From 329a34b626ec26ab6bb7e7f6f8536892a4cb950e Mon Sep 17 00:00:00 2001
From: Nicolas Hug <nicolashug@fb.com>
Date: Tue, 30 Jul 2024 03:24:37 -0700
Subject: [PATCH 1/6] Add 'scan' term to glossary and update docs

---
 docs/source/glossary.rst                      |  5 ++
 src/torchcodec/decoders/_core/_metadata.py    | 52 ++++++++++---------
 .../decoders/_simple_video_decoder.py         |  7 ++-
 3 files changed, 38 insertions(+), 26 deletions(-)

diff --git a/docs/source/glossary.rst b/docs/source/glossary.rst
index cb125f6e4..7afb74a72 100644
--- a/docs/source/glossary.rst
+++ b/docs/source/glossary.rst
@@ -12,3 +12,8 @@ Glossary
        <https://ffmpeg.org/doxygen/trunk/group__lavf__decoding.html#ga757780d38f482deb4d809c6c521fbcc2>`_:
 
         *The best stream is determined according to various heuristics as the most likely to be what the user expects.*
+
+    scan
+       A scan corresponds to an entire pass over a video file, with the purpose
+       of retrieving metadata about the different streams and frames. **It does
+       not involve decoding**, so it is a lot cheaper than decoding the file.
diff --git a/src/torchcodec/decoders/_core/_metadata.py b/src/torchcodec/decoders/_core/_metadata.py
index 83ed807c7..54a8d24c6 100644
--- a/src/torchcodec/decoders/_core/_metadata.py
+++ b/src/torchcodec/decoders/_core/_metadata.py
@@ -24,7 +24,7 @@ class VideoStreamMetadata:
     """Metadata of a single video stream."""
 
     duration_seconds_from_header: Optional[float]
-    """Duration of the stream, in seconds obtained from the header (float or
+    """Duration of the stream, in seconds, obtained from the header (float or
     None). This could be inaccurate."""
     bit_rate: Optional[float]
     """Bit rate of the stream, in seconds (float or None)."""
@@ -38,19 +38,17 @@ class VideoStreamMetadata:
     than ``num_frames_from_header``. We recommend using the
     ``num_frames`` attribute instead. (int or None)."""
     begin_stream_from_content_seconds: Optional[float]
-    """Beginning of the stream in seconds (float or None).
-    This is min(frame.pts) for all frames in this stream."""
+    """Beginning of the stream, in seconds (float or None).
+    This corresponds to the first frame's :term:`pts`. Usually, this is equal to
+    0."""
     end_stream_from_content_seconds: Optional[float]
-    """End of the stream in seconds (float or None).
-    This is max(frame.pts + frame.duration) for all frames in this stream.
-    Note that frames have a pts and duration and the interval defined by
-    [pts, pts + duration) is a half-open interval (the right boundary is open).
-    Therefore no frame is displayed at this time value.
-    Calling
-    SimpleVideoDecoder.get_frame_displayed_at(end_stream_from_content_seconds)
-    will raise a StopIteration exception.
-    If you want to get the last frame you can use [-1] on a SimpleVideoDecoder
-    object."""
+    """End of the stream, in seconds (float or None).
+    This is last_frame.pts + last_frame.duration, so according to our
+    convention, no frame is displayed at this time: read more in
+    :meth:`~torchcodec.decoders.SimpleVideoDecoder.get_frame_displayed_at`.
+    Retrieving the last frame is best done by simply indexing the
+    :class:`~torchcodec.decoders.SimpleVideoDecoder` object with ``[-1]``.
+    """
     codec: Optional[str]
     """Codec (str or None)."""
     width: Optional[int]
@@ -58,15 +56,16 @@ class VideoStreamMetadata:
     height: Optional[int]
     """Height of the frames (int or None)."""
     average_fps_from_header: Optional[float]
-    """Averate fps of the stream (float or None)."""
+    """Averate fps of the stream, obtained from the header (float or None).
+    We recommend using the ``average_fps`` attribute instead."""
     stream_index: int
     """Index of the stream within the video (int)."""
 
