Validate negative sampling range starts

Author: aswanth-07

src/torchcodec/samplers/_index_based.py

@@ -31,6 +31,12 @@ def _validate_sampling_range_index_based(
     sampling_range_end,
     num_frames_in_video,
 ):
+    if sampling_range_start < -num_frames_in_video:
+        raise ValueError(
+            f"sampling_range_start ({sampling_range_start}) must be greater than "
+            f"or equal to {-num_frames_in_video}."
+        )
+
     if sampling_range_start < 0:
         sampling_range_start = num_frames_in_video + sampling_range_start
 

test/test_samplers.py

@@ -536,6 +536,20 @@ def test_index_based_samplers_errors(sampler):
     ):
         sampler(decoder, sampling_range_start=-100, sampling_range_end=-100)
 
+    sampling_range_start = -len(decoder) - 1
+    with pytest.raises(
+        ValueError,
+        match=re.escape(
+            f"sampling_range_start ({sampling_range_start}) must be greater than "
+            f"or equal to {-len(decoder)}"
+        ),
+    ):
+        sampler(
+            decoder,
+            sampling_range_start=sampling_range_start,
+            sampling_range_end=-len(decoder),
+        )
+
     with pytest.raises(
         ValueError, match="We determined that sampling_range_end should"
     ):