Implementation of SkBitmapRegionDecoder using SkAndroidCodec

tools/SkCodecTools.h

Index: tools/SkCodecTools.h
diff --git a/tools/SkCodecTools.h b/tools/SkCodecTools.h
index 097915b5d719711c37559f860791ca3b50936dd7..413f0335f3cadeacf8e0fdf2b24e94383b1f3cb0 100644
--- a/tools/SkCodecTools.h
+++ b/tools/SkCodecTools.h
@@ -12,4 +12,28 @@ inline float get_scale_from_sample_size(uint32_t sampleSize) {
     return 1.0f / (float) sampleSize;
 }
 
+/*
+ * Chooses the correct image subset offsets and dimensions for the partial decode.
+ *
+ * @return true if the subset is completely contained within the image
+ *         false otherwise
+ */
+inline bool set_subset_region(int inputOffset, int inputDimension,
+        int imageOriginalDimension, int* imageSubsetOffset, int* outOffset,
+        int* imageSubsetDimension) {
+
+    // This must be at least zero, we can't start decoding the image at a negative coordinate.
+    *imageSubsetOffset = SkTMax(0, inputOffset);
+
+    // If inputOffset is less than zero, we decode to an offset location in the output bitmap.
+    *outOffset = *imageSubsetOffset - inputOffset;
+
+    // Use imageSusetOffset to make sure we don't decode pixels past the edge of the image.
+    // Use outOffset to make sure we don't decode pixels past the edge of the region.
+    *imageSubsetDimension = SkTMin(imageOriginalDimension - *imageSubsetOffset,
+            inputDimension - *outOffset);
+
+    return (*outOffset == 0) && (*imageSubsetDimension == inputDimension);
+}
+
 #endif // SkCodecTools_DEFINED