     @property
     def num_frames(self) -> Optional[int]:
         """Number of frames in the stream. This corresponds to
-        ``num_frames_from_content`` if it's not None, otherwise it corresponds
-        to ``num_frames_from_header``.
+        ``num_frames_from_content`` if a :term:`scan` was made, otherwise it
+        corresponds to ``num_frames_from_header``.
         """
         if self.num_frames_from_content is not None:
             return self.num_frames_from_content
@@ -76,8 +75,8 @@ def num_frames(self) -> Optional[int]:
     @property
     def duration_seconds(self) -> Optional[float]:
         """Duration of the stream in seconds. We try to calculate the duration
-        from the actual frames if we scanned the frames. Otherwise we fall back
-        to the duration obtained from the header.
+        from the actual frames if a :term:`scan` wsa performed. Otherwise we
+        fall back to ``duration_seconds_from_header``.
         """
         if (
             self.end_stream_from_content_seconds is None
@@ -91,14 +90,17 @@ def duration_seconds(self) -> Optional[float]:
 
     @property
     def average_fps(self) -> Optional[float]:
-        """Average fps of the stream. We try to get the average fps from the
-        actual frames if we scanned the frames. Otherwise we fall back to the
-        fps obtained from the header.
+        """Average fps of the stream. If a :term:`scan` was perfomed, this is
+        computed from the number of frames and the duration of the stream.
+        Otherwise we fall back to ``average_fps_from_header``.
         """
-        if (
-            self.end_stream_from_content_seconds is None
-            or self.begin_stream_from_content_seconds is None
-            or self.num_frames is None
+        if any(
+            attr is None
+            for attr in (
+                self.end_stream_from_content_seconds,
+                self.begin_stream_from_content_seconds,
+                self.num_frames,
+            )
         ):
             return self.average_fps_from_header
         return self.num_frames / (
diff --git a/src/torchcodec/decoders/_simple_video_decoder.py b/src/torchcodec/decoders/_simple_video_decoder.py
index 349fda94f..295164b2f 100644
--- a/src/torchcodec/decoders/_simple_video_decoder.py
+++ b/src/torchcodec/decoders/_simple_video_decoder.py
@@ -77,7 +77,8 @@ def __repr__(self):
 class SimpleVideoDecoder:
     """A single-stream video decoder.
 
-    If the video contains multiple video streams, the :term:`best stream` is used.
+    If the video contains multiple video streams, the :term:`best stream` is
+    used. This decoder always performs a :term:`scan` of the video.
 
     Args:
         source (str, ``Pathlib.path``, ``torch.Tensor``, or bytes): The source of the video.
@@ -266,6 +267,10 @@ def get_frames_at(self, start: int, stop: int, step: int = 1) -> FrameBatch:
     def get_frame_displayed_at(self, seconds: float) -> Frame:
         """Return a single frame displayed at the given timestamp in seconds.
 
+        Each frame has a :term:`pts` and a duration. The convention is that a
+        given frame is displayed during the [frame_pts, frame_pts +
+        frame_duration) interval.
+
         Args:
             seconds (float): The time stamp in seconds when the frame is
                 displayed, i.e. seconds is in

From 9eedbd73430669516371cc8630f568ca2e5a7b2d Mon Sep 17 00:00:00 2001
From: Nicolas Hug <nicolashug@fb.com>
Date: Tue, 30 Jul 2024 03:37:51 -0700
Subject: [PATCH 2/6] mypy garbage

---
 src/torchcodec/decoders/_core/_metadata.py | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/src/torchcodec/decoders/_core/_metadata.py b/src/torchcodec/decoders/_core/_metadata.py
index 54a8d24c6..811f8d939 100644
--- a/src/torchcodec/decoders/_core/_metadata.py
+++ b/src/torchcodec/decoders/_core/_metadata.py
@@ -94,13 +94,10 @@ def average_fps(self) -> Optional[float]:
         computed from the number of frames and the duration of the stream.
         Otherwise we fall back to ``average_fps_from_header``.
         """
-        if any(
-            attr is None
-            for attr in (
-                self.end_stream_from_content_seconds,
-                self.begin_stream_from_content_seconds,
-                self.num_frames,
-            )
+        if (
+            self.end_stream_from_content_seconds is None
+            or self.begin_stream_from_content_seconds is None
+            or self.num_frames is None
         ):
             return self.average_fps_from_header
         return self.num_frames / (

