Creating a new wrapper for gif decoder

src/codec/SkGifInterlaceIter.cpp

+/*
+ * Copyright 2015 The Android Open Source Project
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "SkGifInterlaceIter.h"
+
+static const uint8_t kStartingInterlaceYValue[] = { 0, 4, 2, 1 };

[scroggo, 2015/03/25 19:44:49]

nit: Values, plural.

[msarett, 2015/03/26 19:15:57]

Done.

+static const uint8_t kDeltaInterlaceYValue[] = { 8, 8, 4, 2 };
+
+SkGifInterlaceIter::SkGifInterlaceIter(int height) : fHeight(height) {
+    fStartYPtr = kStartingInterlaceYValue;
+    fDeltaYPtr = kDeltaInterlaceYValue;
+
+    fCurrY = *fStartYPtr++;
+    fDeltaY = *fDeltaYPtr++;
+}
+
+void SkGifInterlaceIter::prepareY() {
+    uint32_t y = fCurrY + fDeltaY;
+
+    // We went from an if statement to a while loop so that we iterate

[scroggo, 2015/03/25 19:44:49]

Are you saying that the old decoder trashed memory?

I would reword this comment to explain why we're doing it this way, without
referencing the other way.

[msarett, 2015/03/26 19:15:57]

This comment is straight from the old decoder.  I assume it was added after a
bug fix.  I thought it might be important and worth preserving.  I added a new
comment that hopefully makes more sense.

+    // through fStartYPtr until a valid row is found. This is so that
+    // images that are smaller than 5x5 will not trash memory.
+    while (y >= fHeight) {
+        if (kStartingInterlaceYValue +
+                SK_ARRAY_COUNT(kStartingInterlaceYValue) == fStartYPtr) {
+            // Now we have iterated over the entire image.  Forbid any
+            // subsequent calls to nextY().
+            SkDEBUGCODE(fStartYPtr = NULL;)
+            SkDEBUGCODE(fDeltaYPtr = NULL;)
+            y = 0;
+        } else {
+            y = *fStartYPtr++;
+            fDeltaY = *fDeltaYPtr++;
+        }
+    }
+    fCurrY = y;
+}
+
+uint32_t SkGifInterlaceIter::nextY() {
+    SkASSERT(fStartYPtr);
+    SkASSERT(fDeltaYPtr);
+    uint32_t y = fCurrY;
+    prepareY();
+    return y;
+}