Move Entropy methods to lossless.c

Move all the Entropy evaluation methods to lossless.c (from histogram.c).
There's slight difference in the way entropy is computed for evaluating
entropy in prediction methods and histogram (literal) for huffman trees.
Plan (later) to merge few (static) methods and reduce the code size.

This change has no impact on the compression speed/density.

Change-Id: Ife3d96a3c4a8d78a91723d9e0a8d1b78c0256a15

src/dsp/lossless.h

@@ -25,6 +25,10 @@
 extern "C" {
 #endif
 
+
+// Not a trivial literal symbol.
+#define VP8L_NON_TRIVIAL_SYM (0xffffffff)
+
 //------------------------------------------------------------------------------
 // Signatures and generic function-pointers
 
@@ -158,6 +162,23 @@ typedef VP8LStreaks (*VP8LCostCombinedCountFunc)(const uint32_t* X,
 extern VP8LCostCountFunc VP8LHuffmanCostCount;
 extern VP8LCostCombinedCountFunc VP8LHuffmanCostCombinedCount;
 
+// Get the symbol entropy for the distribution 'population'.
+// Set 'trivial_sym', if there's only one symbol present in the distribution.
+double VP8LPopulationCost(const uint32_t* const population, int length,
+                          uint32_t* const trivial_sym);
+
+// Get the combined symbol entropy for the distributions 'X' and 'Y'.
+double VP8LGetCombinedEntropy(const uint32_t* const X,
+                              const uint32_t* const Y, int length);
+
+// Estimate how many bits the combined entropy of literals and distance
+// approximately maps to.
+double VP8LHistogramEstimateBits(const VP8LHistogram* const p);
+
+// This function estimates the cost in bits excluding the bits needed to
+// represent the entropy code itself.
+double VP8LHistogramEstimateBitsBulk(const VP8LHistogram* const p);
+
 typedef void (*VP8LHistogramAddFunc)(const VP8LHistogram* const a,
                                      const VP8LHistogram* const b,
                                      VP8LHistogram* const out);