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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 CHROME_RENDERER_NET_PREDICTOR_QUEUE_H__ | |
25 #define CHROME_RENDERER_NET_PREDICTOR_QUEUE_H__ | |
26 | |
27 #include <string> | |
28 | |
29 #include "base/basictypes.h" | |
30 #include "base/memory/scoped_ptr.h" | |
31 | |
32 class DnsQueue { | |
33 public: | |
34 // BufferSize is a signed type used for indexing into a buffer. | |
35 typedef int32 BufferSize; | |
36 | |
37 enum PushResult { SUCCESSFUL_PUSH, OVERFLOW_PUSH, REDUNDANT_PUSH }; | |
38 | |
39 // The size specified in the constructor creates a buffer large enough | |
40 // to hold at most one string of that length, or "many" | |
41 // strings of considerably shorter length. Note that strings | |
42 // are padded internally with a terminal '\0" while stored, | |
43 // so if you are trying to be precise and get N strings of | |
44 // length K to fit, you should actually construct a buffer with | |
45 // an internal size of N*(K+1). | |
46 explicit DnsQueue(BufferSize size); | |
47 ~DnsQueue(void); | |
48 | |
49 size_t Size() const { return size_; } | |
50 void Clear(); | |
51 | |
52 // Push takes an unterminated string of the given length | |
53 // and inserts it into the queue for later | |
54 // extraction by read. For each successful push(), there | |
55 // can later be a corresponding read() to extracted the text. | |
56 // The string must not contain an embedded null terminator | |
57 // Exactly length chars are written, or the push fails (where | |
58 // "fails" means nothing is written). | |
59 // Returns true for success, false for failure (nothing written). | |
60 PushResult Push(const char* source, const size_t length); | |
61 | |
62 PushResult Push(std::string source) { | |
63 return Push(source.c_str(), source.length()); | |
64 } | |
65 | |
66 // Extract the next available string from the buffer. | |
67 // If the buffer is empty, then return false. | |
68 bool Pop(std::string* out_string); | |
69 | |
70 private: | |
71 bool Validate(); // Checks that all internal data is valid. | |
72 | |
73 const scoped_ptr<char[]> buffer_; // Circular buffer, plus extra char ('\0'). | |
74 const BufferSize buffer_size_; // Size one smaller than allocated space. | |
75 const BufferSize buffer_sentinel_; // Index of extra '\0' at end of buffer_. | |
76 | |
77 // If writable_ == readable_, then the buffer is empty. | |
78 BufferSize readable_; // Next readable char in buffer_. | |
79 BufferSize writeable_; // The next space in buffer_ to push. | |
80 | |
81 // Number of queued strings | |
82 size_t size_; | |
83 | |
84 DISALLOW_COPY_AND_ASSIGN(DnsQueue); | |
85 }; // class DnsQueue | |
86 | |
87 #endif // CHROME_RENDERER_NET_PREDICTOR_QUEUE_H__ | |
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