Speedups for unused Huffman groups.

If we know a Huffman group is unused, do not build
it: just check that it is valid.

Change-Id: Ie8223fe1afa1e769085a23ab92e730edd9060176

src/dec/vp8l_dec.c

@@ -362,12 +362,8 @@ static int ReadHuffmanCodes(VP8LDecoder* const dec, int xsize, int ysize,
   VP8LMetadata* const hdr = &dec->hdr_;
   uint32_t* huffman_image = NULL;
   HTreeGroup* htree_groups = NULL;
-  // When reading htrees, some might be unused, as the format allows it.
-  // We will still read them but put them in this htree_group_bogus.
-  HTreeGroup htree_group_bogus;
   HuffmanCode* huffman_tables = NULL;
-  HuffmanCode* huffman_tables_bogus = NULL;
-  HuffmanCode* next = NULL;
+  HuffmanCode* huffman_table = NULL;
   int num_htree_groups = 1;
   int num_htree_groups_max = 1;
   int max_alphabet_size = 0;
@@ -418,12 +414,6 @@ static int ReadHuffmanCodes(VP8LDecoder* const dec, int xsize, int ysize,
         if (*mapped_group == -1) *mapped_group = num_htree_groups++;
         huffman_image[i] = *mapped_group;
       }
-      huffman_tables_bogus = (HuffmanCode*)WebPSafeMalloc(
-          table_size, sizeof(*huffman_tables_bogus));
-      if (huffman_tables_bogus == NULL) {
-        dec->status_ = VP8_STATUS_OUT_OF_MEMORY;
-        goto Error;
-      }
     } else {
       num_htree_groups = num_htree_groups_max;
     }
@@ -453,36 +443,44 @@ static int ReadHuffmanCodes(VP8LDecoder* const dec, int xsize, int ysize,
     goto Error;
   }
 
-  next = huffman_tables;
+  huffman_table = huffman_tables;
   for (i = 0; i < num_htree_groups_max; ++i) {
-    // If the index "i" is unused in the Huffman image, read the coefficients
-    // but store them to a bogus htree_group.
-    const int is_bogus = (mapping != NULL && mapping[i] == -1);
+    // If the index "i" is unused in the Huffman image, just make sure the
+    // coefficients are valid but do not store them.
+    if (mapping != NULL && mapping[i] == -1) {
+      for (j = 0; j < HUFFMAN_CODES_PER_META_CODE; ++j) {
+        int alphabet_size = kAlphabetSize[j];
+        if (j == 0 && color_cache_bits > 0) {
+          alphabet_size += (1 << color_cache_bits);
+        }
+        // Passing in NULL so that nothing gets filled.
+        if (!ReadHuffmanCode(alphabet_size, dec, code_lengths, NULL)) {
+          goto Error;
+        }
+      }
+    } else {
       HTreeGroup* const htree_group =
-        is_bogus ? &htree_group_bogus :
           &htree_groups[(mapping == NULL) ? i : mapping[i]];
       HuffmanCode** const htrees = htree_group->htrees;
-    HuffmanCode* huffman_tables_i = is_bogus ? huffman_tables_bogus : next;
       int size;
       int total_size = 0;
       int is_trivial_literal = 1;
       int max_bits = 0;
       for (j = 0; j < HUFFMAN_CODES_PER_META_CODE; ++j) {
         int alphabet_size = kAlphabetSize[j];
-      htrees[j] = huffman_tables_i;
+        htrees[j] = huffman_table;
         if (j == 0 && color_cache_bits > 0) {
-        alphabet_size += 1 << color_cache_bits;
+          alphabet_size += (1 << color_cache_bits);
         }
-      size =
-          ReadHuffmanCode(alphabet_size, dec, code_lengths, huffman_tables_i);
+        size = ReadHuffmanCode(alphabet_size, dec, code_lengths, huffman_table);
         if (size == 0) {
           goto Error;
         }
         if (is_trivial_literal && kLiteralMap[j] == 1) {
-        is_trivial_literal = (huffman_tables_i->bits == 0);
+          is_trivial_literal = (huffman_table->bits == 0);
         }
-      total_size += huffman_tables_i->bits;
-      huffman_tables_i += size;
+        total_size += huffman_table->bits;
+        huffman_table += size;
         if (j <= ALPHA) {
           int local_max_bits = code_lengths[0];
           int k;
@@ -494,7 +492,6 @@ static int ReadHuffmanCodes(VP8LDecoder* const dec, int xsize, int ysize,
           max_bits += local_max_bits;
         }
       }
-    if (!is_bogus) next = huffman_tables_i;
       htree_group->is_trivial_literal = is_trivial_literal;
       htree_group->is_trivial_code = 0;
       if (is_trivial_literal) {
@@ -511,6 +508,7 @@ static int ReadHuffmanCodes(VP8LDecoder* const dec, int xsize, int ysize,
           !htree_group->is_trivial_code && (max_bits < HUFFMAN_PACKED_BITS);
       if (htree_group->use_packed_table) BuildPackedTable(htree_group);
     }
+  }
   ok = 1;
 
