1 /*
2  * Copyright 2016 Google Inc.
3  *
4  * Use of this source code is governed by a BSD-style license that can be
5  * found in the LICENSE file.
6  */
7 
8 #ifndef SkStreamBuffer_DEFINED
9 #define SkStreamBuffer_DEFINED
10 
11 #include "SkData.h"
12 #include "SkStream.h"
13 #include "SkTypes.h"
14 #include "../private/SkTHash.h"
15 
16 /**
17  *  Helper class for reading from a stream that may not have all its data
18  *  available yet.
19  *
20  *  Used by GIFImageReader, and currently set up for that use case.
21  *
22  *  Buffers up to 256 * 3 bytes (256 colors, with 3 bytes each) to support GIF.
23  *  FIXME (scroggo): Make this more general purpose?
24  */
25 class SkStreamBuffer : SkNoncopyable {
26 public:
27     SkStreamBuffer(std::unique_ptr<SkStream>);
28     ~SkStreamBuffer();
29 
30     /**
31      *  Return a pointer the buffered data.
32      *
33      *  The number of bytes buffered is the number passed to buffer()
34      *  after the last call to flush().
35      */
36     const char* get() const;
37 
38     /**
39      *  Buffer from the stream into our buffer.
40      *
41      *  If this call returns true, get() can be used to access |bytes| bytes
42      *  from the stream. In addition, markPosition() can be called to mark this
43      *  position and enable calling getAtPosition() later to retrieve |bytes|
44      *  bytes.
45      *
46      *  @param bytes Total number of bytes desired.
47      *
48      *  @return Whether all bytes were successfully buffered.
49      */
50     bool buffer(size_t bytes);
51 
52     /**
53      *  Flush the buffer.
54      *
55      *  After this call, no bytes are buffered.
56      */
57     void flush() {
58         if (fHasLengthAndPosition) {
59             if (fTrulyBuffered < fBytesBuffered) {
60                 fStream->move(fBytesBuffered - fTrulyBuffered);
61             }
62             fTrulyBuffered = 0;
63         }
64         fPosition += fBytesBuffered;
65         fBytesBuffered = 0;
66     }
67 
68     /**
69      *  Mark the current position in the stream to return to it later.
70      *
71      *  This is the position of the start of the buffer. After this call, a
72      *  a client can call getDataAtPosition to retrieve all the bytes currently
73      *  buffered.
74      *
75      *  @return size_t Position which can be passed to getDataAtPosition later
76      *      to retrieve the data currently buffered.
77      */
78     size_t markPosition();
79 
80     /**
81      *  Retrieve data at position, as previously marked by markPosition().
82      *
83      *  @param position Position to retrieve data, as marked by markPosition().
84      *  @param length   Amount of data required at position.
85      *  @return SkData The data at position.
86      */
87     sk_sp<SkData> getDataAtPosition(size_t position, size_t length);
88 
89 private:
90     static constexpr size_t kMaxSize = 256 * 3;
91 
92     std::unique_ptr<SkStream>   fStream;
93     size_t                      fPosition;
94     char                        fBuffer[kMaxSize];
95     size_t                      fBytesBuffered;
96     // If the stream has a length and position, we can make two optimizations:
97     // - We can skip buffering
98     // - During parsing, we can store the position and size of data that is
99     //   needed later during decoding.
100     const bool                  fHasLengthAndPosition;
101     // When fHasLengthAndPosition is true, we do not need to actually buffer
102     // inside buffer(). We'll buffer inside get(). This keeps track of how many
103     // bytes we've buffered inside get(), for the (non-existent) case of:
104     //  buffer(n)
105     //  get()
106     //  buffer(n + u)
107     //  get()
108     // The second call to get() needs to only truly buffer the part that was
109     // not already buffered.
110     mutable size_t              fTrulyBuffered;
111     // Only used if !fHasLengthAndPosition. In that case, markPosition will
112     // copy into an SkData, stored here.
113     SkTHashMap<size_t, SkData*> fMarkedData;
114 };
115 #endif // SkStreamBuffer_DEFINED
116 
117