From f209a5481ef106ffb4a1cab4adc5eeb63a4a1e88 Mon Sep 17 00:00:00 2001
From: Vincent Rabaud <vrabaud@google.com>
Date: Thu, 1 Jun 2017 17:08:43 +0200
Subject: [PATCH] Use the plane code and not the distance when computing
 statistics.

As backward references use the plane code when checking the cost
of a distance, statistics used to compute the cost should use it too.
This provides a small compression improvement at no speed cost.

Change-Id: Icade150929ee39ef6dc0d8b1fc85973086ecf41d
---
 src/enc/backward_references_cost_enc.c | 14 +++++++++++---
 src/enc/histogram_enc.c                | 16 ++++++++++++----
 src/enc/histogram_enc.h                |  4 +++-
 3 files changed, 26 insertions(+), 8 deletions(-)

diff --git a/src/enc/backward_references_cost_enc.c b/src/enc/backward_references_cost_enc.c
index 226bb437..cbad3b41 100644
--- a/src/enc/backward_references_cost_enc.c
+++ b/src/enc/backward_references_cost_enc.c
@@ -58,13 +58,21 @@ static void ConvertPopulationCountTableToBitEstimates(
   }
 }
 
-static int CostModelBuild(CostModel* const m, int cache_bits,
+static int CostModelBuild(CostModel* const m, int xsize, int cache_bits,
                           const VP8LBackwardRefs* const refs) {
   int ok = 0;
+  VP8LRefsCursor c = VP8LRefsCursorInit(refs);
   VP8LHistogram* const histo = VP8LAllocateHistogram(cache_bits);
   if (histo == NULL) goto Error;
 
-  VP8LHistogramCreate(histo, refs, cache_bits);
+  // The following code is similar to VP8LHistogramCreate but converts the
+  // distance to plane code.
+  VP8LHistogramInit(histo, cache_bits);
+  while (VP8LRefsCursorOk(&c)) {
+    VP8LHistogramAddSinglePixOrCopy(histo, c.cur_pos, VP8LDistanceToPlaneCode,
+                                    xsize);
+    VP8LRefsCursorNext(&c);
+  }
 
   ConvertPopulationCountTableToBitEstimates(
       VP8LHistogramNumCodes(histo->palette_code_bits_),
@@ -583,7 +591,7 @@ static int BackwardReferencesHashChainDistanceOnly(
     if (!cc_init) goto Error;
   }
 
-  if (!CostModelBuild(cost_model, cache_bits, refs)) {
+  if (!CostModelBuild(cost_model, xsize, cache_bits, refs)) {
     goto Error;
   }
 
diff --git a/src/enc/histogram_enc.c b/src/enc/histogram_enc.c
index beb0c53a..c2806332 100644
--- a/src/enc/histogram_enc.c
+++ b/src/enc/histogram_enc.c
@@ -76,7 +76,7 @@ void VP8LHistogramStoreRefs(const VP8LBackwardRefs* const refs,
                             VP8LHistogram* const histo) {
   VP8LRefsCursor c = VP8LRefsCursorInit(refs);
   while (VP8LRefsCursorOk(&c)) {
-    VP8LHistogramAddSinglePixOrCopy(histo, c.cur_pos);
+    VP8LHistogramAddSinglePixOrCopy(histo, c.cur_pos, NULL, 0);
     VP8LRefsCursorNext(&c);
   }
 }
@@ -138,7 +138,9 @@ VP8LHistogramSet* VP8LAllocateHistogramSet(int size, int cache_bits) {
 // -----------------------------------------------------------------------------
 
 void VP8LHistogramAddSinglePixOrCopy(VP8LHistogram* const histo,
-                                     const PixOrCopy* const v) {
+                                     const PixOrCopy* const v,
+                                     int (*const distance_modifier)(int, int),
+                                     int distance_modifier_arg0) {
   if (PixOrCopyIsLiteral(v)) {
     ++histo->alpha_[PixOrCopyLiteral(v, 3)];
     ++histo->red_[PixOrCopyLiteral(v, 2)];
@@ -152,7 +154,13 @@ void VP8LHistogramAddSinglePixOrCopy(VP8LHistogram* const histo,
     int code, extra_bits;
     VP8LPrefixEncodeBits(PixOrCopyLength(v), &code, &extra_bits);
     ++histo->literal_[NUM_LITERAL_CODES + code];
-    VP8LPrefixEncodeBits(PixOrCopyDistance(v), &code, &extra_bits);
+    if (distance_modifier == NULL) {
+      VP8LPrefixEncodeBits(PixOrCopyDistance(v), &code, &extra_bits);
+    } else {
+      VP8LPrefixEncodeBits(
+          distance_modifier(distance_modifier_arg0, PixOrCopyDistance(v)),
+          &code, &extra_bits);
+    }
     ++histo->distance_[code];
   }
 }
@@ -473,7 +481,7 @@ static void HistogramBuild(
   while (VP8LRefsCursorOk(&c)) {
     const PixOrCopy* const v = c.cur_pos;
     const int ix = (y >> histo_bits) * histo_xsize + (x >> histo_bits);
-    VP8LHistogramAddSinglePixOrCopy(histograms[ix], v);
+    VP8LHistogramAddSinglePixOrCopy(histograms[ix], v, NULL, 0);
     x += PixOrCopyLength(v);
     while (x >= xsize) {
       x -= xsize;
diff --git a/src/enc/histogram_enc.h b/src/enc/histogram_enc.h
index ce563d37..00c73f51 100644
--- a/src/enc/histogram_enc.h
+++ b/src/enc/histogram_enc.h
@@ -90,7 +90,9 @@ VP8LHistogram* VP8LAllocateHistogram(int cache_bits);
 
 // Accumulate a token 'v' into a histogram.
 void VP8LHistogramAddSinglePixOrCopy(VP8LHistogram* const histo,
-                                     const PixOrCopy* const v);
+                                     const PixOrCopy* const v,
+                                     int (*const distance_modifier)(int, int),
+                                     int distance_modifier_arg0);
 
 static WEBP_INLINE int VP8LHistogramNumCodes(int palette_code_bits) {
   return NUM_LITERAL_CODES + NUM_LENGTH_CODES +