fix(datasets): guard Feistel cycle-walking loop against non-convergence

Replace the unbounded while True in EpisodeAwareSampler._permute with a
bounded for loop capped at _MAX_CYCLE_WALK_STEPS (100) and raise
RuntimeError if the cycle-walk fails to land in [0, num_frames). The
loop is expected to converge in <4 steps on the chosen power-of-two
domain, so the bound is a safety net that should never trip in practice
but prevents a pathological infinite loop.

https://claude.ai/code/session_01HQ15tFrBsHYScjGWosEv22

src/lerobot/configs/train.py

@@ -99,6 +99,10 @@ class TrainPipelineConfig(HubMixin):
     batch_size: int = 8
     prefetch_factor: int = 4
     persistent_workers: bool = True
+    # Deterministic data order (pure function of seed and epoch): immune to cross-rank RNG
+    # desync and enables sample-exact resume. Set to false for the legacy RNG-based shuffle.
+    # Ignored when dataset.streaming is enabled.
+    deterministic_sampler: bool = True
     steps: int = 100_000
     eval_freq: int = 20_000
     log_freq: int = 200

src/lerobot/datasets/__init__.py

@@ -50,7 +50,7 @@ from .lerobot_dataset import LeRobotDataset
 from .multi_dataset import MultiLeRobotDataset
 from .pipeline_features import aggregate_pipeline_dataset_features, create_initial_features
 from .pyav_utils import check_video_encoder_parameters_pyav, detect_available_encoders_pyav
-from .sampler import EpisodeAwareSampler
+from .sampler import EpisodeAwareSampler, compute_sampler_state
 from .streaming_dataset import StreamingLeRobotDataset
 from .utils import DEFAULT_EPISODES_PATH, create_lerobot_dataset_card
 from .video_utils import VideoEncodingManager
@@ -82,6 +82,7 @@ __all__ = [
     "aggregate_stats",
     "convert_image_to_video_dataset",
     "create_initial_features",
+    "compute_sampler_state",
     "create_lerobot_dataset_card",
     "column_for_style",
     "delete_episodes",

src/lerobot/datasets/sampler.py

@@ -14,14 +14,49 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import logging
+import math
 from collections.abc import Iterator
 
+import numpy as np
 import torch
 
 logger = logging.getLogger(__name__)
 
+_MASK_64 = (1 << 64) - 1
+_FEISTEL_ROUNDS = 4
+# Cycle-walking converges in <4 expected steps on the chosen domain; this bound is a generous
+# safety net that should never be hit in practice.
+_MAX_CYCLE_WALK_STEPS = 100
+
+
+def _mix64(x: int) -> int:
+    """SplitMix64 finalizer (64-bit integer hash)."""
+    x = (x + 0x9E3779B97F4A7C15) & _MASK_64
+    x ^= x >> 30
+    x = (x * 0xBF58476D1CE4E5B9) & _MASK_64
+    x ^= x >> 27
+    x = (x * 0x94D049BB133111EB) & _MASK_64
+    x ^= x >> 31
+    return x
+
 
