jchuff.cpp

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/* +------------------------------------------------------------------------+
   |                     Mobile Robot Programming Toolkit (MRPT)            |
   |                          http://www.mrpt.org/                          |
   |                                                                        |
   | Copyright (c) 2005-2017, Individual contributors, see AUTHORS file     |
   | See: http://www.mrpt.org/Authors - All rights reserved.                |
   | Released under BSD License. See details in http://www.mrpt.org/License |
   +------------------------------------------------------------------------+ */

#define JPEG_INTERNALS
#include "jinclude.h"
#include "mrpt_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] == nullptr)
                                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] == nullptr)
                                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 == nullptr) ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, tblno);

        /* Allocate a workspace if we haven't already done so. */
        if (*pdtbl == nullptr)
                *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. */
        INT32 put_buffer = (INT32)code;
        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)
{
        int temp, temp2;
        int nbits;
        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[])
{
        int temp;
        int nbits;
        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 == nullptr)
                                *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 == nullptr)
                                *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] = nullptr;
#ifdef ENTROPY_OPT_SUPPORTED
                entropy->dc_count_ptrs[i] = entropy->ac_count_ptrs[i] = nullptr;
#endif
        }
}