From 6374cc977ac15891f9e465a9c3ccd1edf1c19a83 Mon Sep 17 00:00:00 2001
From: Nicolas Hug <nicolashug@fb.com>
Date: Tue, 30 Jul 2024 03:39:29 -0700
Subject: [PATCH 3/6] readme

---
 README.md | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/README.md b/README.md
index 71c920895..344c33b03 100644
--- a/README.md
+++ b/README.md
@@ -23,12 +23,13 @@ from torchcodec.decoders import SimpleVideoDecoder
 decoder = SimpleVideoDecoder("path/to/video.mp4")
 
 decoder.metadata
-# VideoStreamMetadata:  (Truncated output)
+# VideoStreamMetadata:
 #   num_frames: 250
 #   duration_seconds: 10.0
 #   bit_rate: 31315.0
 #   codec: h264
 #   average_fps: 25.0
+#   ... (truncated output)
 
 len(decoder)  # == decoder.metadata.num_frames!
 # 250

From 8faccc65d348755f90d309dd0e1625ca35ce561c Mon Sep 17 00:00:00 2001
From: Nicolas Hug <nicolashug@fb.com>
Date: Tue, 30 Jul 2024 09:20:34 -0700
Subject: [PATCH 4/6] Rename stuff

---
 src/torchcodec/decoders/_core/_metadata.py    | 24 +++++++++----------
 .../decoders/_simple_video_decoder.py         | 20 ++++++++--------
 test/decoders/test_metadata.py                |  4 ++--
 3 files changed, 24 insertions(+), 24 deletions(-)

diff --git a/src/torchcodec/decoders/_core/_metadata.py b/src/torchcodec/decoders/_core/_metadata.py
index 811f8d939..d2fe7fe94 100644
--- a/src/torchcodec/decoders/_core/_metadata.py
+++ b/src/torchcodec/decoders/_core/_metadata.py
@@ -37,11 +37,11 @@ class VideoStreamMetadata:
     content (the scan doesn't involve decoding). This is more accurate
     than ``num_frames_from_header``. We recommend using the
     ``num_frames`` attribute instead. (int or None)."""
-    begin_stream_from_content_seconds: Optional[float]
+    begin_stream_seconds: Optional[float]
     """Beginning of the stream, in seconds (float or None).
     This corresponds to the first frame's :term:`pts`. Usually, this is equal to
     0."""
-    end_stream_from_content_seconds: Optional[float]
+    end_stream_seconds: Optional[float]
     """End of the stream, in seconds (float or None).
     This is last_frame.pts + last_frame.duration, so according to our
     convention, no frame is displayed at this time: read more in
@@ -79,13 +79,13 @@ def duration_seconds(self) -> Optional[float]:
         fall back to ``duration_seconds_from_header``.
         """
         if (
-            self.end_stream_from_content_seconds is None
-            or self.begin_stream_from_content_seconds is None
+            self.end_stream_seconds is None
+            or self.begin_stream_seconds is None
         ):
             return self.duration_seconds_from_header
         return (
-            self.end_stream_from_content_seconds
-            - self.begin_stream_from_content_seconds
+            self.end_stream_seconds
+            - self.begin_stream_seconds
         )
 
     @property
@@ -95,14 +95,14 @@ def average_fps(self) -> Optional[float]:
         Otherwise we fall back to ``average_fps_from_header``.
         """
         if (
-            self.end_stream_from_content_seconds is None
-            or self.begin_stream_from_content_seconds is None
+            self.end_stream_seconds is None
+            or self.begin_stream_seconds is None
             or self.num_frames is None
         ):
             return self.average_fps_from_header
         return self.num_frames / (
-            self.end_stream_from_content_seconds
-            - self.begin_stream_from_content_seconds
+            self.end_stream_seconds
+            - self.begin_stream_seconds
         )
 
     def __repr__(self):
@@ -160,10 +160,10 @@ def get_video_metadata(decoder: torch.Tensor) -> VideoMetadata:
                 # keys with the Python names
                 num_frames_from_header=stream_dict.get("numFrames"),
                 num_frames_from_content=stream_dict.get("numFramesFromScan"),
-                begin_stream_from_content_seconds=stream_dict.get(
+                begin_stream_seconds=stream_dict.get(
                     "minPtsSecondsFromScan"
                 ),
-                end_stream_from_content_seconds=stream_dict.get(
+                end_stream_seconds=stream_dict.get(
                     "maxPtsSecondsFromScan"
                 ),
                 codec=stream_dict.get("codec"),
diff --git a/src/torchcodec/decoders/_simple_video_decoder.py b/src/torchcodec/decoders/_simple_video_decoder.py
index 295164b2f..6d1b29f5f 100644
--- a/src/torchcodec/decoders/_simple_video_decoder.py
+++ b/src/torchcodec/decoders/_simple_video_decoder.py
@@ -141,22 +141,22 @@ def __init__(
             )
         self._num_frames = self.metadata.num_frames_from_content
 
