add diff recording and output code

experimental/skpdiff/SkDiffContext.h

Index: experimental/skpdiff/SkDiffContext.h
diff --git a/experimental/skpdiff/SkDiffContext.h b/experimental/skpdiff/SkDiffContext.h
new file mode 100644
index 0000000000000000000000000000000000000000..545b51512380a02b1fa62dba2f86cc66d4f8d836
--- /dev/null
+++ b/experimental/skpdiff/SkDiffContext.h
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+/*
+ * Copyright 2013 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkDiffContext_DEFINED
+#define SkDiffContext_DEFINED
+
+#include "SkString.h"
+#include "SkTArray.h"
+#include "SkTDArray.h"
+
+class SkWStream;
+class SkImageDiffer;
+
+/**
+ * Collects records of diffs and outputs them as JSON.
+ */
+class SkDiffContext {
+public:
+    SkDiffContext();
+    ~SkDiffContext();
+
+    /**
+     * Sets the differs to be used in each diff. Already started diffs will not retroactively use
+     * these.
+     * @param differs An array of differs to use. The array is copied, but not the differs
+     *                themselves.
+     */
+    void setDiffers(const SkTDArray<SkImageDiffer*>& differs);
+
+    /**
+     * Compares two directories of images with the given differ
+     * @param baselinePath The baseline directory's path
+     * @param testPath     The test directory's path
+     */
+    void diffDirectories(const char baselinePath[], const char testPath[]);
+
+    /**
+     * Compares two sets of images identified by glob style patterns with the given differ
+     * @param baselinePattern A pattern for baseline files
+     * @param testPattern     A pattern for test files that matches each file of the baseline file
+     */
+    void diffPatterns(const char baselinePattern[], const char testPattern[]);
+
+    /**
+     * Compares the images at the given paths
+     * @param baselinePath The baseline file path
+     * @param testPath     The matching test file path
+     */
+    void addDiff(const char* baselinePath, const char* testPath);
+
+    /**
+     * Output the records of each diff in JSON.
+     *
+     * The format of the JSON document is one top level array named "records".
+     * Each record in the array is an object with both a "baselinePath" and "testPath" string field.
+     * They also have an array named "diffs" with each element being one diff record for the two
+     * images indicated in the above field.
+     * A diff record includes:
+     *    "differName"       : string name of the diff metric used
+     *    "result"           : numerical result of the diff
+     *    "pointsOfInterest" : an array of coordinates (stored as a 2-array of ints) of interesting
+     *                         points
+     *
+     * Here is an example:
+     *
+     * {
+     *     "records": [
+     *         {
+     *             "baselinePath": "queue.png",
+     *             "testPath": "queue.png",
+     *             "diffs": [
+     *                 {
+     *                     "differName": "different_pixels",
+     *                     "result": 1,
+     *                     "pointsOfInterest": [
+     *                         [285,279],
+     *                     ]
+     *                 }
+     *             ]
+     *         }
+     *     ]
+     * }
+     *
+     * @param stream The stream to output the diff to
+     */
+    void outputRecords(SkWStream& stream);
+
+private:
+    struct DiffData {
+        const char* fDiffName;
+        double fResult;
+        SkTDArray<SkIPoint> fPointsOfInterest;
+    };
+
+    struct DiffRecord {
+        SkString           fBaselinePath;
+        SkString               fTestPath;
+        SkTArray<DiffData>        fDiffs;
+        DiffRecord*                fNext;
+    };
+
+    // We use linked list for the records so that their pointers remain stable. A resizable array
+    // might change its pointers, which would make it harder for async diffs to record their
+    // results.
+    DiffRecord * fRecords;
+
+    SkImageDiffer** fDiffers;
+    int fDifferCount;
+};
+
+#endif