Initial public release of nearby library

Signed-off-by: Alexey Polyudov <apolyudov@google.com>
Change-Id: I85d490e448375aa4eb7b09680757c02fff1420b4

cpp/platform/prng.cc

@@ -0,0 +1,59 @@
+// Copyright 2020 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "platform/prng.h"
+
+#include <limits>
+
+#include "absl/time/clock.h"
+
+namespace location {
+namespace nearby {
+
+#define UNSIGNED_INT_BITMASK (std::numeric_limits<unsigned int>::max())
+
+Prng::Prng() {
+  // absl::GetCurrentTimeNanos() returns 64 bits, but srand() wants an unsigned
+  // int, so we may have to lose some of those 64 bits.
+  //
+  // The lower bits of the current-time-in-nanos are likely to have more entropy
+  // than the upper bits, so choose the former.
+  srand(static_cast<unsigned int>(absl::GetCurrentTimeNanos() &
+                                  UNSIGNED_INT_BITMASK));
+}
+
+Prng::~Prng() {
+  // Nothing to do.
+}
+
+#define RANDOM_BYTE (rand() & 0x0FF)  // NOLINT
+
+std::int32_t Prng::nextInt32() {
+  return (static_cast<std::int32_t>(RANDOM_BYTE) << 24) |
+         (static_cast<std::int32_t>(RANDOM_BYTE) << 16) |
+         (static_cast<std::int32_t>(RANDOM_BYTE) << 8) |
+         (static_cast<std::int32_t>(RANDOM_BYTE));
+}
+
+std::uint32_t Prng::nextUInt32() {
+  return static_cast<std::uint32_t>(nextInt32());
+}
+
+std::int64_t Prng::nextInt64() {
+  return (static_cast<std::int64_t>(nextInt32()) << 32) |
+         (static_cast<std::int64_t>(nextInt32()));
+}
+
+}  // namespace nearby
+}  // namespace location