer = state.free_in_buffer;

  ASSIGN_STATE(entropy->saved, state.cur);

}

/*

 * Huffman coding optimization.

 *

 * 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 a Huffman coding tree for the observed counts.

 * Symbols which are not needed at all for the particular image are not

 * assigned any code, which saves space in the DHT marker as well as in

 * the compressed data.

 */

#ifdef ENTROPY_OPT_SUPPORTED

/* Process a single block's worth of coefficients */

LOCAL(void)

htest_one_block (j_compress_ptr cinfo, JCOEFPTR block, int last_dc_val,

		 long dc_counts[], long ac_counts[])

{

  register int temp;

  register int nbits;

  register int k, r;

  /* Encode the DC coefficient difference per section F.1.2.1 */

  temp = block[0] - last_dc_val;

  if (temp < 0)

    temp = -temp;

  /* Find the number of bits needed for the magnitude of the coefficient */

  nbits = 0;

  while (temp) {

    nbits++;

    temp >>= 1;

  }

  /* Check for out-of-range coefficient values.

   * Since we're encoding a difference, the range limit is twice as much.

   */

  if (nbits > MAX_COEF_BITS+1)

    ERREXIT(cinfo, JERR_BAD_DCT_COEF);

  /* 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[jpeg_natural_order[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;

      /* 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++;

      /* Check for out-of-range coefficient values */

      if (nbits > MAX_COEF_BITS)

	ERREXIT(cinfo, JERR_BAD_DCT_COEF);

      /* 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.

 * No data is actually output, so no suspension return is possible.

 */

METHODDEF(boolean)

encode_mcu_gather (j_compress_ptr cinfo, JBLOCKROW *MCU_data)

{

  huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;

  int blkn, ci;

  jpeg_component_info * compptr;

  /* Take care of restart intervals if needed */

  if (cinfo->restart_interval) {

    if (entropy->restarts_to_go == 0) {

      /* Re-initialize DC predictions to 0 */

      for (ci = 0; ci < cinfo->comps_in_scan; ci++)

	entropy->saved.last_dc_val[ci] = 0;

      /* Update restart state */

      entropy->restarts_to_go = cinfo->restart_interval;

    }

    entropy->restarts_to_go--;

  }

  for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {

    ci = cinfo->MCU_membership[blkn];

    compptr = cinfo->cur_comp_info[ci];

    htest_one_block(cinfo, MCU_data[blkn][0], entropy->saved.last_dc_val[ci],

		    entropy->dc_count_ptrs[compptr->dc_tbl_no],

		    entropy->ac_count_ptrs[compptr->ac_tbl_no]);

    entropy->saved.last_dc_val[ci] = MCU_data[blkn][0][0];

  }

  return TRUE;

}

/*

 * Generate the best Huffman code table for the given counts, fill htbl.

 * Note this is also used by jcphuff.c.

 *

 * The JPEG standard requires that no symbol be assigned a codeword of all

 * one bits (so that padding bits added at the end of a compressed segment

 * can't look like a valid code).  Because of the canonical ordering of

 * codewords, this just means that there must be an unused slot in the

 * longest codeword length category.  Section K.2 of the JPEG spec suggests

 * reserving such a slot by pretending that symbol 256 is a valid symbol

 * with count 1.  In theory that's not optimal; giving it count zero but

 * including it in the symbol set anyway should give a better Huffman code.

 * But the theoretically better code actually seems to come out worse in

 * practice, because it produces more all-ones bytes (which incur stuffed

 * zero bytes in the final file).  In any case the difference is tiny.

 *

 * The JPEG standard requires Huffman codes to be no more than 16 bits long.

 * If some symbols have a very small but nonzero probability, the Huffman tree

 * must be adjusted to meet the code length restriction.  We currently use

 * the adjustment method suggested in JPEG section K.2.  This method is *not*

 * optimal; it may not choose the best possible limited-length code.  But

 * typically only very-low-frequency symbols will be given less-than-optimal

 * lengths, so the code is almost optimal.  Experimental comparisons against

 * an optimal limited-length-code algorithm indicate that the difference is

 * microscopic --- usually less than a hundredth of a percent of total size.

 * So the extra complexity of an optimal algorithm doesn't seem worthwhile.

 */

GLOBAL(void)

jpeg_gen_optimal_table (j_compress_ptr cinfo, JHUFF_TBL * htbl, long freq[])

{

#define MAX_CLEN 32		/* assumed maximum initial code length */

  UINT8 bits[MAX_CLEN+1];	/* bits[k] = # of symbols with code length k */

  int codesize[257];		/* codesize[k] = code length of symbol k */

  int 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(bits, SIZEOF(bits));

  MEMZERO(codesize, SIZEOF(codesize));

  for (i = 0; i < 257; i++)

    others[i] = -1;		/* init links to empty */

  freq[256] = 1;		/* make sure 256 has 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 last 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, JERR_HUFF_CLEN_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) */

  MEMCOPY(htbl->bits, 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++;

      }

    }

  }

  /* Set sent_table FALSE so updated table will be written to JPEG file. */

  htbl->sent_table = FALSE;

}

/*

 * Finish up a statistics-gathering pass and create the new Huffman tables.

 */

METHODDEF(void)

finish_pass_gather (j_compress_ptr cinfo)

{

  huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;

  int ci, dctbl, actbl;

  jpeg_component_info * compptr;

  JHUFF_TBL **htblptr;

  boolean did_dc[NUM_HUFF_TBLS];

  boolean did_ac[NUM_HUFF_TBLS];

  /* It's important not to apply jpeg_gen_optimal_table more than once

   * per table, because it clobbers the input frequency counts!

   */

  MEMZERO(did_dc, SIZEOF(did_dc));

  MEMZERO(did_ac, SIZEOF(did_ac));

  for (ci = 0; ci < cinfo->comps_in_scan; ci++) {

    compptr = cinfo->cur_comp_info[ci];

    dctbl = compptr->dc_tbl_no;

    actbl = compptr->ac_tbl_no;

    if (! did_dc[dctbl]) {

      htblptr = & cinfo->dc_huff_tbl_ptrs[dctbl];

      if (*htblptr == NULL)

	*htblptr = jpeg_alloc_huff_table((j_common_ptr) cinfo);

      jpeg_gen_optimal_table(cinfo, *htblptr, entropy->dc_count_ptrs[dctbl]);

      did_dc[dctbl] = TRUE;

    }

    if (! did_ac[actbl]) {

      htblptr = & cinfo->ac_huff_tbl_ptrs[actbl];

      if (*htblptr == NULL)

	*htblptr = jpeg_alloc_huff_table((j_common_ptr) cinfo);

      jpeg_gen_optimal_table(cinfo, *htblptr, entropy->ac_count_ptrs[actbl]);

      did_ac[actbl] = TRUE;

    }

  }