Add backward_ref, histogram & huffman encode modules from lossless.

Change-Id: Iac056d27972956782defa182caa3ea400cdb77f8

src/utils/huffman_encode.h

@@ -0,0 +1,52 @@
+// Copyright 2011 Google Inc. All Rights Reserved.
+//
+// This code is licensed under the same terms as WebM:
+//  Software License Agreement:  http://www.webmproject.org/license/software/
+//  Additional IP Rights Grant:  http://www.webmproject.org/license/additional/
+// -----------------------------------------------------------------------------
+//
+// Author: jyrki@google.com (Jyrki Alakuijala)
+//
+// Flate like entropy encoding (Huffman) for webp lossless
+
+#ifndef WEBP_UTILS_ENTROPY_ENCODE_H_
+#define WEBP_UTILS_ENTROPY_ENCODE_H_
+
+#include <stdint.h>
+
+#if defined(__cplusplus) || defined(c_plusplus)
+extern "C" {
+#endif
+
+// This function will create a Huffman tree.
+//
+// The (data,length) contains the population counts.
+// The tree_limit is the maximum bit depth of the Huffman codes.
+//
+// The depth contains the tree, i.e., how many bits are used for
+// the symbol.
+//
+// See http://en.wikipedia.org/wiki/Huffman_coding
+//
+// Returns 0 when an error has occured.
+int CreateHuffmanTree(const int* data,
+                      const int length,
+                      const int tree_limit,
+                      uint8_t* depth);
+
+// Write a huffman tree from bit depths into the deflate representation
+// of a Huffman tree. In deflate, the generated Huffman tree is to be
+// compressed once more using a Huffman tree.
+void CreateCompressedHuffmanTree(const uint8_t* depth, int len,
+                                 int* num_symbols,
+                                 uint8_t* tree,
+                                 uint8_t* extra_bits_data);
+
+// Get the actual bit values for a tree of bit depths.
+void ConvertBitDepthsToSymbols(const uint8_t* depth, int len, uint16_t* bits);
+
+#if defined(__cplusplus) || defined(c_plusplus)
+}
+#endif
+
+#endif  // WEBP_UTILS_ENTROPY_ENCODE_H_