   // All OK. Finalize pointers.
@@ -521,7 +519,6 @@ static int ReadHuffmanCodes(VP8LDecoder* const dec, int xsize, int ysize,
 
  Error:
   WebPSafeFree(code_lengths);
-  WebPSafeFree(huffman_tables_bogus);
   WebPSafeFree(mapping);
   if (!ok) {
     WebPSafeFree(huffman_image);

src/utils/huffman_utils.c

@@ -91,7 +91,8 @@ static int BuildHuffmanTable(HuffmanCode* const root_table, int root_bits,
 
   assert(code_lengths_size != 0);
   assert(code_lengths != NULL);
-  assert(root_table != NULL);
+  assert((root_table != NULL && sorted != NULL) ||
+         (root_table == NULL && sorted == NULL));
   assert(root_bits > 0);
 
   // Build histogram of code lengths.
@@ -120,16 +121,22 @@ static int BuildHuffmanTable(HuffmanCode* const root_table, int root_bits,
   for (symbol = 0; symbol < code_lengths_size; ++symbol) {
     const int symbol_code_length = code_lengths[symbol];
     if (code_lengths[symbol] > 0) {
+      if (sorted != NULL) {
         sorted[offset[symbol_code_length]++] = symbol;
+      } else {
+        offset[symbol_code_length]++;
+      }
     }
   }
 
   // Special case code with only one value.
   if (offset[MAX_ALLOWED_CODE_LENGTH] == 1) {
+    if (sorted != NULL) {
       HuffmanCode code;
       code.bits = 0;
       code.value = (uint16_t)sorted[0];
       ReplicateValue(table, 1, total_size, code);
+    }
     return total_size;
   }
 
@@ -151,6 +158,7 @@ static int BuildHuffmanTable(HuffmanCode* const root_table, int root_bits,
       if (num_open < 0) {
         return 0;
       }
+      if (root_table == NULL) continue;
       for (; count[len] > 0; --count[len]) {
         HuffmanCode code;
         code.bits = (uint8_t)len;
@@ -169,6 +177,7 @@ static int BuildHuffmanTable(HuffmanCode* const root_table, int root_bits,
       if (num_open < 0) {
         return 0;
       }
+      if (root_table == NULL) continue;
       for (; count[len] > 0; --count[len]) {
         HuffmanCode code;
         if ((key & mask) != low) {
@@ -206,7 +215,10 @@ int VP8LBuildHuffmanTable(HuffmanCode* const root_table, int root_bits,
                           const int code_lengths[], int code_lengths_size) {
   int total_size;
   assert(code_lengths_size <= MAX_CODE_LENGTHS_SIZE);
-  if (code_lengths_size <= SORTED_SIZE_CUTOFF) {
+  if (root_table == NULL) {
+    total_size = BuildHuffmanTable(NULL, root_bits,
+                                   code_lengths, code_lengths_size, NULL);
+  } else if (code_lengths_size <= SORTED_SIZE_CUTOFF) {
     // use local stack-allocated array.
     uint16_t sorted[SORTED_SIZE_CUTOFF];
     total_size = BuildHuffmanTable(root_table, root_bits,

src/utils/huffman_utils.h

@@ -78,6 +78,8 @@ void VP8LHtreeGroupsFree(HTreeGroup* const htree_groups);
 // the huffman table.
 // Returns built table size or 0 in case of error (invalid tree or
 // memory error).
+// If root_table is NULL, it returns 0 if a lookup cannot be built, something
+// > 0 otherwise (but not the table size).
 int VP8LBuildHuffmanTable(HuffmanCode* const root_table, int root_bits,
                           const int code_lengths[], int code_lengths_size);