 class EpisodeAwareSampler:
+    """Sampler over episode frames with O(num_episodes) memory.
+
+    Only episode boundaries are stored; logical positions map to frame indices on the fly, so
+    memory does not grow with the number of frames.
+
+    By default (`deterministic=True`) shuffling uses a seeded Feistel permutation over
+    `[0, num_frames)`: the data order is a pure function of `(seed, epoch)`, needs no RNG
+    synchronization across distributed ranks, and any position can be sought in O(1), enabling
+    sample-exact resume via `state_dict` / `load_state_dict`. Each completed `__iter__`
+    advances the epoch. The shuffle is pseudo-random rather than truly uniform — the standard
+    large-scale trade-off. During a resumed epoch, `__len__` still reports the full length.
+
+    With `deterministic=False`, shuffling falls back to `torch.randperm` driven by `generator`
+    (accelerate synchronizes the generator across ranks when preparing the dataloader).
+    """
+
     def __init__(
         self,
         dataset_from_indices: list[int],
@@ -31,62 +66,160 @@ class EpisodeAwareSampler:
         drop_n_last_frames: int = 0,
         shuffle: bool = False,
         generator: torch.Generator | None = None,
+        deterministic: bool = True,
+        seed: int = 0,
     ):
-        """Sampler that optionally incorporates episode boundary information.
-
+        """
         Args:
-            dataset_from_indices: List of indices containing the start of each episode in the dataset.
-            dataset_to_indices: List of indices containing the end of each episode in the dataset.
-            episode_indices_to_use: List of episode indices to use. If None, all episodes are used.
-                                    Assumes that episodes are indexed from 0 to N-1.
-            drop_n_first_frames: Number of frames to drop from the start of each episode.
-            drop_n_last_frames: Number of frames to drop from the end of each episode.
+            dataset_from_indices: Start index of each episode in the dataset.
+            dataset_to_indices: End index of each episode in the dataset.
+            episode_indices_to_use: Episode indices to use; None means all.
+            drop_n_first_frames: Frames to drop from the start of each episode.
+            drop_n_last_frames: Frames to drop from the end of each episode.
             shuffle: Whether to shuffle the indices.
-            generator: Generator used for shuffling. Exposing this attribute (even when None) lets
-                       `accelerate` register it as the synchronized RNG in distributed training, so
-                       every rank draws the same permutation and batch shards stay disjoint. When
-                       None, shuffling falls back to the global torch RNG.
+            generator: Generator for non-deterministic shuffling (global torch RNG when None).
+            deterministic: Use the seeded Feistel permutation instead of `torch.randperm`.
+            seed: Seed the deterministic permutation is derived from (together with the epoch).
         """
         if drop_n_first_frames < 0:
             raise ValueError(f"drop_n_first_frames must be >= 0, got {drop_n_first_frames}")
         if drop_n_last_frames < 0:
             raise ValueError(f"drop_n_last_frames must be >= 0, got {drop_n_last_frames}")
+        if deterministic and generator is not None:
+            raise ValueError("generator is unused in deterministic mode; pass seed instead.")
 
-        indices = []
-        for episode_idx, (start_index, end_index) in enumerate(
-            zip(dataset_from_indices, dataset_to_indices, strict=True)
-        ):
-            if episode_indices_to_use is None or episode_idx in episode_indices_to_use:
-                ep_length = end_index - start_index
-                if drop_n_first_frames + drop_n_last_frames >= ep_length:
-                    logger.warning(
-                        "Episode %d has %d frames but drop_n_first_frames=%d and "
-                        "drop_n_last_frames=%d removes all frames. Skipping.",
-                        episode_idx,
-                        ep_length,
-                        drop_n_first_frames,
-                        drop_n_last_frames,
-                    )
-                    continue
-                indices.extend(range(start_index + drop_n_first_frames, end_index - drop_n_last_frames))
+        from_indices = np.asarray(dataset_from_indices, dtype=np.int64)
+        to_indices = np.asarray(dataset_to_indices, dtype=np.int64)
+        if from_indices.shape != to_indices.shape:
+            raise ValueError(
+                f"dataset_from_indices and dataset_to_indices must have the same length, "
+                f"got {len(from_indices)} and {len(to_indices)}"
+            )
 
-        if not indices:
+        used = np.ones(len(from_indices), dtype=bool)
+        if episode_indices_to_use is not None:
+            used = np.zeros(len(from_indices), dtype=bool)
+            used[np.asarray(episode_indices_to_use, dtype=np.int64)] = True
+
+        starts = from_indices + drop_n_first_frames
+        lengths = to_indices - drop_n_last_frames - starts
+        for episode_idx in np.flatnonzero(used & (lengths <= 0)):
+            logger.warning(
+                "Episode %d has %d frames but drop_n_first_frames=%d and "
+                "drop_n_last_frames=%d removes all frames. Skipping.",
+                episode_idx,
+                to_indices[episode_idx] - from_indices[episode_idx],
+                drop_n_first_frames,
+                drop_n_last_frames,
+            )
+        used &= lengths > 0
+        if not used.any():
             raise ValueError(
                 "No valid frames remain after applying drop_n_first_frames and drop_n_last_frames. "
                 "All episodes were either filtered out or had too few frames."
             )
 
