First-order-FEC recoverability calculation

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 762813385632171397377dc6845808a22cc3495b..94b916c6c54bc62eb6a6fb5c2585d4bfa0980c53 100644
--- a/webrtc/test/fuzzers/transport_feedback_packet_loss_tracker_fuzzer.cc
+++ b/webrtc/test/fuzzers/transport_feedback_packet_loss_tracker_fuzzer.cc
@@ -19,6 +19,15 @@ namespace webrtc {
 
 namespace {
 
+template <typename T>
+T FuzzInput(const uint8_t*& data, size_t& size) {
+  RTC_CHECK(size >= sizeof(T));
+  T rc = ByteReader<T>::ReadBigEndian(data);
+  data += sizeof(T);
+  size -= sizeof(T);
+  return rc;
+}
+
 using TransportFeedback = rtcp::TransportFeedback;
 
 class FuzzTransportFeedback final : public rtcp::TransportFeedbackInterface {
@@ -116,25 +125,33 @@ class FuzzTransportFeedback final : public rtcp::TransportFeedbackInterface {
 }  // namespace
 
 void FuzzOneInput(const uint8_t* data, size_t size) {
-  if (size < sizeof(uint32_t)) {
+  if (size < 3 * sizeof(uint16_t)) {
     return;
   }
   constexpr size_t kSeqNumHalf = 0x8000u;
+
+  // Produce min-window, max-window and min_pairs_num_for_rplr, such that:
+  // a. min <= max <= kSeqNumHalf
+  // b. 1 <= min_pairs_num_for_rplr <= max - 1 (fencepost)
+  // Because of /b/, both min and max can't be below 2.
+
   const size_t window_size_1 = std::min(
       kSeqNumHalf,
-      static_cast<size_t>(std::max(static_cast<uint16_t>(1),
-                                   ByteReader<uint16_t>::ReadBigEndian(data))));
-  data += sizeof(uint16_t);
+      static_cast<size_t>(std::max(static_cast<uint16_t>(2),
+                                   FuzzInput<uint16_t>(data, size))));
   const size_t window_size_2 = std::min(
       kSeqNumHalf,
+      static_cast<size_t>(std::max(static_cast<uint16_t>(2),
+                                   FuzzInput<uint16_t>(data, size))));
+  const size_t min_window_size = std::min(window_size_1, window_size_2);
+  const size_t max_window_size = std::max(window_size_1, window_size_2);
+  const size_t min_pairs_num_for_rplr = std::min(
+      max_window_size - 1,
       static_cast<size_t>(std::max(static_cast<uint16_t>(1),
-                                   ByteReader<uint16_t>::ReadBigEndian(data))));
-  data += sizeof(uint16_t);
-  size -= 2 * sizeof(uint16_t);
+                                   FuzzInput<uint16_t>(data, size))));
 
   TransportFeedbackPacketLossTracker tracker(
-      std::min(window_size_1, window_size_2),
-      std::max(window_size_1, window_size_2));
+      min_window_size, max_window_size, min_pairs_num_for_rplr);
   FuzzTransportFeedback feedback(rtc::ArrayView<const uint8_t>(data, size));
   while (!feedback.ended()) {
     tracker.OnReceivedTransportFeedback(feedback);