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 kStartingInterlaceYValues[] = { 0, 4, 2, 1 };
+static const uint8_t kDeltaInterlaceYValues[] = { 8, 8, 4, 2 };
+
+SkGifInterlaceIter::SkGifInterlaceIter(int height) : fHeight(height) {
+    fStartYPtr = kStartingInterlaceYValues;
+    fDeltaYPtr = kDeltaInterlaceYValues;
+
+    fCurrY = *fStartYPtr++;
+    fDeltaY = *fDeltaYPtr++;
+}
+
+void SkGifInterlaceIter::prepareY() {
+    int32_t y = fCurrY + fDeltaY;
+
+    // Iterate through fStartYPtr until a valid row is found.
+    // This ensures that we do not move past the height of the small images.
+    while (y >= fHeight) {
+        if (kStartingInterlaceYValues +
+                SK_ARRAY_COUNT(kStartingInterlaceYValues) == 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;
+}
+
+int32_t SkGifInterlaceIter::nextY() {
+    SkASSERT(fStartYPtr);
+    SkASSERT(fDeltaYPtr);
+    int32_t y = fCurrY;
+    prepareY();
+    return y;
+}