fix asserts in SkMulDiv255Round, and add test

tests/MathTest.cpp

 
 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 #include "Test.h"
 #include "SkFloatBits.h"
 #include "SkFloatingPoint.h"
@@ skipping 30 matching lines @@
     };
 
     for (size_t i = 0; i < SK_ARRAY_COUNT(gVals); ++i) {
         test_floor_value(reporter, gVals[i]);
 //        test_floor_value(reporter, -gVals[i]);
     }
 }
 
 ///////////////////////////////////////////////////////////////////////////////
 
+// test that SkMul16ShiftRound and SkMulDiv255Round return the same result
+static void test_muldivround(skiatest::Reporter* reporter) {
+#if 0
+    // this "complete" test is too slow, so we test a random sampling of it
+
+    for (int a = 0; a <= 32767; ++a) {
+        for (int b = 0; b <= 32767; ++b) {
+            unsigned prod0 = SkMul16ShiftRound(a, b, 8);
+            unsigned prod1 = SkMulDiv255Round(a, b);
+            SkASSERT(prod0 == prod1);
+        }
+    }
+#endif
+
+    SkRandom rand;
+    for (int i = 0; i < 10000; ++i) {
+        unsigned a = rand.nextU() & 0x7FFF;
+        unsigned b = rand.nextU() & 0x7FFF;
+
+        unsigned prod0 = SkMul16ShiftRound(a, b, 8);
+        unsigned prod1 = SkMulDiv255Round(a, b);
+
+        REPORTER_ASSERT(reporter, prod0 == prod1);
+    }
+}
+
 static float float_blend(int src, int dst, float unit) {
     return dst + (src - dst) * unit;
 }
 
 static int blend31(int src, int dst, int a31) {
     return dst + ((src - dst) * a31 * 2114 >> 16);
     //    return dst + ((src - dst) * a31 * 33 >> 10);
 }
 
 static int blend31_slow(int src, int dst, int a31) {
@@ skipping 527 matching lines @@
 #endif
 
 #ifdef SK_SCALAR_IS_FLOAT
     test_blend(reporter);
 #endif
 
     if (false) test_floor(reporter);
 
     // disable for now
     if (false) test_blend31();  // avoid bit rot, suppress warning
+
+    test_muldivround(reporter);
 }
 
 #include "TestClassDef.h"
 DEFINE_TESTCLASS("Math", MathTestClass, TestMath)