R/PLR calculation - time-based window

webrtc/test/fuzzers/transport_feedback_packet_loss_tracker_fuzzer.cc

Index: webrtc/test/fuzzers/transport_feedback_packet_loss_tracker_fuzzer.cc
diff --git a/webrtc/test/fuzzers/transport_feedback_packet_loss_tracker_fuzzer.cc b/webrtc/test/fuzzers/transport_feedback_packet_loss_tracker_fuzzer.cc
index 5c3cfcba289b44103b7326cfc8b09c11f2fa7ead..c8e261982d12cb9ce23fc0da69d9a74b1e98e428 100644
--- a/webrtc/test/fuzzers/transport_feedback_packet_loss_tracker_fuzzer.cc
+++ b/webrtc/test/fuzzers/transport_feedback_packet_loss_tracker_fuzzer.cc
@@ -109,16 +109,12 @@ bool Setup(const uint8_t** data,
 
   constexpr size_t kSeqNumHalf = 0x8000u;
 
-  // 0x8000 >= max_window_size >= plr_min_num_packets > rplr_min_num_pairs >= 1
-  // (The distribution isn't uniform, but it's enough; more would be overkill.)
-  const size_t max_window_size = FuzzInRange(data, size, 2, kSeqNumHalf);
-  const size_t plr_min_num_packets =
-      FuzzInRange(data, size, 2, max_window_size);
-  const size_t rplr_min_num_pairs =
-      FuzzInRange(data, size, 1, plr_min_num_packets - 1);
+  const int64_t max_window_size_ms = FuzzInRange(data, size, 1, 1 << 16);
+  const size_t plr_min_num_packets = FuzzInRange(data, size, 1, kSeqNumHalf);
+  const size_t rplr_min_num_pairs = FuzzInRange(data, size, 1, kSeqNumHalf - 1);
 
   tracker->reset(new TransportFeedbackPacketLossTracker(
-      max_window_size, plr_min_num_packets, rplr_min_num_pairs));
+      max_window_size_ms, plr_min_num_packets, rplr_min_num_pairs));
 
   return true;
 }
@@ -140,10 +136,56 @@ bool FuzzSequenceNumberDelta(const uint8_t** data,
   return true;
 }
 
+bool FuzzClockAdvancement(const uint8_t** data,
+                          size_t* size,
+                          int64_t* time_ms) {
+  // Fuzzing 64-bit worth of delta would be extreme overkill, as 32-bit is
+  // already ~49 days long. We'll fuzz deltas up to a smaller value, and this
+  // way also guarantee that wrap-around is impossible, as in real life.
+
+  // Higher likelihood for more likely cases:
+  //  5% chance of delta = 0.
+  // 20% chance of delta in range [1 : 10] (uniformly distributed)
+  // 55% chance of delta in range [11 : 500] (uniformly distributed)
+  // 20% chance of delta in range [501 : 10000] (uniformly distributed)
+  struct ProbabilityDistribution {
+    float probability;
+    size_t lower;
+    size_t higher;
+  };
+  constexpr ProbabilityDistribution clock_probability_distribution[] = {
+      {0.05, 0, 0}, {0.20, 1, 10}, {0.55, 11, 500}, {0.20, 501, 10000}};
+
+  if (*size < sizeof(uint8_t)) {
+    return false;
+  }
+  const float fuzzed = FuzzInput<uint8_t>(data, size) / 256.0f;
+
+  float cumulative_probability = 0;
+  for (const auto& dist : clock_probability_distribution) {
+    cumulative_probability += dist.probability;
+    if (fuzzed < cumulative_probability) {
+      if (dist.lower == dist.higher) {
+        *time_ms += dist.lower;
+        return true;
+      } else if (*size < sizeof(uint16_t)) {
+        return false;
+      } else {
+        *time_ms += FuzzInRange(data, size, dist.lower, dist.higher);
+        return true;
+      }
+    }
+  }
+
+  RTC_NOTREACHED();
+  return false;
+}
+
 bool FuzzPacketSendBlock(
     std::unique_ptr<TransportFeedbackPacketLossTracker>& tracker,
     const uint8_t** data,
-    size_t* size) {
+    size_t* size,
+    int64_t* time_ms) {
   // We want to test with block lengths between 1 and 2^16, inclusive.
   if (*size < sizeof(uint8_t)) {
     return false;
@@ -155,16 +197,24 @@ bool FuzzPacketSendBlock(
     return false;
   }
   uint16_t seq_num = FuzzInput<uint16_t>(data, size);
-  tracker->OnPacketAdded(seq_num);
+  tracker->OnPacketAdded(seq_num, *time_ms);
   tracker->Validate();
 
+  bool may_continue = FuzzClockAdvancement(data, size, time_ms);
+  if (!may_continue) {
+    return false;
+  }
+
   for (size_t i = 1; i < packet_block_len; i++) {
     uint16_t delta;
-    bool may_continue = FuzzSequenceNumberDelta(data, size, &delta);
+    may_continue = FuzzSequenceNumberDelta(data, size, &delta);
+    if (!may_continue)
+      return false;
+    may_continue = FuzzClockAdvancement(data, size, time_ms);
     if (!may_continue)
       return false;
     seq_num += delta;
-    tracker->OnPacketAdded(seq_num);
+    tracker->OnPacketAdded(seq_num, *time_ms);
     tracker->Validate();
   }
 
@@ -206,8 +256,15 @@ void FuzzOneInput(const uint8_t* data, size_t size) {
 
   may_continue = Setup(&data, &size, &tracker);
 
+  // We never expect this to wrap around, so it makes sense to just start with
+  // a sane value, and keep on incrementing by a fuzzed delta.
+  if (size < sizeof(uint32_t)) {
+    return;
+  }
+  int64_t time_ms = FuzzInput<uint32_t>(&data, &size);
+
   while (may_continue) {
-    may_continue = FuzzPacketSendBlock(tracker, &data, &size);
+    may_continue = FuzzPacketSendBlock(tracker, &data, &size, &time_ms);
     if (!may_continue) {
       return;
     }