Add a buffered SkStream class.

src/utils/SkFrontBufferedStream.cpp

Index: src/utils/SkFrontBufferedStream.cpp
diff --git a/src/utils/SkFrontBufferedStream.cpp b/src/utils/SkFrontBufferedStream.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..12030da8fbb437c1ec3752859e8fc817b789faf0
--- /dev/null
+++ b/src/utils/SkFrontBufferedStream.cpp
@@ -0,0 +1,147 @@
+/*
+ * 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 "SkFrontBufferedStream.h"
+
+SkStreamRewindable* SkFrontBufferedStream::Create(SkStream* stream, size_t bufferSize) {
+    if (NULL == stream) {
+        return NULL;
+    }
+    return SkNEW_ARGS(SkFrontBufferedStream, (stream, bufferSize));
+}
+
+SkFrontBufferedStream::SkFrontBufferedStream(SkStream* stream, size_t bufferSize)
+    : fStream(SkRef(stream))
+    , fOffset(0)
+    , fBufferedSoFar(0)
+    , fBufferSize(bufferSize)
+    , fBuffer(bufferSize) {}
+
+bool SkFrontBufferedStream::isAtEnd() const {
+    if (fOffset < fBufferedSoFar) {
+        // Even if the underlying stream is at the end, this stream has been
+        // rewound after buffering, so it is not at the end.
+        return false;
+    }
+
+    return fStream->isAtEnd();
+}
+
+bool SkFrontBufferedStream::rewind() {
+    // Only allow a rewind if we have not exceeded the buffer.
+    if (fOffset <= fBufferSize) {
+        fOffset = 0;
+        return true;
+    }
+    return false;
+}
+
+bool SkFrontBufferedStream::hasLength() const {
+    return fStream->hasLength();
+}
+
+size_t SkFrontBufferedStream::getLength() const {
+    return fStream->getLength();
+}
+
+size_t SkFrontBufferedStream::readFromBuffer(char* dst, size_t size) {
+    SkASSERT(fOffset < fBufferedSoFar);
+    // Some data has already been copied to fBuffer. Read up to the
+    // lesser of the size requested and the remainder of the buffered
+    // data.
+    const size_t bytesToCopy = SkTMin(size, fBufferedSoFar - fOffset);
+    if (dst != NULL) {
+        memcpy(dst, fBuffer + fOffset, bytesToCopy);
+    }
+
+    // Update fOffset to the new position. It is guaranteed to be
+    // within the buffered data.
+    fOffset += bytesToCopy;
+    SkASSERT(fOffset <= fBufferedSoFar);
+
+    return bytesToCopy;
+}
+
+size_t SkFrontBufferedStream::bufferAndWriteTo(char* dst, size_t size) {
+    SkASSERT(size > 0);
+    SkASSERT(fOffset >= fBufferedSoFar);
+    // Data needs to be buffered. Buffer up to the lesser of the size requested
+    // and the remainder of the max buffer size.
+    const size_t bytesToBuffer = SkTMin(size, fBufferSize - fBufferedSoFar);
+    char* buffer = fBuffer + fOffset;
+    const size_t buffered = fStream->read(buffer, bytesToBuffer);
+
+    fBufferedSoFar += buffered;
+    fOffset = fBufferedSoFar;
+    SkASSERT(fBufferedSoFar <= fBufferSize);
+
+    // Copy the buffer to the destination buffer and update the amount read.
+    if (dst != NULL) {
+        memcpy(dst, buffer, buffered);
+    }
+
+    return buffered;
+}
+
+size_t SkFrontBufferedStream::readDirectlyFromStream(char* dst, size_t size) {
+    SkASSERT(size > 0);
+    // If we get here, we have buffered all that can be buffered.
+    SkASSERT(fBufferSize == fBufferedSoFar && fOffset >= fBufferSize);
+
+    const size_t bytesReadDirectly = fStream->read(dst, size);
+    fOffset += bytesReadDirectly;
+
+    // If we have read past the end of the buffer, rewinding is no longer
+    // supported, so we can go ahead and free the memory.
+    if (bytesReadDirectly > 0) {

[scroggo, 2013/09/26 17:26:04]

In patch set 9 I accidentally removed this line, which broke the intended
behavior that an SkFrontBufferedStream which has buffered exactly the entire
stream can be rewound. It also passed my tests, unfortunately, so I added a test
to catch that case.

+        fBuffer.reset(0);
+    }
+
+    return bytesReadDirectly;
+}
+
+size_t SkFrontBufferedStream::read(void* voidDst, size_t size) {
+    // Cast voidDst to a char* for easy addition.
+    char* dst = reinterpret_cast<char*>(voidDst);
+    SkDEBUGCODE(const size_t totalSize = size;)
+    const size_t start = fOffset;
+
+    // First, read any data that was previously buffered.
+    if (fOffset < fBufferedSoFar) {
+        const size_t bytesCopied = this->readFromBuffer(dst, size);
+
+        // Update the remaining number of bytes needed to read
+        // and the destination buffer.
+        size -= bytesCopied;
+        SkASSERT(size + (fOffset - start) == totalSize);
+        if (dst != NULL) {
+            dst += bytesCopied;
+        }
+    }
+
+    // Buffer any more data that should be buffered, and copy it to the
+    // destination.
+    if (size > 0 && fBufferedSoFar < fBufferSize) {
+        const size_t buffered = this->bufferAndWriteTo(dst, size);
+
+        // Update the remaining number of bytes needed to read
+        // and the destination buffer.
+        size -= buffered;
+        SkASSERT(size + (fOffset - start) == totalSize);
+        if (dst != NULL) {
+            dst += buffered;
+        }
+    }
+
+    if (size > 0 && !fStream->isAtEnd()) {
+        const size_t bytesReadDirectly = this->readDirectlyFromStream(dst, size);
+        SkDEBUGCODE(size -= bytesReadDirectly;)
+        SkASSERT(size + (fOffset - start) == totalSize);
+    }
+
+    return fOffset - start;
+}