1// Copyright 2015 Google Inc.
2//
3// Licensed under the Apache License, Version 2.0 (the "License");
4// you may not use this file except in compliance with the License.
5// You may obtain a copy of the License at
6//
7// http://www.apache.org/licenses/LICENSE-2.0
8//
9// Unless required by applicable law or agreed to in writing, software
10// distributed under the License is distributed on an "AS IS" BASIS,
11// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12// See the License for the specific language governing permissions and
13// limitations under the License.
14//
15////////////////////////////////////////////////////////////////////////////////
16//
17// The cache layer works as follows:
18// The cache is implemented as a vector (of size 'cache_size') of shared
19// pointers to pages recently used. The least recently used page is stored
20// at the begining of the vector, the most recent at the end.
21
22#include "src/binary_parse/cached_paged_byte_array.h"
23
24#include <cstddef>
25
26namespace piex {
27namespace binary_parse {
28
29CachedPagedByteArray::CachedPagedByteArray(
30 const PagedByteArray* paged_byte_array, size_t cache_size)
31 : paged_byte_array_(paged_byte_array), cache_size_(cache_size) {}
32
33void CachedPagedByteArray::getPage(size_t page_index,
34 const unsigned char** begin,
35 const unsigned char** end,
36 PagedByteArray::PagePtr* page) const {
37 std::lock_guard<std::mutex> lock(mutex_);
38 size_t cache_index;
39 if (getFromCache(page_index, &cache_index)) {
40 // Cache hit, retrieve the page from the cache.
41 *begin = cached_pages_[cache_index].begin;
42 *end = cached_pages_[cache_index].end;
43 *page = cached_pages_[cache_index].page;
44
45 // Remove the page to insert it at the end of the cache later.
46 cached_pages_.erase(cached_pages_.begin() +
47 static_cast<std::ptrdiff_t>(cache_index));
48 } else {
49 // Cache miss, ask PagedByteArray to load the page.
50 paged_byte_array_->getPage(page_index, begin, end, page);
51
52 // If the cache is full, remove the first (least recently used) page.
53 if (cached_pages_.size() >= cache_size_) {
54 cached_pages_.erase(cached_pages_.begin());
55 }
56 }
57
58 // Cache the most recently used page to the end of the vector.
59 CachedPage cache_page;
60 cache_page.index = page_index;
61 cache_page.page = *page;
62 cache_page.begin = *begin;
63 cache_page.end = *end;
64 cached_pages_.push_back(cache_page);
65}
66
67bool CachedPagedByteArray::getFromCache(size_t page_index,
68 size_t* cache_index) const {
69 for (size_t i = 0; i < cached_pages_.size(); ++i) {
70 if (cached_pages_[i].index == page_index) {
71 *cache_index = i;
72 return true;
73 }
74 }
75 return false;
76}
77
78} // namespace binary_parse
79} // namespace piex
80