-        self.indices = indices
+        self._starts = starts[used]
+        self._cum_lengths = np.cumsum(lengths[used])
+        self._num_frames = int(self._cum_lengths[-1])
         self.shuffle = shuffle
         self.generator = generator
+        self.deterministic = deterministic
+        self.seed = seed
+        self._epoch = 0
+        self._start_index = 0
+
+        # Smallest even-bit-width power-of-two domain >= num_frames: equal Feistel halves,
+        # cycle-walking converges in <4 expected steps.
+        bits = max((self._num_frames - 1).bit_length(), 2)
+        self._half_bits = (bits + 1) // 2
+        self._half_mask = (1 << self._half_bits) - 1
+
+    @property
+    def indices(self) -> list[int]:
+        """Materialized frame indices in unshuffled order; O(num_frames), introspection only."""
+        return [self._frame_index(k) for k in range(self._num_frames)]
+
+    def set_epoch(self, epoch: int) -> None:
+        self._require_deterministic("set_epoch")
+        self._epoch = epoch
+
+    def state_dict(self) -> dict:
+        self._require_deterministic("state_dict")
+        return {"epoch": self._epoch, "start_index": self._start_index}
+
+    def load_state_dict(self, state: dict) -> None:
+        self._require_deterministic("load_state_dict")
+        self._epoch = state["epoch"]
+        self._start_index = state["start_index"]
+
+    def _require_deterministic(self, method: str) -> None:
+        if not self.deterministic:
+            raise RuntimeError(f"{method} requires deterministic=True: an RNG order cannot be sought.")
+
+    def _round_keys(self, epoch: int) -> list[int]:
+        state = _mix64(_mix64(self.seed) ^ _mix64(epoch))
+        keys = []
+        for _ in range(_FEISTEL_ROUNDS):
+            state = _mix64(state)
+            keys.append(state)
+        return keys
+
+    def _permute(self, index: int, keys: list[int]) -> int:
+        # Feistel network with cycle-walking: a bijection on [0, num_frames).
+        half_bits, half_mask = self._half_bits, self._half_mask
+        for _ in range(_MAX_CYCLE_WALK_STEPS):
+            left, right = index >> half_bits, index & half_mask
+            for key in keys:
+                left, right = right, left ^ (_mix64(right ^ key) & half_mask)
+            index = (left << half_bits) | right
+            if index < self._num_frames:
+                return index
+        raise RuntimeError(
+            f"Feistel cycle-walking did not converge within {_MAX_CYCLE_WALK_STEPS} steps; "
+            "this should never happen for a valid domain."
+        )
+
+    def _frame_index(self, position: int) -> int:
+        episode = int(np.searchsorted(self._cum_lengths, position, side="right"))
+        position_in_episode = position - (int(self._cum_lengths[episode - 1]) if episode > 0 else 0)
+        return int(self._starts[episode]) + position_in_episode
 
     def __iter__(self) -> Iterator[int]:
+        if not self.deterministic:
+            return self._iter_default()
+        # Advance epoch state eagerly, not on first consumption of the generator.
+        epoch, start = self._epoch, self._start_index
+        self._epoch += 1
+        self._start_index = 0
+        return self._iter_deterministic_epoch(epoch, start)
+
+    def _iter_default(self) -> Iterator[int]:
         if self.shuffle:
-            for i in torch.randperm(len(self.indices), generator=self.generator):
-                yield self.indices[i]
+            for i in torch.randperm(self._num_frames, generator=self.generator):
+                yield self._frame_index(int(i))
         else:
-            for i in self.indices:
-                yield i
+            for k in range(self._num_frames):
+                yield self._frame_index(k)
+
+    def _iter_deterministic_epoch(self, epoch: int, start: int) -> Iterator[int]:
+        keys = self._round_keys(epoch) if self.shuffle else None
+        for k in range(start, self._num_frames):
+            yield self._frame_index(self._permute(k, keys) if self.shuffle else k)
 
