components/dns_prefetch/renderer/predictor_queue.h

   1 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 // DnsQueue is implemented as an almost FIFO circular buffer for text
   6 // strings that don't have embedded nulls ('\0').  The "almost" element is that
   7 // some duplicate strings may be removed (i.e., the string won't really be
   8 // pushed *if* the class happens to notice that a duplicate is already in the
   9 // queue).
  10 // The buffers internal format is null terminated character strings
  11 // (a.k.a., c_strings).
  12 // It is written to be as fast as possible during push() operations, so
  13 // that there will be minimal performance impact on a supplier thread.
  14 // The push() operation will not block, and no memory allocation is involved
  15 // (internally) during the push operations.
  16 // The one caveat is that if there is insufficient space in the buffer to
  17 // accept additional string via a push(), then the push() will fail, and
  18 // the buffer will be unmodified.
  19
  20 // This class was designed for use in DNS prefetch operations.  During
  21 // rendering, the supplier is the renderer (typically), and the consumer
  22 // is a thread that sends messages to an async DNS resolver.
  23
  24 #ifndef COMPONENTS_DNS_PREFETCH_RENDERER_PREDICTOR_QUEUE_H__
  25 #define COMPONENTS_DNS_PREFETCH_RENDERER_PREDICTOR_QUEUE_H__
  26
  27 #include <string>
  28
  29 #include "base/basictypes.h"
  30 #include "base/memory/scoped_ptr.h"
  31
  32 namespace dns_prefetch {
  33
  34 class DnsQueue {
  35  public:
  36   // BufferSize is a signed type used for indexing into a buffer.
  37   typedef int32 BufferSize;
  38
  39   enum PushResult { SUCCESSFUL_PUSH, OVERFLOW_PUSH, REDUNDANT_PUSH };
  40
  41   // The size specified in the constructor creates a buffer large enough
  42   // to hold at most one string of that length, or "many"
  43   // strings of considerably shorter length.  Note that strings
  44   // are padded internally with a terminal '\0" while stored,
  45   // so if you are trying to be precise and get N strings of
  46   // length K to fit, you should actually construct a buffer with
  47   // an internal size of N*(K+1).
  48   explicit DnsQueue(BufferSize size);
  49   ~DnsQueue(void);
  50
  51   size_t Size() const { return size_; }
  52   void Clear();
  53
  54   // Push takes an unterminated string of the given length
  55   // and inserts it into the queue for later
  56   // extraction by read.  For each successful push(), there
  57   // can later be a corresponding read() to extracted the text.
  58   // The string must not contain an embedded null terminator
  59   // Exactly length chars are written, or the push fails (where
  60   // "fails" means nothing is written).
  61   // Returns true for success, false for failure (nothing written).
  62   PushResult Push(const char* source, const size_t length);
  63
  64   PushResult Push(std::string source) {
  65     return Push(source.c_str(), source.length());
  66   }
  67
  68   // Extract the next available string from the buffer.
  69   // If the buffer is empty, then return false.
  70   bool Pop(std::string* out_string);
  71
  72  private:
  73   bool Validate();  // Checks that all internal data is valid.
  74
  75   const scoped_ptr<char[]> buffer_;  // Circular buffer, plus extra char ('\0').
  76   const BufferSize buffer_size_;  // Size one smaller than allocated space.
  77   const BufferSize buffer_sentinel_;  // Index of extra '\0' at end of buffer_.
  78
  79   // If writable_ == readable_, then the buffer is empty.
  80   BufferSize readable_;  // Next readable char in buffer_.
  81   BufferSize writeable_;  // The next space in buffer_ to push.
  82
  83   // Number of queued strings
  84   size_t size_;
  85
  86   DISALLOW_COPY_AND_ASSIGN(DnsQueue);
  87 };  // class DnsQueue
  88
  89 }  // namespace dns_prefetch
  90
  91 #endif  // COMPONENTS_DNS_PREFETCH_RENDERER_PREDICTOR_QUEUE_H__