Hash .pngs instead of SkBitmaps.

tests/UnpremultiplyTest.cpp

Index: tests/UnpremultiplyTest.cpp
diff --git a/tests/UnpremultiplyTest.cpp b/tests/UnpremultiplyTest.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..7846a1b370ad4ba7c7fef47ad2a5deed5148c76c
--- /dev/null
+++ b/tests/UnpremultiplyTest.cpp
@@ -0,0 +1,38 @@
+/*
+ * Copyright 2014 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "SkColorPriv.h"
+#include "SkUnPreMultiply.h"
+#include "Test.h"
+
+DEF_TEST(Unpremultiply, reporter) {
+    // Here we test that unpremultiplication is injective:
+    // no two distinct premul colors map to the same unpremul color.
+
+    // DM exploits this fact to safely hash .pngs instead of the original bitmaps.
+
+    // It is sufficient to test red.  Green and blue follow the same rules.
+    // This means we have at most 256*256 possible colors to deal with.
+    int hits[256*256];
+    for (size_t i = 0; i < SK_ARRAY_COUNT(hits); i++) {
+        hits[i] = 0;
+    }
+
+    for (int a = 0; a < 256; a++) {
+        for (int r = 0; r <= a; r++) {
+            SkPMColor pm = SkPackARGB32(a, r, 0, 0);
+            SkColor upm = SkUnPreMultiply::PMColorToColor(pm);
+
+            // ARGB -> AR
+            hits[upm >> 16]++;
+        }
+    }
+
+    for (size_t i = 0; i < SK_ARRAY_COUNT(hits); i++) {
+        REPORTER_ASSERT(reporter, hits[i] < 2);
+    }
+}