1 //
2 // Copyright 2012 The Android Open Source Project
3 //
4 // Manage a resource ID cache.
5
6 #define LOG_TAG "ResourceIdCache"
7
8 #include <utils/String16.h>
9 #include <utils/Log.h>
10 #include "ResourceIdCache.h"
11 #include <map>
12
13 static size_t mHits = 0;
14 static size_t mMisses = 0;
15 static size_t mCollisions = 0;
16
17 static const size_t MAX_CACHE_ENTRIES = 2048;
18 static const android::String16 TRUE16("1");
19 static const android::String16 FALSE16("0");
20
21 struct CacheEntry {
22 // concatenation of the relevant strings into a single instance
23 android::String16 hashedName;
24 uint32_t id;
25
CacheEntryCacheEntry26 CacheEntry() {}
CacheEntryCacheEntry27 CacheEntry(const android::String16& name, uint32_t resId) : hashedName(name), id(resId) { }
28 };
29
30 static std::map< uint32_t, CacheEntry > mIdMap;
31
32
33 // djb2; reasonable choice for strings when collisions aren't particularly important
hashround(uint32_t hash,int c)34 static inline uint32_t hashround(uint32_t hash, int c) {
35 return ((hash << 5) + hash) + c; /* hash * 33 + c */
36 }
37
hash(const android::String16 & hashableString)38 static uint32_t hash(const android::String16& hashableString) {
39 uint32_t hash = 5381;
40 const char16_t* str = hashableString.string();
41 while (int c = *str++) hash = hashround(hash, c);
42 return hash;
43 }
44
45 namespace android {
46
makeHashableName(const android::String16 & package,const android::String16 & type,const android::String16 & name,bool onlyPublic)47 static inline String16 makeHashableName(const android::String16& package,
48 const android::String16& type,
49 const android::String16& name,
50 bool onlyPublic) {
51 String16 hashable = String16(name);
52 hashable += type;
53 hashable += package;
54 hashable += (onlyPublic ? TRUE16 : FALSE16);
55 return hashable;
56 }
57
lookup(const android::String16 & package,const android::String16 & type,const android::String16 & name,bool onlyPublic)58 uint32_t ResourceIdCache::lookup(const android::String16& package,
59 const android::String16& type,
60 const android::String16& name,
61 bool onlyPublic) {
62 const String16 hashedName = makeHashableName(package, type, name, onlyPublic);
63 const uint32_t hashcode = hash(hashedName);
64 std::map<uint32_t, CacheEntry>::iterator item = mIdMap.find(hashcode);
65 if (item == mIdMap.end()) {
66 // cache miss
67 mMisses++;
68 return 0;
69 }
70
71 // legit match?
72 if (hashedName == (*item).second.hashedName) {
73 mHits++;
74 return (*item).second.id;
75 }
76
77 // collision
78 mCollisions++;
79 mIdMap.erase(hashcode);
80 return 0;
81 }
82
83 // returns the resource ID being stored, for callsite convenience
store(const android::String16 & package,const android::String16 & type,const android::String16 & name,bool onlyPublic,uint32_t resId)84 uint32_t ResourceIdCache::store(const android::String16& package,
85 const android::String16& type,
86 const android::String16& name,
87 bool onlyPublic,
88 uint32_t resId) {
89 if (mIdMap.size() < MAX_CACHE_ENTRIES) {
90 const String16 hashedName = makeHashableName(package, type, name, onlyPublic);
91 const uint32_t hashcode = hash(hashedName);
92 mIdMap[hashcode] = CacheEntry(hashedName, resId);
93 }
94 return resId;
95 }
96
dump()97 void ResourceIdCache::dump() {
98 printf("ResourceIdCache dump:\n");
99 printf("Size: %zd\n", mIdMap.size());
100 printf("Hits: %zd\n", mHits);
101 printf("Misses: %zd\n", mMisses);
102 printf("(Collisions: %zd)\n", mCollisions);
103 }
104
105 }
106