audio_codec_pair_id.cc (3592B)
1 /* 2 * Copyright (c) 2018 The WebRTC project authors. All Rights Reserved. 3 * 4 * Use of this source code is governed by a BSD-style license 5 * that can be found in the LICENSE file in the root of the source 6 * tree. An additional intellectual property rights grant can be found 7 * in the file PATENTS. All contributing project authors may 8 * be found in the AUTHORS file in the root of the source tree. 9 */ 10 11 #include "api/audio_codecs/audio_codec_pair_id.h" 12 13 #include <atomic> 14 #include <cstdint> 15 16 #include "rtc_base/checks.h" 17 18 namespace webrtc { 19 20 namespace { 21 22 // Returns a new value that it has never returned before. You may call it at 23 // most 2^63 times in the lifetime of the program. Note: The returned values 24 // may be easily predictable. 25 uint64_t GetNextId() { 26 static std::atomic<uint64_t> next_id(0); 27 28 // Atomically increment `next_id`, and return the previous value. Relaxed 29 // memory order is sufficient, since all we care about is that different 30 // callers return different values. 31 const uint64_t new_id = next_id.fetch_add(1, std::memory_order_relaxed); 32 33 // This check isn't atomic with the increment, so if we start 2^63 + 1 34 // invocations of GetNextId() in parallel, the last one to do the atomic 35 // increment could return the ID 0 before any of the others had time to 36 // trigger this DCHECK. We blithely assume that this won't happen. 37 RTC_DCHECK_LT(new_id, uint64_t{1} << 63) << "Used up all ID values"; 38 39 return new_id; 40 } 41 42 // Make an integer ID more unpredictable. This is a 1:1 mapping, so you can 43 // feed it any value, but the idea is that you can feed it a sequence such as 44 // 0, 1, 2, ... and get a new sequence that isn't as trivially predictable, so 45 // that users won't rely on it being consecutive or increasing or anything like 46 // that. 47 constexpr uint64_t ObfuscateId(uint64_t id) { 48 // Any nonzero coefficient that's relatively prime to 2^64 (that is, any odd 49 // number) and any constant will give a 1:1 mapping. These high-entropy 50 // values will prevent the sequence from being trivially predictable. 51 // 52 // Both the multiplication and the addition going to overflow almost always, 53 // but that's fine---we *want* arithmetic mod 2^64. 54 return uint64_t{0x85fdb20e1294309a} + uint64_t{0xc516ef5c37462469} * id; 55 } 56 57 // The first ten values. Verified against the Python function 58 // 59 // def f(n): 60 // return (0x85fdb20e1294309a + 0xc516ef5c37462469 * n) % 2**64 61 // 62 // Callers should obviously not depend on these exact values... 63 // 64 // (On Visual C++, we have to disable warning C4307 (integral constant 65 // overflow), even though unsigned integers have perfectly well-defined 66 // overflow behavior.) 67 #ifdef _MSC_VER 68 #pragma warning(push) 69 #pragma warning(disable : 4307) 70 #endif 71 static_assert(ObfuscateId(0) == uint64_t{0x85fdb20e1294309a}, ""); 72 static_assert(ObfuscateId(1) == uint64_t{0x4b14a16a49da5503}, ""); 73 static_assert(ObfuscateId(2) == uint64_t{0x102b90c68120796c}, ""); 74 static_assert(ObfuscateId(3) == uint64_t{0xd5428022b8669dd5}, ""); 75 static_assert(ObfuscateId(4) == uint64_t{0x9a596f7eefacc23e}, ""); 76 static_assert(ObfuscateId(5) == uint64_t{0x5f705edb26f2e6a7}, ""); 77 static_assert(ObfuscateId(6) == uint64_t{0x24874e375e390b10}, ""); 78 static_assert(ObfuscateId(7) == uint64_t{0xe99e3d93957f2f79}, ""); 79 static_assert(ObfuscateId(8) == uint64_t{0xaeb52cefccc553e2}, ""); 80 static_assert(ObfuscateId(9) == uint64_t{0x73cc1c4c040b784b}, ""); 81 #ifdef _MSC_VER 82 #pragma warning(pop) 83 #endif 84 85 } // namespace 86 87 AudioCodecPairId AudioCodecPairId::Create() { 88 return AudioCodecPairId(ObfuscateId(GetNextId())); 89 } 90 91 } // namespace webrtc