Add a buffered SkStream class.

src/utils/SkBufferedStream.cpp

+/*
+ * 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 "SkBufferedStream.h"
+
+SkBufferedStream* SkBufferedStream::Create(SkStream* stream, size_t bufferSize) {
+    if (NULL == stream) {
+        return NULL;
+    }
+    return SkNEW_ARGS(SkBufferedStream, (stream, bufferSize));
+}
+
+SkBufferedStream::SkBufferedStream(SkStream* stream, size_t bufferSize)
+    : fStream(SkRef(stream))
+    , fOffset(0)
+    , fBufferedSoFar(0)
+    , fBufferSize(bufferSize)
+    , fBuffer(bufferSize) {}
+
+bool SkBufferedStream::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 SkBufferedStream::rewind() {
+    // Only allow a rewind if we have not exceeded the buffer.
+    if (fOffset <= fBufferSize) {
+        fOffset = 0;
+        return true;
+    }
+    return false;
+}
+
+bool SkBufferedStream::hasLength() const {
+    return fStream->hasLength();
+}
+
+size_t SkBufferedStream::getLength() const {
+    return fStream->getLength();
+}
+
+size_t SkBufferedStream::read(void* voidDst, size_t size) {
+    char* dst = reinterpret_cast<char*>(voidDst);
+    SkDEBUGCODE(const size_t totalSize = size;)
+    size_t readSoFar = 0;
+    if (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);
+        memcpy(dst, this->getBufferAtOffset(), bytesToCopy);
+
+        // Update fOffset to the new position. It is guaranteed to be
+        // within the buffered data.
+        fOffset += bytesToCopy;
+        SkASSERT(fOffset <= fBufferedSoFar);
+
+        if (bytesToCopy == size) {
+            // The entire requested read was inside the already buffered
+            // data.
+            SkASSERT(fOffset <= fBufferedSoFar);
+            return size;
+        }
+
+        // The requested read extends beyond the buffered data. Update
+        // the remaining number of bytes needed to read, the number of
+        // bytes read so far, and the destination buffer.
+        size -= bytesToCopy;
+        readSoFar += bytesToCopy;
+        SkASSERT(size + readSoFar == totalSize);
+        dst += bytesToCopy;
+    }
+
+    // If we got here, we have read everything that was already buffered.
+    SkASSERT(fOffset >= fBufferedSoFar);
+
+    if (fBufferedSoFar < fBufferSize) {
+        // 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 = this->getBufferAtOffset();
+        const size_t buffered = fStream->read(buffer, bytesToBuffer);
+        fBufferedSoFar += buffered;
+        fOffset = fBufferedSoFar;
+
+        // Copy the buffer to the destination buffer and update the amount read.
+        memcpy(dst, buffer, buffered);
+        readSoFar += buffered;
+
+        if (buffered == size || fStream->isAtEnd()) {
+            // We were able to buffer all of the data requested (or all the data
+            // remaining in the stream) and provide it to the caller.
+            SkASSERT(fBufferedSoFar <= fBufferSize);
+            SkASSERT(totalSize == readSoFar || fStream->isAtEnd());
+            return readSoFar;
+        }
+
+        // The requested read extends beyond the length of the buffer. Update
+        // the remaining number of bytes needed to read and the destination
+        // buffer.
+        size -= buffered;
+        SkASSERT(size + readSoFar == totalSize);
+        dst += buffered;
+    }
+
+    // If we get here, we have buffered all that can be buffered.
+    SkASSERT(fBufferSize == fBufferedSoFar && fOffset >= fBufferSize);
+
+    // Read directly from the stream.
+    const size_t bytesReadDirectly = fStream->read(dst, size);
+    fOffset += bytesReadDirectly;
+    return bytesReadDirectly + readSoFar;
+}