add diff recording and output code

experimental/skpdiff/SkDiffContext.h

+/*
+ * 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
+     * @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;
+    size_t fDifferCount;

[bsalomon, 2013/07/02 13:37:15]

Does this really need to be 64 bits on a 64 bit machine?

Google/Skia convention is to use plain old "int" for counts (unless they are
byte counts).

+};
+
+#endif