Certificate Transparency: Code for unpacking EV cert hashes whitelist

chrome/browser/net/bit_stream_reader.h

+// Copyright 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 CHROME_BROWSER_NET_BIT_STREAM_READER_H_
+#define CHROME_BROWSER_NET_BIT_STREAM_READER_H_
+
+#include <stdint.h>
+
+#include "base/strings/string_piece.h"
+
+namespace internal {
+
+// A class for reading individual bits from a buffer of bytes they are packed
+// into. Bits are read MSB-first from the stream.
+// It is limited to 64-bit reads and is inefficient as a design choice - Since
+// it is used infrequently to unpack the Golomb-coded EV certificate hashes
+// whitelist in a blocking thread.

[Ryan Sleevi, 2014/10/20 19:18:24]

This comment needs rewording. You shouldn't discuss how it's used (a layering
violation) or about the implementation details ("in a blocking thread")

// A class for reading individual bits from a packed buffer. Bits are read
MSB-first from the
// stream.
// It is limited to 64-bit reads and is inefficient as a design choice. This
class should not
// be used frequently.
//
// [There probably needs to be something here about how this reader encodes
data, otherwise, why is it just not a byte, etc]

[Eran Messeri, 2014/10/21 14:59:59]

Done.

+class BitStreamReader {
+ public:
+  explicit BitStreamReader(const base::StringPiece& source);
+
+  // Reads unary-encoded number into |out|. Returns true if
+  // there was at least one bit to read, false otherwise.
+  bool ReadUnaryEncoding(uint64_t* out);
+
+  // Reads |num_bits| (up to 64) into |out|. |out| is filled from the MSB to the
+  // LSB. If |num_bits| is less than 64, the most significant |64 - num_bits|
+  // bits are unused and left as zeros. Returns true if the stream had the
+  // requested |num_bits|, false otherwise.
+  bool ReadBits(uint8_t num_bits, uint64_t* out);
+
+  // Returns the number of bits left in the stream.
+  uint64_t BitsLeft() const;
+
+ private:
+  // Reads a single bit. Within a byte, the bits are read from the MSB to the
+  // LSB.
+  uint8_t ReadBit();
+
+  const base::StringPiece source_;
+
+  // Index of the byte currently being read from.
+  uint64_t current_byte_;
+
+  // Index of the last bit read within |current_byte_|. Since bits are read
+  // from the MSB to the LSB, this value is initialized to 7 and decremented
+  // after each read.
+  int8 current_bit_;
+};

[Ryan Sleevi, 2014/10/20 19:18:25]

DISALLOW_COPY_AND_ASSIGN

[Eran Messeri, 2014/10/21 14:59:59]

Done.

+
+}  // namespace internal
+
+#endif  // CHROME_BROWSER_NET_BIT_STREAM_READER_H_