/*
* 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
	}
}
