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