Fix FlatMapMapDataset length overflow when combined with infinite rep…

…eat.

PiperOrigin-RevId: 706724840

grain/_src/python/dataset/transformations/flatmap.py

@@ -13,6 +13,7 @@
 # limitations under the License.
 """Flatmap transformation for MapDataset."""
 import functools
+import sys
 from typing import Any, Callable, Sequence, TypeVar
 
 from grain._src.core import transforms
@@ -37,6 +38,11 @@ def __init__(
     self._transform = transform
 
   def __len__(self) -> int:
+    # If the parent dataset is on infinite repeat, its length is
+    # sys.maxsize and would result in overflows if further increased.
+    # In this case, we just keep the length as sys.maxsize.
+    if len(self._parent) >= sys.maxsize / self._transform.max_fan_out:
+      return sys.maxsize
     return self._transform.max_fan_out * len(self._parent)
 
   def __str__(self) -> str:

grain/_src/python/dataset/transformations/flatmap_test.py

@@ -15,6 +15,7 @@
 
 import dataclasses
 import itertools
+import sys
 from typing import Any, Sequence
 
 from absl.testing import absltest
@@ -73,6 +74,13 @@ def test_fixed_fan_out_size(self):
     )
     self.assertLen(flatmap_ds, self.fan_out * len(self.range_ds))
 
+  def test_flatmap_ds_length_after_repeat(self):
+    flatmap_ds = flatmap.FlatMapMapDataset(
+        self.range_ds.repeat(),
+        FixedSizeSplitWithNoTransform(max_fan_out=self.fan_out),
+    )
+    self.assertLen(flatmap_ds, sys.maxsize)
+
   def test_fixed_fan_out_data_no_transform(self):
     flatmap_ds = flatmap.FlatMapMapDataset(
         self.range_ds, FixedSizeSplitWithNoTransform(max_fan_out=self.fan_out)