From f57bead0f239a0f89d6d7e35d6ed6595d93c2e5b Mon Sep 17 00:00:00 2001
From: Molly Xu <mollyxu@fb.com>
Date: Mon, 20 Oct 2025 12:27:21 -0700
Subject: [PATCH] modified getNumFrames

---
 src/torchcodec/_core/SingleStreamDecoder.cpp | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/src/torchcodec/_core/SingleStreamDecoder.cpp b/src/torchcodec/_core/SingleStreamDecoder.cpp
index d06c47922..4f25dc56a 100644
--- a/src/torchcodec/_core/SingleStreamDecoder.cpp
+++ b/src/torchcodec/_core/SingleStreamDecoder.cpp
@@ -1583,7 +1583,16 @@ std::optional<int64_t> SingleStreamDecoder::getNumFrames(
     case SeekMode::exact:
       return streamMetadata.numFramesFromContent.value();
     case SeekMode::approximate: {
-      return streamMetadata.numFramesFromHeader;
+      if (streamMetadata.numFramesFromHeader.has_value()) {
+        return streamMetadata.numFramesFromHeader;
+      } else if (
+          streamMetadata.averageFpsFromHeader.has_value() &&
+          streamMetadata.durationSecondsFromHeader.has_value()) {
+        return static_cast<int64_t>(std::lround(
+            streamMetadata.averageFpsFromHeader.value() *
+            streamMetadata.durationSecondsFromHeader.value()));
+      }
+      return std::nullopt;
     }
     default:
       TORCH_CHECK(false, "Unknown SeekMode");