diff options
Diffstat (limited to 'third_party/libwebrtc/api/audio_codecs/audio_codec_pair_id.cc')
-rw-r--r-- | third_party/libwebrtc/api/audio_codecs/audio_codec_pair_id.cc | 91 |
1 files changed, 91 insertions, 0 deletions
diff --git a/third_party/libwebrtc/api/audio_codecs/audio_codec_pair_id.cc b/third_party/libwebrtc/api/audio_codecs/audio_codec_pair_id.cc new file mode 100644 index 0000000000..6cb51ed6b7 --- /dev/null +++ b/third_party/libwebrtc/api/audio_codecs/audio_codec_pair_id.cc @@ -0,0 +1,91 @@ +/* + * Copyright (c) 2018 The WebRTC project authors. All Rights Reserved. + * + * Use of this source code is governed by a BSD-style license + * that can be found in the LICENSE file in the root of the source + * tree. An additional intellectual property rights grant can be found + * in the file PATENTS. All contributing project authors may + * be found in the AUTHORS file in the root of the source tree. + */ + +#include "api/audio_codecs/audio_codec_pair_id.h" + +#include <atomic> +#include <cstdint> + +#include "rtc_base/checks.h" + +namespace webrtc { + +namespace { + +// Returns a new value that it has never returned before. You may call it at +// most 2^63 times in the lifetime of the program. Note: The returned values +// may be easily predictable. +uint64_t GetNextId() { + static std::atomic<uint64_t> next_id(0); + + // Atomically increment `next_id`, and return the previous value. Relaxed + // memory order is sufficient, since all we care about is that different + // callers return different values. + const uint64_t new_id = next_id.fetch_add(1, std::memory_order_relaxed); + + // This check isn't atomic with the increment, so if we start 2^63 + 1 + // invocations of GetNextId() in parallel, the last one to do the atomic + // increment could return the ID 0 before any of the others had time to + // trigger this DCHECK. We blithely assume that this won't happen. + RTC_DCHECK_LT(new_id, uint64_t{1} << 63) << "Used up all ID values"; + + return new_id; +} + +// Make an integer ID more unpredictable. This is a 1:1 mapping, so you can +// feed it any value, but the idea is that you can feed it a sequence such as +// 0, 1, 2, ... and get a new sequence that isn't as trivially predictable, so +// that users won't rely on it being consecutive or increasing or anything like +// that. +constexpr uint64_t ObfuscateId(uint64_t id) { + // Any nonzero coefficient that's relatively prime to 2^64 (that is, any odd + // number) and any constant will give a 1:1 mapping. These high-entropy + // values will prevent the sequence from being trivially predictable. + // + // Both the multiplication and the addition going to overflow almost always, + // but that's fine---we *want* arithmetic mod 2^64. + return uint64_t{0x85fdb20e1294309a} + uint64_t{0xc516ef5c37462469} * id; +} + +// The first ten values. Verified against the Python function +// +// def f(n): +// return (0x85fdb20e1294309a + 0xc516ef5c37462469 * n) % 2**64 +// +// Callers should obviously not depend on these exact values... +// +// (On Visual C++, we have to disable warning C4307 (integral constant +// overflow), even though unsigned integers have perfectly well-defined +// overflow behavior.) +#ifdef _MSC_VER +#pragma warning(push) +#pragma warning(disable : 4307) +#endif +static_assert(ObfuscateId(0) == uint64_t{0x85fdb20e1294309a}, ""); +static_assert(ObfuscateId(1) == uint64_t{0x4b14a16a49da5503}, ""); +static_assert(ObfuscateId(2) == uint64_t{0x102b90c68120796c}, ""); +static_assert(ObfuscateId(3) == uint64_t{0xd5428022b8669dd5}, ""); +static_assert(ObfuscateId(4) == uint64_t{0x9a596f7eefacc23e}, ""); +static_assert(ObfuscateId(5) == uint64_t{0x5f705edb26f2e6a7}, ""); +static_assert(ObfuscateId(6) == uint64_t{0x24874e375e390b10}, ""); +static_assert(ObfuscateId(7) == uint64_t{0xe99e3d93957f2f79}, ""); +static_assert(ObfuscateId(8) == uint64_t{0xaeb52cefccc553e2}, ""); +static_assert(ObfuscateId(9) == uint64_t{0x73cc1c4c040b784b}, ""); +#ifdef _MSC_VER +#pragma warning(pop) +#endif + +} // namespace + +AudioCodecPairId AudioCodecPairId::Create() { + return AudioCodecPairId(ObfuscateId(GetNextId())); +} + +} // namespace webrtc |