Split the bit reader functionalities from the byte stream provider.

media/base/bit_reader_core.h

+// Copyright (c) 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef MEDIA_BASE_BIT_READER_CORE_H_
+#define MEDIA_BASE_BIT_READER_CORE_H_
+
+#include "base/basictypes.h"
+#include "base/logging.h"
+#include "media/base/media_export.h"
+
+namespace media {
+
+class MEDIA_EXPORT BitReaderCore {
+ public:
+  class ByteStreamProvider {
+   public:
+    ByteStreamProvider();
+    virtual ~ByteStreamProvider();
+
+    // Consume at most the following |max_n| bytes of the stream
+    // and return the number n of bytes actually consumed.
+    // Set |*array| to point to a memory buffer containing those n bytes.
+    // Note: |*array| must be valid until the next call to GetBytes
+    // but there is no guarantee it is valid after.
+    virtual int GetBytes(int max_n, const uint8** array) = 0;
+  };
+
+  // Lifetime of |byte_stream_provider| must be longer than BitReaderCore.
+  explicit BitReaderCore(ByteStreamProvider* byte_stream_provider);
+  ~BitReaderCore();
+
+  // Read one bit from the stream and return it as a boolean in |*out|.
+  // Remark: we do not use the template version for reading a bool
+  // since it generates some optimization warnings during compilation
+  // on Windows platforms.
+  bool ReadBits(int num_bits, bool* out) {
+    DCHECK_EQ(num_bits, 1);
+    return ReadFlag(out);
+  }
+
+  // Read |num_bits| next bits from stream and return in |*out|, first bit
+  // from the stream starting at |num_bits| position in |*out|,
+  // bits of |*out| whose position is strictly greater than |num_bits|
+  // are all set to zero.
+  // Notes:
+  // - |num_bits| cannot be larger than the bits the type can hold.
+  // - From the above description, passing a signed type in |T| does not
+  //   mean the first bit read from the stream gives the sign of the value.
+  // Return false if the given number of bits cannot be read (not enough
+  // bits in the stream), true otherwise. When return false, the stream will
+  // enter a state where further ReadBits/SkipBits operations will always
+  // return false unless |num_bits| is 0. The type |T| has to be a primitive
+  // integer type.
+  template<typename T> bool ReadBits(int num_bits, T* out) {
+    DCHECK_LE(num_bits, static_cast<int>(sizeof(T) * 8));
+    uint64 temp;
+    bool ret = ReadBitsInternal(num_bits, &temp);
+    *out = static_cast<T>(temp);
+    return ret;
+  }
+
+  // Read one bit from the stream and return it as a boolean in |*flag|.
+  bool ReadFlag(bool* flag);
+
+  // Retrieve some bits without actually consuming them.
+  // Bits returned in |*out| are shifted so the most significant bit contains
+  // the next bit that can be read from the stream.
+  // Return the number of bits actually written in |out|.
+  // Note: |num_bits| is just a suggestion of how many bits the caller
+  // wish to get in |*out| and must be less than 64:
+  // - The number of bits returned can be more than |num_bits|.
+  // - However, it will be strictly less than |num_bits|
+  //   if and only if there are not enough bits left in the stream.
+  int PeekBitsMsbAligned(int num_bits, uint64* out);
+
+  // Skip |num_bits| next bits from stream. Return false if the given number of
+  // bits cannot be skipped (not enough bits in the stream), true otherwise.
+  // When return false, the stream will enter a state where further
+  // ReadBits/ReadFlag/SkipBits operations
+  // will always return false unless |num_bits| is 0.
+  bool SkipBits(int num_bits);
+
+  // Returns the number of bits read so far.
+  int bits_read() const;
+
+ private:
+  // Help function used by ReadBits to avoid inlining the bit reading logic.
+  bool ReadBitsInternal(int num_bits, uint64* out);
+
+  // Refill bit registers to have at least |min_nbits| bits available.
+  // Return true if the mininimum bit count condition is met after the refill.
+  bool Refill(int min_nbits);
+
+  // Refill the current bit register from the next bit register.
+  void RefillCurrentRegister();
+
+  ByteStreamProvider* const byte_stream_provider_;
+
+  // Number of bits read so far.
+  int bits_read_;
+
+  // Number of bits in |reg_| that have not been consumed yet.
+  // Note: bits are consumed from MSB to LSB.
+  int nbits_;
+  uint64 reg_;
+
+  // Number of bits in |reg_next_| that have not been consumed yet.
+  // Note: bits are consumed from MSB to LSB.
+  int nbits_next_;
+  uint64 reg_next_;
+
+  DISALLOW_COPY_AND_ASSIGN(BitReaderCore);
+};
+
+}  // namespace media
+
+#endif  // MEDIA_BASE_BIT_READER_CORE_H_