Add a buffered SkStream class.

This is used by Android to buffer an input stream which may not
otherwise be able to rewind.

Add a test for the new class.

R=bungeman@google.com, mtklein@google.com, reed@google.com

Review URL: https://codereview.chromium.org/23717055

git-svn-id: http://skia.googlecode.com/svn/trunk/include@11488 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/utils/SkFrontBufferedStream.h b/utils/SkFrontBufferedStream.h
new file mode 100644
index 0000000..e3eb4db
--- /dev/null
+++ b/utils/SkFrontBufferedStream.h
@@ -0,0 +1,77 @@
+/*
+ * Copyright 2013 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "SkStream.h"
+#include "SkTemplates.h"
+
+/**
+ *  Specialized stream that only buffers the first X bytes of a stream,
+ *  where X is passed in by the user. Note that unlike some buffered
+ *  stream APIs, once more than those bytes are read, no more buffering
+ *  is done. This stream is designed for a use case where the caller
+ *  knows that rewind will only be called from within X bytes (inclusive),
+ *  and the wrapped stream is not necessarily able to rewind at all.
+ */
+class SkFrontBufferedStream : public SkStreamRewindable {
+public:
+    /**
+     *  Creates a new stream that wraps and buffers SkStream.
+     *  @param stream SkStream to buffer. If NULL, NULL is returned. After
+     *      this call, unref stream and do not refer to it.
+     *      SkFrontBufferedStream is expected to be its only owner.
+     *  @param bufferSize Exact size of the buffer to be used.
+     *  @return An SkStream that can buffer up to bufferSize.
+     */
+    static SkStreamRewindable* Create(SkStream* stream, size_t bufferSize);
+
+    virtual size_t read(void* buffer, size_t size) SK_OVERRIDE;
+
+    virtual bool isAtEnd() const SK_OVERRIDE;
+
+    virtual bool rewind() SK_OVERRIDE;
+
+    virtual bool hasPosition() const SK_OVERRIDE { return true; }
+
+    virtual size_t getPosition() const SK_OVERRIDE { return fOffset; }
+
+    virtual bool hasLength() const SK_OVERRIDE;
+
+    virtual size_t getLength() const SK_OVERRIDE;
+
+    virtual SkStreamRewindable* duplicate() const SK_OVERRIDE { return NULL; }
+
+private:
+    SkAutoTUnref<SkStream>  fStream;
+    // Current offset into the stream. Always >= 0.
+    size_t                  fOffset;
+    // Amount that has been buffered by calls to read. Will always be less than
+    // fBufferSize.
+    size_t                  fBufferedSoFar;
+    // Total size of the buffer.
+    const size_t            fBufferSize;
+    SkAutoTMalloc<char>     fBuffer;
+
+    // Private. Use Create.
+    SkFrontBufferedStream(SkStream*, size_t bufferSize);
+
+    // Read up to size bytes from already buffered data, and copy to
+    // dst, if non-NULL. Updates fOffset. Assumes that fOffset is less
+    // than fBufferedSoFar.
+    size_t readFromBuffer(char* dst, size_t size);
+
+    // Buffer up to size bytes from the stream, and copy to dst if non-
+    // NULL. Updates fOffset and fBufferedSoFar. Assumes that fOffset is
+    // less than fBufferedSoFar, and size is greater than 0.
+    size_t bufferAndWriteTo(char* dst, size_t size);
+
+    // Read up to size bytes directly from the stream and into dst if non-
+    // NULL. Updates fOffset. Assumes fOffset is at or beyond the buffered
+    // data, and size is greater than 0.
+    size_t readDirectlyFromStream(char* dst, size_t size);
+
+    typedef SkStream INHERITED;
+};