SkSafeMath for tracking size_t overflow

Do multiply (mul) and add while tracking that the
calculation does not overflow, which can be checked with
ok().

The new unit test shows a couple examples.

Author:  Herb Derby <herb@google.com>
Change-Id: I7e67671d2488d67f21d47d9618736a6bae8f23c3
Reviewed-on: https://skia-review.googlesource.com/33721
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Herb Derby <herb@google.com>

1 files changed, 51 insertions, 0 deletions

diff --git a/tests/SafeMathTest.cpp b/tests/SafeMathTest.cpp
new file mode 100644
index 0000000000..7f597393d0
--- /dev/null
+++ b/tests/SafeMathTest.cpp
@@ -0,0 +1,51 @@
+/*
+ * Copyright 2017 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 "SkSafeMath.h"
+
+DEF_TEST(SafeMath, r) {
+    size_t max = std::numeric_limits<size_t>::max();
+
+    {
+        size_t halfMax = max >> 1;
+        size_t halfMaxPlus1 = halfMax + 1;
+        SkSafeMath safe;
+        REPORTER_ASSERT(r, safe.add(halfMax, halfMax) == 2 * halfMax);
+        REPORTER_ASSERT(r, safe);
+        REPORTER_ASSERT(r, safe.add(halfMax, halfMaxPlus1) == max);
+        REPORTER_ASSERT(r, safe);
+        REPORTER_ASSERT(r, safe.add(max, 1) == 0);
+        REPORTER_ASSERT(r, !safe);
+    }
+
+    {
+        SkSafeMath safe;
+        (void) safe.add(max, max);
+        REPORTER_ASSERT(r, !safe);
+    }
+
+    {
+        size_t bits = (sizeof(size_t) * 8);
+        size_t halfBits = bits / 2;
+        size_t sqrtMax = max >> halfBits;
+        size_t sqrtMaxPlus1 = sqrtMax + 1;
+        SkSafeMath safe;
+        REPORTER_ASSERT(r, safe.mul(sqrtMax, sqrtMax) == sqrtMax * sqrtMax);
+        REPORTER_ASSERT(r, safe);
+        REPORTER_ASSERT(r, safe.mul(sqrtMax, sqrtMaxPlus1) == sqrtMax << halfBits);
+        REPORTER_ASSERT(r, safe);
+        REPORTER_ASSERT(r, safe.mul(sqrtMaxPlus1, sqrtMaxPlus1) == 0);
+        REPORTER_ASSERT(r, !safe);
+    }
+
+    {
+        SkSafeMath safe;
+        (void) safe.mul(max, max);
+        REPORTER_ASSERT(r, !safe);
+    }
+}