The Tor Browser: media/libjpeg/jchuff.c@b8a032363ba2 (annotated)

media/libjpeg/jchuff.c@b8a032363ba2 (annotated)

media/libjpeg/jchuff.c

Thu, 22 Jan 2015 13:21:57 +0100

author: Michael Schloh von Bennewitz <michael@schloh.com>
date: Thu, 22 Jan 2015 13:21:57 +0100
branch: TOR_BUG_9701
changeset 15: b8a032363ba2
permissions: -rw-r--r--

Incorporate requested changes from Mozilla in review:
https://bugzilla.mozilla.org/show_bug.cgi?id=1123480#c6

 /*
  * jchuff.c
  *
  * This file was part of the Independent JPEG Group's software:
  * Copyright (C) 1991-1997, Thomas G. Lane.
  * libjpeg-turbo Modifications:
  * Copyright (C) 2009-2011, D. R. Commander.
  * 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 */
 #include <limits.h>
 static const unsigned char jpeg_nbits_table[65536] = {
 /* Number i needs jpeg_nbits_table[i] bits to be represented. */
 #include "jpeg_nbits_table.h"
 };
 #ifndef min
  #define min(a,b) ((a)<(b)?(a):(b))
 #endif
 /* 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 {
   size_t 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,
 * 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,
 * 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 */
 /* These macros perform the same task as the emit_bits() function in the
  * original libjpeg code.  In addition to reducing overhead by explicitly
  * inlining the code, additional performance is achieved by taking into
  * account the size of the bit buffer and waiting until it is almost full
  * before emptying it.  This mostly benefits 64-bit platforms, since 6
  * bytes can be stored in a 64-bit bit buffer before it has to be emptied.
  */
 #define EMIT_BYTE() { \
   JOCTET c; \
   put_bits -= 8; \
   c = (JOCTET)GETJOCTET(put_buffer >> put_bits); \
   *buffer++ = c; \
   if (c == 0xFF)  /* need to stuff a zero byte? */ \
     *buffer++ = 0; \
  }
 #define PUT_BITS(code, size) { \
   put_bits += size; \
   put_buffer = (put_buffer << size) | code; \
 }
 #define CHECKBUF15() { \
   if (put_bits > 15) { \
     EMIT_BYTE() \
     EMIT_BYTE() \
   } \
 }
 #define CHECKBUF31() { \
   if (put_bits > 31) { \
     EMIT_BYTE() \
     EMIT_BYTE() \
     EMIT_BYTE() \
     EMIT_BYTE() \
   } \
 }
 #define CHECKBUF47() { \
   if (put_bits > 47) { \
     EMIT_BYTE() \
     EMIT_BYTE() \
     EMIT_BYTE() \
     EMIT_BYTE() \
     EMIT_BYTE() \
     EMIT_BYTE() \
   } \
 }
 #if __WORDSIZE==64 || defined(_WIN64)
 #define EMIT_BITS(code, size) { \
   CHECKBUF47() \
   PUT_BITS(code, size) \
 }
 #define EMIT_CODE(code, size) { \
   temp2 &= (((INT32) 1)<<nbits) - 1; \
   CHECKBUF31() \
   PUT_BITS(code, size) \
   PUT_BITS(temp2, nbits) \
  }
 #else
 #define EMIT_BITS(code, size) { \
   PUT_BITS(code, size) \
   CHECKBUF15() \
 }
 #define EMIT_CODE(code, size) { \
   temp2 &= (((INT32) 1)<<nbits) - 1; \
   PUT_BITS(code, size) \
   CHECKBUF15() \
   PUT_BITS(temp2, nbits) \
   CHECKBUF15() \
  }
 #endif
 #define BUFSIZE (DCTSIZE2 * 2)
 #define LOAD_BUFFER() { \
   if (state->free_in_buffer < BUFSIZE) { \
     localbuf = 1; \
     buffer = _buffer; \
   } \
   else buffer = state->next_output_byte; \
  }
 #define STORE_BUFFER() { \
   if (localbuf) { \
     bytes = buffer - _buffer; \
     buffer = _buffer; \
     while (bytes > 0) { \
       bytestocopy = min(bytes, state->free_in_buffer); \
       MEMCOPY(state->next_output_byte, buffer, bytestocopy); \
       state->next_output_byte += bytestocopy; \
       buffer += bytestocopy; \
       state->free_in_buffer -= bytestocopy; \
       if (state->free_in_buffer == 0) \
         if (! dump_buffer(state)) return FALSE; \
       bytes -= bytestocopy; \
     } \
   } \
   else { \
     state->free_in_buffer -= (buffer - state->next_output_byte); \
     state->next_output_byte = buffer; \
   } \
  }
 LOCAL(boolean)
 flush_bits (working_state * state)
 {
   JOCTET _buffer[BUFSIZE], *buffer;
   size_t put_buffer;  int put_bits;
   size_t bytes, bytestocopy;  int localbuf = 0;
   put_buffer = state->cur.put_buffer;
   put_bits = state->cur.put_bits;
   LOAD_BUFFER()
   /* fill any partial byte with ones */
   PUT_BITS(0x7F, 7)
   while (put_bits >= 8) EMIT_BYTE()
   state->cur.put_buffer = 0;	/* and reset bit-buffer to empty */
   state->cur.put_bits = 0;
   STORE_BUFFER()
   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, temp3;
   int nbits;
   int r, code, size;
   JOCTET _buffer[BUFSIZE], *buffer;
   size_t put_buffer;  int put_bits;
   int code_0xf0 = actbl->ehufco[0xf0], size_0xf0 = actbl->ehufsi[0xf0];
   size_t bytes, bytestocopy;  int localbuf = 0;
   put_buffer = state->cur.put_buffer;
   put_bits = state->cur.put_bits;
   LOAD_BUFFER()
   /* Encode the DC coefficient difference per section F.1.2.1 */
   temp = temp2 = block[0] - last_dc_val;
  /* This is a well-known technique for obtaining the absolute value without a
   * branch.  It is derived from an assembly language technique presented in
   * "How to Optimize for the Pentium Processors", Copyright (c) 1996, 1997 by
   * Agner Fog.
   */
   temp3 = temp >> (CHAR_BIT * sizeof(int) - 1);
   temp ^= temp3;
   temp -= temp3;
   /* For a negative input, want temp2 = bitwise complement of abs(input) */
   /* This code assumes we are on a two's complement machine */
   temp2 += temp3;
   /* Find the number of bits needed for the magnitude of the coefficient */
   nbits = jpeg_nbits_table[temp];
   /* Emit the Huffman-coded symbol for the number of bits */
   code = dctbl->ehufco[nbits];
   size = dctbl->ehufsi[nbits];
   PUT_BITS(code, size)
   CHECKBUF15()
   /* Mask off any extra bits in code */
   temp2 &= (((INT32) 1)<<nbits) - 1;
   /* Emit that number of bits of the value, if positive, */
   /* or the complement of its magnitude, if negative. */
   PUT_BITS(temp2, nbits)
   CHECKBUF15()
   /* Encode the AC coefficients per section F.1.2.2 */
   r = 0;			/* r = run length of zeros */
 /* Manually unroll the k loop to eliminate the counter variable.  This
  * improves performance greatly on systems with a limited number of
  * registers (such as x86.)
  */
 #define kloop(jpeg_natural_order_of_k) {  \
   if ((temp = block[jpeg_natural_order_of_k]) == 0) { \
     r++; \
   } else { \
     temp2 = temp; \
     /* Branch-less absolute value, bitwise complement, etc., same as above */ \
     temp3 = temp >> (CHAR_BIT * sizeof(int) - 1); \
     temp ^= temp3; \
     temp -= temp3; \
     temp2 += temp3; \
     nbits = jpeg_nbits_table[temp]; \
     /* if run length > 15, must emit special run-length-16 codes (0xF0) */ \
     while (r > 15) { \
       EMIT_BITS(code_0xf0, size_0xf0) \
       r -= 16; \
     } \
     /* Emit Huffman symbol for run length / number of bits */ \
     temp3 = (r << 4) + nbits;  \
     code = actbl->ehufco[temp3]; \
     size = actbl->ehufsi[temp3]; \
     EMIT_CODE(code, size) \
     r = 0;  \
   } \
 }
   /* One iteration for each value in jpeg_natural_order[] */
   kloop(1);   kloop(8);   kloop(16);  kloop(9);   kloop(2);   kloop(3);
   kloop(10);  kloop(17);  kloop(24);  kloop(32);  kloop(25);  kloop(18);
   kloop(11);  kloop(4);   kloop(5);   kloop(12);  kloop(19);  kloop(26);
   kloop(33);  kloop(40);  kloop(48);  kloop(41);  kloop(34);  kloop(27);
   kloop(20);  kloop(13);  kloop(6);   kloop(7);   kloop(14);  kloop(21);
   kloop(28);  kloop(35);  kloop(42);  kloop(49);  kloop(56);  kloop(57);
   kloop(50);  kloop(43);  kloop(36);  kloop(29);  kloop(22);  kloop(15);
   kloop(23);  kloop(30);  kloop(37);  kloop(44);  kloop(51);  kloop(58);
   kloop(59);  kloop(52);  kloop(45);  kloop(38);  kloop(31);  kloop(39);
   kloop(46);  kloop(53);  kloop(60);  kloop(61);  kloop(54);  kloop(47);
   kloop(55);  kloop(62);  kloop(63);
   /* If the last coef(s) were zero, emit an end-of-block code */
   if (r > 0) {
     code = actbl->ehufco[0];
     size = actbl->ehufsi[0];
     EMIT_BITS(code, size)
   }
   state->cur.put_buffer = put_buffer;
   state->cur.put_bits = put_bits;
   STORE_BUFFER()
   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
   }
 }

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media/libjpeg/jchuff.c