1 /*
2  * Copyright (C) 2021 The Android Open Source Project
3  *
4  * Licensed under the Apache License, Version 2.0 (the "License");
5  * you may not use this file except in compliance with the License.
6  * You may obtain a copy of the License at
7  *
8  *      http://www.apache.org/licenses/LICENSE-2.0
9  *
10  * Unless required by applicable law or agreed to in writing, software
11  * distributed under the License is distributed on an "AS IS" BASIS,
12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13  * See the License for the specific language governing permissions and
14  * limitations under the License.
15  */
16 
17 #ifndef INCLUDE_PERFETTO_TRACING_INTERNAL_COMPILE_TIME_HASH_H_
18 #define INCLUDE_PERFETTO_TRACING_INTERNAL_COMPILE_TIME_HASH_H_
19 
20 #include <stddef.h>
21 #include <stdint.h>
22 
23 namespace perfetto {
24 namespace internal {
25 
26 // A helper class which computes a 64-bit hash of the input data at compile
27 // time. The algorithm used is FNV-1a as it is fast and easy to implement and
28 // has relatively few collisions.
29 // WARNING: This hash function should not be used for any cryptographic purpose.
30 class CompileTimeHash {
31  public:
32   // Creates an empty hash object
CompileTimeHash()33   constexpr inline CompileTimeHash() {}
34 
35   // Hashes a byte array.
Update(const char * data,size_t size)36   constexpr inline CompileTimeHash Update(const char* data, size_t size) const {
37     return CompileTimeHash(HashRecursively(kFnv1a64OffsetBasis, data, size));
38   }
39 
digest()40   constexpr inline uint64_t digest() const { return result_; }
41 
42  private:
CompileTimeHash(uint64_t result)43   constexpr inline CompileTimeHash(uint64_t result) : result_(result) {}
44 
HashRecursively(uint64_t value,const char * data,size_t size)45   static constexpr inline uint64_t HashRecursively(uint64_t value,
46                                                    const char* data,
47                                                    size_t size) {
48     return !size ? value
49                  : HashRecursively(
50                        (value ^ static_cast<uint8_t>(*data)) * kFnv1a64Prime,
51                        data + 1, size - 1);
52   }
53 
54   static constexpr uint64_t kFnv1a64OffsetBasis = 0xcbf29ce484222325;
55   static constexpr uint64_t kFnv1a64Prime = 0x100000001b3;
56 
57   uint64_t result_ = kFnv1a64OffsetBasis;
58 };
59 
60 }  // namespace internal
61 }  // namespace perfetto
62 
63 #endif  // INCLUDE_PERFETTO_TRACING_INTERNAL_COMPILE_TIME_HASH_H_
64