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 837ae883f6c83f213c9951d0f21832d363359cc7..38452ade18a5c2ca9e264c71df475f2a8011b75a 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;
+}
+
 class TransportFeedbackGenerator {
  public:
   explicit TransportFeedbackGenerator(rtc::ArrayView<const uint8_t> data)
@@ -81,23 +90,30 @@ class TransportFeedbackGenerator {
 }  // 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<size_t>(
       kSeqNumHalf,
-      std::max<uint16_t>(1, ByteReader<uint16_t>::ReadBigEndian(data)));
-  data += sizeof(uint16_t);
+      std::max<uint16_t>(2, FuzzInput<uint16_t>(&data, &size)));
   const size_t window_size_2 = std::min<size_t>(
       kSeqNumHalf,
-      std::max<uint16_t>(1, ByteReader<uint16_t>::ReadBigEndian(data)));
-  data += sizeof(uint16_t);
-  size -= 2 * sizeof(uint16_t);
+      std::max<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<size_t>(
+      max_window_size - 1,
+      std::max<uint16_t>(1, 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);
   TransportFeedbackGenerator feedback_generator(
       rtc::ArrayView<const uint8_t>(data, size));
   while (!feedback_generator.ended()) {