-        if self.metadata.begin_stream_from_content_seconds is None:
+        if self.metadata.begin_stream_seconds is None:
             raise ValueError(
                 "The minimum pts value in seconds is unknown. "
                 + _ERROR_REPORTING_INSTRUCTIONS
             )
-        self._begin_stream_from_content_seconds = (
-            self.metadata.begin_stream_from_content_seconds
+        self._begin_stream_seconds = (
+            self.metadata.begin_stream_seconds
         )
 
-        if self.metadata.end_stream_from_content_seconds is None:
+        if self.metadata.end_stream_seconds is None:
             raise ValueError(
                 "The maximum pts value in seconds is unknown. "
                 + _ERROR_REPORTING_INSTRUCTIONS
             )
-        self._end_stream_from_content_seconds = (
-            self.metadata.end_stream_from_content_seconds
+        self._end_stream_seconds = (
+            self.metadata.end_stream_seconds
         )
 
     def __len__(self) -> int:
@@ -280,14 +280,14 @@ def get_frame_displayed_at(self, seconds: float) -> Frame:
             Frame: The frame that is displayed at ``seconds``.
         """
         if (
-            not self._begin_stream_from_content_seconds
+            not self._begin_stream_seconds
             <= seconds
-            < self._end_stream_from_content_seconds
+            < self._end_stream_seconds
         ):
             raise IndexError(
                 f"Invalid pts in seconds: {seconds}. "
-                f"It must be greater than or equal to {self._begin_stream_from_content_seconds} "
-                f"and less than {self._end_stream_from_content_seconds}."
+                f"It must be greater than or equal to {self._begin_stream_seconds} "
+                f"and less than {self._end_stream_seconds}."
             )
         data, pts_seconds, duration_seconds = core.get_frame_at_pts(
             self._decoder, seconds
diff --git a/test/decoders/test_metadata.py b/test/decoders/test_metadata.py
index a9375fea8..1ccceb627 100644
--- a/test/decoders/test_metadata.py
+++ b/test/decoders/test_metadata.py
@@ -92,8 +92,8 @@ def test_num_frames_fallback(
         bit_rate=123,
         num_frames_from_header=num_frames_from_header,
         num_frames_from_content=num_frames_from_content,
-        begin_stream_from_content_seconds=0,
-        end_stream_from_content_seconds=4,
+        begin_stream_seconds=0,
+        end_stream_seconds=4,
         codec="whatever",
         width=123,
         height=321,

From cfb8918fa14673371d71fd913bb2875da9f9b002 Mon Sep 17 00:00:00 2001
From: Nicolas Hug <nicolashug@fb.com>
Date: Tue, 30 Jul 2024 09:28:43 -0700
Subject: [PATCH 5/6] More stuff

---
 examples/basic_example.py                     |  9 ++---
 src/torchcodec/decoders/_core/_metadata.py    | 40 +++++++------------
 .../decoders/_simple_video_decoder.py         | 22 ++--------
 3 files changed, 22 insertions(+), 49 deletions(-)

diff --git a/examples/basic_example.py b/examples/basic_example.py
index 31f54825b..212f73638 100644
--- a/examples/basic_example.py
+++ b/examples/basic_example.py
@@ -150,14 +150,11 @@ def plot(frames: torch.Tensor, title : Optional[str] = None):
 # -------------------------
 #
 # So far, we have retrieved frames based on their index. We can also retrieve
-# frames based on *when* they are displayed.  The available method are
-# :meth:`~torchcodec.decoders.SimpleVideoDecoder.get_frame_displayed_at` and
-# :meth:`~torchcodec.decoders.SimpleVideoDecoder.get_frames_displayed_at`, which
-# also return :class:`~torchcodec.decoders.Frame` and
-# :class:`~torchcodec.decoders.FrameBatch` objects respectively.
+# frames based on *when* they are displayed with
+# :meth:`~torchcodec.decoders.SimpleVideoDecoder.get_frame_displayed_at`, which
+# also returns :class:`~torchcodec.decoders.Frame` and
 
