nearby: snapshot of cl/313536507

Signed-off-by: Alexey Polyudov <apolyudov@google.com>
Change-Id: I8936b527079074d5c3af5531b9245063767cc4a7

cpp/platform_v2/base/prng.cc

@@ -0,0 +1,45 @@
+#include "platform_v2/base/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