From 8dfc4c6f1740c442bbe332ef1a7a078cb673bf85 Mon Sep 17 00:00:00 2001
From: Pascal Massimino <pascal.massimino@gmail.com>
Date: Wed, 4 May 2011 16:26:47 -0700
Subject: [PATCH] factorize and unify GetAlpha() between the C and SSE2 version

patch by Christian Duvivier (cduvivier at google dot com)

Change-Id: I47ac75010aa4036cf09f13d23043e654c4966a00
---
 src/enc/dsp.c      | 24 +++++++++++++++---------
 src/enc/dsp_sse2.c | 16 +---------------
 src/enc/vp8enci.h  |  2 ++
 3 files changed, 18 insertions(+), 24 deletions(-)

diff --git a/src/enc/dsp.c b/src/enc/dsp.c
index 27bbc50b..cbdc9388 100644
--- a/src/enc/dsp.c
+++ b/src/enc/dsp.c
@@ -24,13 +24,14 @@ static int ClipAlpha(int alpha) {
   return alpha < 0 ? 0 : alpha > 255 ? 255 : alpha;
 }
 
-static int GetAlpha(const int histo[MAX_COEFF_THRESH]) {
+int VP8GetAlpha(const int histo[MAX_COEFF_THRESH + 1]) {
   int num = 0, den = 0, val = 0;
   int k;
   int alpha;
+  // note: changing this loop to avoid the numerous "k + 1" slows things down.
   for (k = 0; k < MAX_COEFF_THRESH; ++k) {
-    if (histo[k]) {
-      val += histo[k];
+    if (histo[k + 1]) {
+      val += histo[k + 1];
       num += val * (k + 1);
       den += (k + 1) * (k + 1);
     }
@@ -42,20 +43,25 @@ static int GetAlpha(const int histo[MAX_COEFF_THRESH]) {
 
 static int CollectHistogram(const uint8_t* ref, const uint8_t* pred,
                             int start_block, int end_block) {
-  int histo[MAX_COEFF_THRESH] = { 0 };
+  int histo[MAX_COEFF_THRESH + 1] = { 0 };
   int16_t out[16];
   int j, k;
   for (j = start_block; j < end_block; ++j) {
     VP8FTransform(ref + VP8Scan[j], pred + VP8Scan[j], out);
+
+    // Convert coefficients to bin (within out[]).
     for (k = 0; k < 16; ++k) {
       const int v = abs(out[k]) >> 2;
-      if (v) {
-        const int bin = (v > MAX_COEFF_THRESH) ? MAX_COEFF_THRESH : v;
-        histo[bin - 1]++;
-      }
+      out[k] = (v > MAX_COEFF_THRESH) ? MAX_COEFF_THRESH : v;
+    }
+
+    // Use bin to update histogram.
+    for (k = 0; k < 16; ++k) {
+      histo[out[k]]++;
     }
   }
-  return GetAlpha(histo);
+
+  return VP8GetAlpha(histo);
 }
 
 //-----------------------------------------------------------------------------
diff --git a/src/enc/dsp_sse2.c b/src/enc/dsp_sse2.c
index ac180c04..9a1749cd 100644
--- a/src/enc/dsp_sse2.c
+++ b/src/enc/dsp_sse2.c
@@ -61,21 +61,7 @@ static int CollectHistogramSSE2(const uint8_t* ref, const uint8_t* pred,
     }
   }
 
-  {
-    int num = 0, den = 0, val = 0;
-    int alpha;
-    for (k = 0; k < MAX_COEFF_THRESH; ++k) {
-      if (histo[k + 1]) {
-        val += histo[k + 1];
-        num += val * (k + 1);
-        den += (k + 1) * (k + 1);
-      }
-    }
-    // we scale the value to a usable [0..255] range
-    alpha = den ? 10 * num / den - 5 : 0;
-    alpha = alpha < 0 ? 0 : alpha > 255 ? 255 : alpha;
-    return alpha;
-  }
+  return VP8GetAlpha(histo);
 }
 
 //-----------------------------------------------------------------------------
diff --git a/src/enc/vp8enci.h b/src/enc/vp8enci.h
index 14667354..0f8d71f4 100644
--- a/src/enc/vp8enci.h
+++ b/src/enc/vp8enci.h
@@ -446,6 +446,8 @@ int VP8EncFinishLayer(VP8Encoder* const enc);    // finalize coding
 void VP8EncDeleteLayer(VP8Encoder* enc);         // reclaim memory
 
   // in dsp.c
+int VP8GetAlpha(const int histo[MAX_COEFF_THRESH + 1]);
+
 // Transforms
 // VP8Idct: Does one of two inverse transforms. If do_two is set, the transforms
 //          will be done for (ref, in, dst) and (ref + 4, in + 16, dst + 4).