Add jpeg_skip_scanlines() API to libjpeg-turbo

jdarith.c

 /*
  * jdarith.c
  *
+ * This file is part of the Independent JPEG Group's software:
  * Developed 1997-2009 by Guido Vollbeding.
- * This file is part of the Independent JPEG Group's software.
+ * libjpeg-turbo Modifications:
+ * Copyright (C) 2015, D. R. Commander.
  * For conditions of distribution and use, see the accompanying README file.
  *
  * This file contains portable arithmetic entropy decoding routines for JPEG
  * (implementing the ISO/IEC IS 10918-1 and CCITT Recommendation ITU-T T.81).
  *
  * Both sequential and progressive modes are supported in this single module.
  *
  * Suspension is not currently supported in this module.
  */
 
@@ skipping 491 matching lines @@
     if (entropy->restarts_to_go == 0)
       process_restart(cinfo);
     entropy->restarts_to_go--;
   }
 
   if (entropy->ct == -1) return TRUE;   /* if error do nothing */
 
   /* Outer loop handles each block in the MCU */
 
   for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {
-    block = MCU_data[blkn];
+    block = MCU_data ? MCU_data[blkn] : NULL;
     ci = cinfo->MCU_membership[blkn];
     compptr = cinfo->cur_comp_info[ci];
 
     /* Sections F.2.4.1 & F.1.4.4.1: Decoding of DC coefficients */
 
     tbl = compptr->dc_tbl_no;
 
     /* Table F.4: Point to statistics bin S0 for DC coefficient coding */
     st = entropy->dc_stats[tbl] + entropy->dc_context[ci];
 
@@ skipping 26 matching lines @@
         entropy->dc_context[ci] = 4 + (sign * 4);  /* small diff category */
       v = m;
       /* Figure F.24: Decoding the magnitude bit pattern of v */
       st += 14;
       while (m >>= 1)
         if (arith_decode(cinfo, st)) v |= m;
       v += 1; if (sign) v = -v;
       entropy->last_dc_val[ci] += v;
     }
 
-    (*block)[0] = (JCOEF) entropy->last_dc_val[ci];
+    if (block)
+      (*block)[0] = (JCOEF) entropy->last_dc_val[ci];
 
     /* Sections F.2.4.2 & F.1.4.4.2: Decoding of AC coefficients */
 
     tbl = compptr->ac_tbl_no;
 
     /* Figure F.20: Decode_AC_coefficients */
     for (k = 1; k <= DCTSIZE2 - 1; k++) {
       st = entropy->ac_stats[tbl] + 3 * (k - 1);
       if (arith_decode(cinfo, st)) break;       /* EOB flag */
       while (arith_decode(cinfo, st + 1) == 0) {
@@ skipping 23 matching lines @@
             st += 1;
           }
         }
       }
       v = m;
       /* Figure F.24: Decoding the magnitude bit pattern of v */
       st += 14;
       while (m >>= 1)
         if (arith_decode(cinfo, st)) v |= m;
       v += 1; if (sign) v = -v;
-      (*block)[jpeg_natural_order[k]] = (JCOEF) v;
+      if (block)
+        (*block)[jpeg_natural_order[k]] = (JCOEF) v;
     }
   }
 
   return TRUE;
 }
 
 
 /*
  * Initialize for an arithmetic-compressed scan.
  */
@@ skipping 133 matching lines @@
     int *coef_bit_ptr, ci;
     cinfo->coef_bits = (int (*)[DCTSIZE2])
       (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
                                   cinfo->num_components*DCTSIZE2*SIZEOF(int));
     coef_bit_ptr = & cinfo->coef_bits[0][0];
     for (ci = 0; ci < cinfo->num_components; ci++) 
       for (i = 0; i < DCTSIZE2; i++)
         *coef_bit_ptr++ = -1;
   }
 }