     def __len__(self) -> int:
-        return len(self.indices)
+        return self._num_frames
+
+
+def compute_sampler_state(step: int, num_frames: int, batch_size: int, num_processes: int) -> dict:
+    """Map an optimization step to an `EpisodeAwareSampler` state for sample-exact resume.
+
+    Under accelerate's batch sharding, one step consumes `batch_size * num_processes` sampler
+    positions and each rank sees `ceil(ceil(num_frames / batch_size) / num_processes)` batches
+    per epoch (`even_batches` padding included). The start index provably stays below
+    `num_frames`; the `min` is defensive.
+    """
+    batches_per_epoch = math.ceil(math.ceil(num_frames / batch_size) / num_processes)
+    epoch, batches_into_epoch = divmod(step, batches_per_epoch)
+    start_index = min(batches_into_epoch * batch_size * num_processes, num_frames)
+    return {"epoch": epoch, "start_index": start_index}

src/lerobot/scripts/lerobot_train.py

@@ -43,7 +43,7 @@ from lerobot.common.train_utils import (
 from lerobot.common.wandb_utils import WandBLogger
 from lerobot.configs import parser
 from lerobot.configs.train import TrainPipelineConfig
-from lerobot.datasets import EpisodeAwareSampler, make_dataset
+from lerobot.datasets import EpisodeAwareSampler, compute_sampler_state, make_dataset
 from lerobot.envs import close_envs, make_env, make_env_pre_post_processors
 from lerobot.optim.factory import make_optimizer_and_scheduler
 from lerobot.policies import PreTrainedPolicy, make_policy, make_pre_post_processors
@@ -387,11 +387,30 @@ def train(cfg: TrainPipelineConfig, accelerator: "Accelerator | None" = None):
         logging.info(f"{num_total_params=} ({format_big_number(num_total_params)})")
 
     # create dataloader for offline training
-    if hasattr(active_cfg, "drop_n_last_frames"):
+    if cfg.deterministic_sampler and not cfg.dataset.streaming:
+        # Deterministic data order: no cross-rank RNG sync needed, sample-exact resume.
         shuffle = False
-        # A dedicated generator (rather than the global torch RNG) lets accelerator.prepare
-        # synchronize the shuffle permutation across ranks, keeping batch shards disjoint even
-        # when ranks consume the global RNG asymmetrically (e.g. eval on the main process only).
+        sampler = EpisodeAwareSampler(
+            dataset.meta.episodes["dataset_from_index"],
+            dataset.meta.episodes["dataset_to_index"],
+            episode_indices_to_use=dataset.episodes,
+            drop_n_last_frames=getattr(active_cfg, "drop_n_last_frames", 0),
+            shuffle=True,
+            seed=cfg.seed if cfg.seed is not None else 0,
+        )
+        if cfg.resume and step > 0:
+            sampler_state = compute_sampler_state(
+                step, len(sampler), cfg.batch_size, accelerator.num_processes
+            )
+            sampler.load_state_dict(sampler_state)
+            if is_main_process:
+                logging.info(
+                    f"Resuming data order at epoch {sampler_state['epoch']}, "
+                    f"sample {sampler_state['start_index']}"
+                )
+    elif hasattr(active_cfg, "drop_n_last_frames"):
+        shuffle = False
+        # Legacy RNG shuffle: a dedicated generator lets accelerate synchronize it across ranks.
         sampler_generator = torch.Generator()
         if cfg.seed is not None:
             sampler_generator.manual_seed(cfg.seed)
@@ -401,6 +420,7 @@ def train(cfg: TrainPipelineConfig, accelerator: "Accelerator | None" = None):
             episode_indices_to_use=dataset.episodes,
             drop_n_last_frames=active_cfg.drop_n_last_frames,
             shuffle=True,
+            deterministic=False,
             generator=sampler_generator,
         )
     else:

tests/datasets/test_sampler.py

@@ -118,12 +118,18 @@ def test_shuffle_with_generator_is_deterministic():
     # Two samplers shuffling with same-seed generators must yield identical permutations.
     # This is what keeps batch shards disjoint across ranks in distributed training, where
     # accelerate synchronizes the sampler's generator state instead of the global torch RNG.
-    sampler_a = EpisodeAwareSampler([0], [6], shuffle=True, generator=torch.Generator().manual_seed(42))
-    sampler_b = EpisodeAwareSampler([0], [6], shuffle=True, generator=torch.Generator().manual_seed(42))
+    sampler_a = EpisodeAwareSampler(
+        [0], [6], shuffle=True, deterministic=False, generator=torch.Generator().manual_seed(42)
+    )
+    sampler_b = EpisodeAwareSampler(
+        [0], [6], shuffle=True, deterministic=False, generator=torch.Generator().manual_seed(42)
+    )
     assert list(sampler_a) == list(sampler_b)
 
