update pr to reflect refactor

Author: Molly Xu

src/torchcodec/_core/Metadata.cpp

@@ -29,6 +29,9 @@ std::optional<double> StreamMetadata::getDurationSeconds(
         return static_cast<double>(numFramesFromHeader.value()) /
             averageFpsFromHeader.value();
       }
+      if (durationSecondsFromContainer.has_value()) {
+        return durationSecondsFromContainer.value();
+      }
       return std::nullopt;
     default:
       TORCH_CHECK(false, "Unknown SeekMode");
@@ -80,13 +83,13 @@ std::optional<int64_t> StreamMetadata::getNumFrames(SeekMode seekMode) const {
           numFramesFromContent.has_value(), "Missing numFramesFromContent");
       return numFramesFromContent.value();
     case SeekMode::approximate: {
+      auto durationSeconds = getDurationSeconds(seekMode);
       if (numFramesFromHeader.has_value()) {
         return numFramesFromHeader.value();
       }
-      if (averageFpsFromHeader.has_value() &&
-          durationSecondsFromHeader.has_value()) {
+      if (averageFpsFromHeader.has_value() && durationSeconds.has_value()) {
         return static_cast<int64_t>(
-            averageFpsFromHeader.value() * durationSecondsFromHeader.value());
+            averageFpsFromHeader.value() * durationSeconds.value());
       }
       return std::nullopt;
     }

src/torchcodec/_core/Metadata.h

@@ -35,6 +35,9 @@ struct StreamMetadata {
   std::optional<double> averageFpsFromHeader;
   std::optional<double> bitRate;
 
+  // Used as fallback in approximate mode when stream duration is unavailable.
+  std::optional<double> durationSecondsFromContainer;
+
   // More accurate duration, obtained by scanning the file.
   // These presentation timestamps are in time base.
   std::optional<int64_t> beginStreamPtsFromContent;

src/torchcodec/_core/SingleStreamDecoder.cpp

@@ -158,16 +158,6 @@ void SingleStreamDecoder::initializeDecoder() {
         ptsToSeconds(formatContext_->duration, defaultTimeBase);
   }
 
-  // Use container duration as fallback for streams missing stream duration
-  if (containerMetadata_.durationSecondsFromHeader.has_value()) {
-    for (auto& streamMetadata : containerMetadata_.allStreamMetadata) {
-      if (!streamMetadata.durationSecondsFromHeader.has_value()) {
-        streamMetadata.durationSecondsFromHeader =
-            containerMetadata_.durationSecondsFromHeader;
-      }
-    }
-  }
-
   if (formatContext_->bit_rate > 0) {
     containerMetadata_.bitRate = formatContext_->bit_rate;
   }
@@ -182,6 +172,13 @@ void SingleStreamDecoder::initializeDecoder() {
     containerMetadata_.bestAudioStreamIndex = bestAudioStream;
   }
 
+  if (containerMetadata_.durationSecondsFromHeader.has_value()) {
+    for (auto& streamMetadata : containerMetadata_.allStreamMetadata) {
+      streamMetadata.durationSecondsFromContainer =
+          containerMetadata_.durationSecondsFromHeader;
+    }
+  }
+
   if (seekMode_ == SeekMode::exact) {
     scanFileAndUpdateMetadataAndIndex();
   }