Make the zero handling in extend_to_full_distribution more explicit.

The old workaround "p = 0 ? 0 : p -1" is misleading.

?: happens before =
assigning back to p truncates to one byte.

Therefore it is equivalent to (p - 1) & 0xFF, but the check just exists
to work around a first pass bug, so let's make the work around more
clear.

https://bugs.chromium.org/p/webm/issues/detail?id=1089

Change-Id: I587c44dd61c1f3767543c0126376f881889935af

vp10/common/entropy.c

diff --git a/vp10/common/entropy.c b/vp10/common/entropy.c
index 56dd73a..3da08a6 100644
--- a/vp10/common/entropy.c
+++ b/vp10/common/entropy.c
@@ -403,7 +403,6 @@
   {255, 241, 243, 255, 236, 255, 252, 254},
   {255, 243, 245, 255, 237, 255, 252, 254},
   {255, 246, 247, 255, 239, 255, 253, 255},
-  {255, 246, 247, 255, 239, 255, 253, 255},
 };
 
 static const vp10_coeff_probs_model default_coef_probs_4x4[PLANE_TYPES] = {
@@ -743,7 +742,9 @@
 };
 
 static void extend_to_full_distribution(vpx_prob *probs, vpx_prob p) {
-  memcpy(probs, vp10_pareto8_full[p = 0 ? 0 : p - 1],
+  // TODO(aconverse): model[PIVOT_NODE] should never be zero.
+  // https://code.google.com/p/webm/issues/detail?id=1089
+  memcpy(probs, vp10_pareto8_full[p == 0 ? 254 : p - 1],
          MODEL_NODES * sizeof(vpx_prob));
 }
 

vp10/common/entropy.h

diff --git a/vp10/common/entropy.h b/vp10/common/entropy.h
index fba7020..2f93cb3 100644
--- a/vp10/common/entropy.h
+++ b/vp10/common/entropy.h
@@ -153,7 +153,7 @@
 // 1, 3, 5, 7, ..., 253, 255
 // In between probabilities are interpolated linearly
 
-#define COEFF_PROB_MODELS 256
+#define COEFF_PROB_MODELS 255
 
 #define UNCONSTRAINED_NODES         3
 

vp9/common/vp9_entropy.c

diff --git a/vp9/common/vp9_entropy.c b/vp9/common/vp9_entropy.c
index 579857b..1c81581 100644
--- a/vp9/common/vp9_entropy.c
+++ b/vp9/common/vp9_entropy.c
@@ -403,7 +403,6 @@
   {255, 241, 243, 255, 236, 255, 252, 254},
   {255, 243, 245, 255, 237, 255, 252, 254},
   {255, 246, 247, 255, 239, 255, 253, 255},
-  {255, 246, 247, 255, 239, 255, 253, 255},
 };
 
 static const vp9_coeff_probs_model default_coef_probs_4x4[PLANE_TYPES] = {
@@ -743,7 +742,9 @@
 };
 
 static void extend_to_full_distribution(vpx_prob *probs, vpx_prob p) {
-  memcpy(probs, vp9_pareto8_full[p = 0 ? 0 : p - 1],
+  // TODO(aconverse): model[PIVOT_NODE] should never be zero.
+  // https://code.google.com/p/webm/issues/detail?id=1089
+  memcpy(probs, vp9_pareto8_full[p == 0 ? 254 : p - 1],
          MODEL_NODES * sizeof(vpx_prob));
 }
 

vp9/common/vp9_entropy.h

diff --git a/vp9/common/vp9_entropy.h b/vp9/common/vp9_entropy.h
index 21611ed..63b3bff 100644
--- a/vp9/common/vp9_entropy.h
+++ b/vp9/common/vp9_entropy.h
@@ -138,7 +138,7 @@
 // 1, 3, 5, 7, ..., 253, 255
 // In between probabilities are interpolated linearly
 
-#define COEFF_PROB_MODELS 256
+#define COEFF_PROB_MODELS 255
 
 #define UNCONSTRAINED_NODES         3