MCL/sf/mw/qt: comparison src/3rdparty/libjpeg/jchuff.c

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+:1918ee327afb
+/*
+* 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)<<size) - 1; /* mask off any extra bits in code */
+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;
+}
+}
+}
+#endif /* ENTROPY_OPT_SUPPORTED */
+/*
+* Module initialization routine for Huffman entropy encoding.
+*/
+GLOBAL(void)
+jinit_huff_encoder (j_compress_ptr cinfo)
+{
+huff_entropy_ptr entropy;
+int i;
+entropy = (huff_entropy_ptr)
+(*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
+				SIZEOF(huff_entropy_encoder));
+cinfo->entropy = (struct jpeg_entropy_encoder *) entropy;
+entropy->pub.start_pass = start_pass_huff;
+/* Mark tables unallocated */
+for (i = 0; i < NUM_HUFF_TBLS; i++) {
+entropy->dc_derived_tbls[i] = entropy->ac_derived_tbls[i] = NULL;
+#ifdef ENTROPY_OPT_SUPPORTED
+entropy->dc_count_ptrs[i] = entropy->ac_count_ptrs[i] = NULL;
+#endif
+}
+}