 frame_at_2_seconds = decoder.get_frame_displayed_at(seconds=2)
 print(f"{type(frame_at_2_seconds) = }")
 print(frame_at_2_seconds)
 plot(frame_at_2_seconds.data, "Frame displayed at 2 seconds")
-# TODO_BEFORE_RELEASE: illustrate get_frames_displayed_at
diff --git a/src/torchcodec/decoders/_core/_metadata.py b/src/torchcodec/decoders/_core/_metadata.py
index d2fe7fe94..8218a8b85 100644
--- a/src/torchcodec/decoders/_core/_metadata.py
+++ b/src/torchcodec/decoders/_core/_metadata.py
@@ -39,15 +39,18 @@ class VideoStreamMetadata:
     ``num_frames`` attribute instead. (int or None)."""
     begin_stream_seconds: Optional[float]
     """Beginning of the stream, in seconds (float or None).
-    This corresponds to the first frame's :term:`pts`. Usually, this is equal to
-    0."""
+    Conceptually, this corresponds to the first frame's :term:`pts`. It is
+    computed as min(frame.pts) across all frames in the stream. Usually, this is
+    equal to 0."""
     end_stream_seconds: Optional[float]
     """End of the stream, in seconds (float or None).
-    This is last_frame.pts + last_frame.duration, so according to our
-    convention, no frame is displayed at this time: read more in
-    :meth:`~torchcodec.decoders.SimpleVideoDecoder.get_frame_displayed_at`.
-    Retrieving the last frame is best done by simply indexing the
-    :class:`~torchcodec.decoders.SimpleVideoDecoder` object with ``[-1]``.
+    Conceptually, this corresponds to last_frame.pts + last_frame.duration. It
+    is computed as max(frame.pts + frame.duration) across all frames in the
+    stream. Note that no frame is displayed at this time value, so calling
+    :meth:`~torchcodec.decoders.SimpleVideoDecoder.get_frame_displayed_at` with
+    this value would result in an error. Retrieving the last frame is best done
+    by simply indexing the :class:`~torchcodec.decoders.SimpleVideoDecoder`
+    object with ``[-1]``.
     """
     codec: Optional[str]
     """Codec (str or None)."""
@@ -78,15 +81,9 @@ def duration_seconds(self) -> Optional[float]:
         from the actual frames if a :term:`scan` wsa performed. Otherwise we
         fall back to ``duration_seconds_from_header``.
         """
-        if (
-            self.end_stream_seconds is None
-            or self.begin_stream_seconds is None
-        ):
+        if self.end_stream_seconds is None or self.begin_stream_seconds is None:
             return self.duration_seconds_from_header
-        return (
-            self.end_stream_seconds
-            - self.begin_stream_seconds
-        )
+        return self.end_stream_seconds - self.begin_stream_seconds
 
     @property
     def average_fps(self) -> Optional[float]:
@@ -100,10 +97,7 @@ def average_fps(self) -> Optional[float]:
             or self.num_frames is None
         ):
             return self.average_fps_from_header
-        return self.num_frames / (
-            self.end_stream_seconds
-            - self.begin_stream_seconds
-        )
+        return self.num_frames / (self.end_stream_seconds - self.begin_stream_seconds)
 
