Add jpeg_skip_scanlines() API to libjpeg-turbo

jdhuff.c

 /*
  * jdhuff.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.
+ * Copyright (C) 2009-2011, 2015, D. R. Commander.
  * For conditions of distribution and use, see the accompanying README file.
  *
  * This file contains Huffman entropy decoding routines.
  *
  * Much of the complexity here has to do with supporting input suspension.
  * If the data source 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
  * storage only upon successful completion of an MCU.
  */
@@ skipping 536 matching lines @@
   BITREAD_STATE_VARS;
   int blkn;
   savable_state state;
   /* Outer loop handles each block in the MCU */
 
   /* Load up working state */
   BITREAD_LOAD_STATE(cinfo,entropy->bitstate);
   ASSIGN_STATE(state, entropy->saved);
 
   for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {
-    JBLOCKROW block = MCU_data[blkn];
+    JBLOCKROW block = MCU_data ? MCU_data[blkn] : NULL;
     d_derived_tbl * dctbl = entropy->dc_cur_tbls[blkn];
     d_derived_tbl * actbl = entropy->ac_cur_tbls[blkn];
     register int s, k, r;
 
     /* Decode a single block's worth of coefficients */
 
     /* Section F.2.2.1: decode the DC coefficient difference */
     HUFF_DECODE(s, br_state, dctbl, return FALSE, label1);
     if (s) {
       CHECK_BIT_BUFFER(br_state, s, return FALSE);
       r = GET_BITS(s);
       s = HUFF_EXTEND(r, s);
     }
 
     if (entropy->dc_needed[blkn]) {
       /* Convert DC difference to actual value, update last_dc_val */
       int ci = cinfo->MCU_membership[blkn];
       s += state.last_dc_val[ci];
       state.last_dc_val[ci] = s;
-      /* Output the DC coefficient (assumes jpeg_natural_order[0] = 0) */
-      (*block)[0] = (JCOEF) s;
+      if (block) {
+        /* Output the DC coefficient (assumes jpeg_natural_order[0] = 0) */
+        (*block)[0] = (JCOEF) s;
+      }
     }
 
-    if (entropy->ac_needed[blkn]) {
+    if (entropy->ac_needed[blkn] && block) {
 
       /* Section F.2.2.2: decode the AC coefficients */
       /* Since zeroes are skipped, output area must be cleared beforehand */
       for (k = 1; k < DCTSIZE2; k++) {
         HUFF_DECODE(s, br_state, actbl, return FALSE, label2);
 
         r = s >> 4;
         s &= 15;
       
         if (s) {
@@ skipping 68 matching lines @@
     if (s) {
       FILL_BIT_BUFFER_FAST
       r = GET_BITS(s);
       s = HUFF_EXTEND(r, s);
     }
 
     if (entropy->dc_needed[blkn]) {
       int ci = cinfo->MCU_membership[blkn];
       s += state.last_dc_val[ci];
       state.last_dc_val[ci] = s;
-      (*block)[0] = (JCOEF) s;
+      if (block)
+        (*block)[0] = (JCOEF) s;
     }
 
-    if (entropy->ac_needed[blkn]) {
+    if (entropy->ac_needed[blkn] && block) {
 
       for (k = 1; k < DCTSIZE2; k++) {
         HUFF_DECODE_FAST(s, l, actbl, slow_decode_mcu);
         r = s >> 4;
         s &= 15;
       
         if (s) {
           k += r;
           FILL_BIT_BUFFER_FAST
           r = GET_BITS(s);
@@ skipping 110 matching lines @@
                                 SIZEOF(huff_entropy_decoder));
   cinfo->entropy = (struct jpeg_entropy_decoder *) entropy;
   entropy->pub.start_pass = start_pass_huff_decoder;
   entropy->pub.decode_mcu = decode_mcu;
 
   /* Mark tables unallocated */
   for (i = 0; i < NUM_HUFF_TBLS; i++) {
     entropy->dc_derived_tbls[i] = entropy->ac_derived_tbls[i] = NULL;
   }
 }