Avoid seeking checks when decoding frames sequentially (#1028)

Author: NicolasHug

src/torchcodec/_core/SingleStreamDecoder.cpp

@@ -626,9 +626,17 @@ FrameOutput SingleStreamDecoder::getFrameAtIndexInternal(
   }
   validateFrameIndex(streamMetadata, frameIndex);
 
-  int64_t pts = getPts(frameIndex);
-  setCursorPtsInSeconds(ptsToSeconds(pts, streamInfo.timeBase));
-  return getNextFrameInternal(preAllocatedOutputTensor);
+  // Only set cursor if we're not decoding sequentially: when decoding
+  // sequentially, we don't need to seek anywhere, so by *not* setting the
+  // cursor we allow canWeAvoidSeeking() to return true early.
+  if (frameIndex != lastDecodedFrameIndex_ + 1) {
+    int64_t pts = getPts(frameIndex);
+    setCursorPtsInSeconds(ptsToSeconds(pts, streamInfo.timeBase));
+  }
+
+  auto result = getNextFrameInternal(preAllocatedOutputTensor);
+  lastDecodedFrameIndex_ = frameIndex;
+  return result;
 }
 
 FrameBatchOutput SingleStreamDecoder::getFramesAtIndices(

src/torchcodec/_core/SingleStreamDecoder.h

@@ -346,6 +346,7 @@ class SingleStreamDecoder {
   bool cursorWasJustSet_ = false;
   int64_t lastDecodedAvFramePts_ = 0;
   int64_t lastDecodedAvFrameDuration_ = 0;
+  int64_t lastDecodedFrameIndex_ = INT64_MIN;
 
   // Stores various internal decoding stats.
   DecodeStats decodeStats_;