Fix the integer overflow when converting PTS to rational type (#1185)

`AVRational` stores the data as a tuple of int (`int32_t`). 
`AVPacket::pts` stores PTS as `int64_t`.

There are cases where the `AVPacket::pts` value is larger than`INT32_MAX`.
In such case, we cannot simply cast the variable type.
We need to perform reduction, otherwise it causes interger overflow.

The test will be added in a subsequent PR.

Author: mthrok

src/libspdl/core/detail/ffmpeg/rational_utils.h

@@ -0,0 +1,42 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#pragma once
+
+#include "libspdl/core/detail/logging.h"
+
+extern "C" {
+#include <libavutil/rational.h>
+}
+
+namespace spdl::core::detail {
+// Check if a given AVRational value falls within a specified window [start,
+// end). Returns true if start <= val < end (half-open interval).
+inline bool is_within_window(
+    const AVRational& val,
+    const AVRational& start,
+    const AVRational& end) {
+  return (av_cmp_q(start, val) <= 0) && (av_cmp_q(val, end) < 0);
+}
+
+inline AVRational to_rational(int64_t val, const AVRational time_base) {
+  AVRational ret;
+  if (av_reduce(
+          &ret.num, &ret.den, val * time_base.num, time_base.den, INT32_MAX)) {
+    // Warn once that reduced PTS may be inexact due to rational reduction
+    // constraints.
+    static bool warned_inexact_pts = false;
+    if (!warned_inexact_pts) {
+      LOG(WARNING) << "PTS estimation was not exact during rational reduction; "
+                      "timestamps might be slightly inaccurate.";
+      warned_inexact_pts = true;
+    }
+  }
+  return ret;
+}
+} // namespace spdl::core::detail

src/libspdl/core/packets.cpp

@@ -9,6 +9,7 @@
 #include <libspdl/core/packets.h>
 
 #include "libspdl/core/detail/ffmpeg/logging.h"
+#include "libspdl/core/detail/ffmpeg/rational_utils.h"
 #include "libspdl/core/detail/tracing.h"
 
 #include <fmt/core.h>
@@ -313,11 +314,12 @@ std::vector<double> get_timestamps(const Packets<media>& packets, bool raw) {
 
   auto [start, end] = packets.timestamp.value_or(NO_WINDOW);
 
+  auto s = av_d2q(start, AV_TIME_BASE);
+  auto e = av_d2q(end, AV_TIME_BASE);
   for (const auto& pkt : pkts) {
-    auto pts = AVRational{static_cast<int>(pkt->pts), 1};
-    auto ts = av_q2d(av_mul_q(pts, packets.time_base));
-    if (raw || (start <= ts && ts < end)) {
-      ret.emplace_back(ts);
+    AVRational pts = detail::to_rational(pkt->pts, packets.time_base);
+    if (raw || detail::is_within_window(pts, s, e)) {
+      ret.emplace_back(av_q2d(pts));
     }
   }
   if (!raw) {