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