     def __repr__(self):
         # Overridden because properites are not printed by default.
@@ -160,12 +154,8 @@ def get_video_metadata(decoder: torch.Tensor) -> VideoMetadata:
                 # keys with the Python names
                 num_frames_from_header=stream_dict.get("numFrames"),
                 num_frames_from_content=stream_dict.get("numFramesFromScan"),
-                begin_stream_seconds=stream_dict.get(
-                    "minPtsSecondsFromScan"
-                ),
-                end_stream_seconds=stream_dict.get(
-                    "maxPtsSecondsFromScan"
-                ),
+                begin_stream_seconds=stream_dict.get("minPtsSecondsFromScan"),
+                end_stream_seconds=stream_dict.get("maxPtsSecondsFromScan"),
                 codec=stream_dict.get("codec"),
                 width=stream_dict.get("width"),
                 height=stream_dict.get("height"),
diff --git a/src/torchcodec/decoders/_simple_video_decoder.py b/src/torchcodec/decoders/_simple_video_decoder.py
index 6d1b29f5f..4a8ec4494 100644
--- a/src/torchcodec/decoders/_simple_video_decoder.py
+++ b/src/torchcodec/decoders/_simple_video_decoder.py
@@ -146,18 +146,14 @@ def __init__(
                 "The minimum pts value in seconds is unknown. "
                 + _ERROR_REPORTING_INSTRUCTIONS
             )
-        self._begin_stream_seconds = (
-            self.metadata.begin_stream_seconds
-        )
+        self._begin_stream_seconds = self.metadata.begin_stream_seconds
 
         if self.metadata.end_stream_seconds is None:
             raise ValueError(
                 "The maximum pts value in seconds is unknown. "
                 + _ERROR_REPORTING_INSTRUCTIONS
             )
-        self._end_stream_seconds = (
-            self.metadata.end_stream_seconds
-        )
+        self._end_stream_seconds = self.metadata.end_stream_seconds
 
     def __len__(self) -> int:
         return self._num_frames
@@ -267,23 +263,13 @@ def get_frames_at(self, start: int, stop: int, step: int = 1) -> FrameBatch:
     def get_frame_displayed_at(self, seconds: float) -> Frame:
         """Return a single frame displayed at the given timestamp in seconds.
 
-        Each frame has a :term:`pts` and a duration. The convention is that a
-        given frame is displayed during the [frame_pts, frame_pts +
-        frame_duration) interval.
-
         Args:
-            seconds (float): The time stamp in seconds when the frame is
-                displayed, i.e. seconds is in
-                [:term:`pts`, :term:`pts` + duration).
+            seconds (float): The time stamp in seconds when the frame is displayed.
 
         Returns:
             Frame: The frame that is displayed at ``seconds``.
         """
-        if (
-            not self._begin_stream_seconds
-            <= seconds
-            < self._end_stream_seconds
-        ):
+        if not self._begin_stream_seconds <= seconds < self._end_stream_seconds:
             raise IndexError(
                 f"Invalid pts in seconds: {seconds}. "
                 f"It must be greater than or equal to {self._begin_stream_seconds} "

From f299effdea9c7d912671040a6e999a9e0cb0fd05 Mon Sep 17 00:00:00 2001
From: Nicolas Hug <nicolashug@fb.com>
Date: Tue, 30 Jul 2024 09:57:07 -0700
Subject: [PATCH 6/6] Address comments

---
 examples/basic_example.py                  | 2 +-
 src/torchcodec/decoders/_core/_metadata.py | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/examples/basic_example.py b/examples/basic_example.py
index 212f73638..735e1464d 100644
--- a/examples/basic_example.py
+++ b/examples/basic_example.py
@@ -152,7 +152,7 @@ def plot(frames: torch.Tensor, title : Optional[str] = None):
 # So far, we have retrieved frames based on their index. We can also retrieve
 # frames based on *when* they are displayed with
 # :meth:`~torchcodec.decoders.SimpleVideoDecoder.get_frame_displayed_at`, which
-# also returns :class:`~torchcodec.decoders.Frame` and
+# also returns :class:`~torchcodec.decoders.Frame`.
 
 frame_at_2_seconds = decoder.get_frame_displayed_at(seconds=2)
 print(f"{type(frame_at_2_seconds) = }")
diff --git a/src/torchcodec/decoders/_core/_metadata.py b/src/torchcodec/decoders/_core/_metadata.py
index 8218a8b85..8db223959 100644
--- a/src/torchcodec/decoders/_core/_metadata.py
+++ b/src/torchcodec/decoders/_core/_metadata.py
@@ -78,7 +78,7 @@ def num_frames(self) -> Optional[int]:
     @property
     def duration_seconds(self) -> Optional[float]:
         """Duration of the stream in seconds. We try to calculate the duration
-        from the actual frames if a :term:`scan` wsa performed. Otherwise we
+        from the actual frames if a :term:`scan` was performed. Otherwise we
         fall back to ``duration_seconds_from_header``.
         """
         if self.end_stream_seconds is None or self.begin_stream_seconds is None: