Implementation of SkBitmapRegionDecoder using SkAndroidCodec

tools/SkCodecTools.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkCodecTools_DEFINED
 #define SkCodecTools_DEFINED
 
 inline float get_scale_from_sample_size(uint32_t sampleSize) {
     return 1.0f / (float) sampleSize;
 }
 
+enum SubsetType {
+    kFullyInside_SubsetType,
+    kPartiallyInside_SubsetType,
+    kOutside_SubsetType,
+};
+
+/*
+ * Corrects image subset offsets and dimensions in order to perform a valid decode.
+ * Also indicates if the image subset should be placed at an offset within the
+ * output bitmap.
+ *
+ * Values of output variables are undefined if the SubsetType is kInvalid.
+ *
+ * @param imageDims Original image dimensions.
+ * @param subset    As input, the subset that the client requested.
+ *                  As output, the image subset that we will decode.
+ * @param outX      The left offset of the image subset within the output bitmap.
+ * @param outY      The top offset of the image subset within the output bitmap.
+ *
+ * @return An indication of how the subset is contained in the image.
+ *         If the return value is kInvalid, values of output variables are undefined.
+ */
+inline SubsetType adjust_subset_rect(const SkISize& imageDims, SkIRect* subset, int* outX,
+        int* outY) {
+    // These must be at least zero, we can't start decoding the image at a negative coordinate.
+    int left = SkTMax(0, subset->fLeft);
+    int top = SkTMax(0, subset->fTop);
+
+    // If input offsets are less than zero, we decode to an offset location in the output bitmap.
+    *outX = left - subset->fLeft;
+    *outY = top - subset->fTop;
+
+    // Make sure we don't decode pixels past the edge of the image or past the edge of the subset.
+    int width = SkTMin(imageDims.width() - left, subset->width() - *outX);
+    int height = SkTMin(imageDims.height() - top, subset->height() - *outY);
+    if (width <= 0 || height <= 0) {
+        return SubsetType::kOutside_SubsetType;
+    }
+
+    subset->setXYWH(left, top, width, height);
+    if ((*outX != 0) || (*outY != 0) || (width != subset->width()) ||
+            (height != subset->height())) {
+        return SubsetType::kPartiallyInside_SubsetType;
+    }
+
+    return SubsetType::kFullyInside_SubsetType;
+}
+
 #endif // SkCodecTools_DEFINED