     # Desyncing the global RNG must not affect the permutation.
-    sampler_c = EpisodeAwareSampler([0], [6], shuffle=True, generator=torch.Generator().manual_seed(42))
+    sampler_c = EpisodeAwareSampler(
+        [0], [6], shuffle=True, deterministic=False, generator=torch.Generator().manual_seed(42)
+    )
     order_before = list(sampler_c)
     sampler_c.generator.manual_seed(42)
     torch.randperm(1000)  # consume global RNG, as rank-asymmetric code (e.g. eval) would
@@ -133,7 +139,7 @@ def test_shuffle_with_generator_is_deterministic():
 def test_generator_attribute_defaults_to_none():
     # accelerate detects synchronizable samplers via `hasattr(sampler, "generator")`,
     # so the attribute must exist even when no generator is passed.
-    sampler = EpisodeAwareSampler([0], [6], shuffle=True)
+    sampler = EpisodeAwareSampler([0], [6], shuffle=True, deterministic=False)
     assert sampler.generator is None
     assert set(sampler) == {0, 1, 2, 3, 4, 5}
 
@@ -161,3 +167,127 @@ def test_partial_episode_drop_warns(caplog):
     # Episode 0 is skipped (1 frame, drop 1), Episode 1 keeps frames 2-5
     assert sampler.indices == [2, 3, 4, 5]
     assert "Episode 0" in caplog.text
+
+
+# --- deterministic mode (seeded Feistel permutation) ---
+
+from functools import partial  # noqa: E402
+
+from lerobot.datasets.sampler import compute_sampler_state  # noqa: E402
+
+deterministic_sampler = partial(EpisodeAwareSampler, deterministic=True)
+
+
+EPISODE_BOUNDS = ([0, 2, 3], [2, 3, 6])  # episodes of 2, 1 and 3 frames
+
+
+def test_deterministic_mode_unshuffled_matches_default_mode():
+    for kwargs in (
+        {},
+        {"drop_n_first_frames": 1},
+        {"drop_n_last_frames": 1},
+        {"episode_indices_to_use": [0, 2]},
+    ):
+        reference = EpisodeAwareSampler(*EPISODE_BOUNDS, shuffle=False, **kwargs)
+        sampler = deterministic_sampler(*EPISODE_BOUNDS, shuffle=False, **kwargs)
+        assert list(sampler) == list(reference), kwargs
+        assert len(sampler) == len(reference), kwargs
+
+
+def test_deterministic_mode_rejects_generator():
+    with pytest.raises(ValueError, match="generator is unused in deterministic mode"):
+        deterministic_sampler(*EPISODE_BOUNDS, shuffle=True, generator=torch.Generator())
+
+
+def test_state_methods_require_deterministic_mode():
+    sampler = EpisodeAwareSampler(*EPISODE_BOUNDS, shuffle=True, deterministic=False)
+    with pytest.raises(RuntimeError, match="deterministic=True"):
+        sampler.set_epoch(1)
+    with pytest.raises(RuntimeError, match="deterministic=True"):
+        sampler.state_dict()
+
+
+@pytest.mark.parametrize("num_frames", [1, 2, 3, 37, 64, 100])
+def test_deterministic_sampler_shuffle_is_permutation(num_frames):
+    for seed in (0, 1, 1234):
+        sampler = deterministic_sampler([0], [num_frames], shuffle=True, seed=seed)
+        assert sorted(sampler) == list(range(num_frames))
+
+
+def test_deterministic_sampler_epochs_reproduce_and_differ():
+    sampler_a = deterministic_sampler([0], [100], shuffle=True, seed=42)
+    sampler_b = deterministic_sampler([0], [100], shuffle=True, seed=42)
+    epoch_0 = list(sampler_a)
+    assert list(sampler_b) == epoch_0  # same (seed, epoch) -> same order on any process
+    epoch_1 = list(sampler_a)  # __iter__ auto-advances the epoch
+    assert epoch_1 != epoch_0
+    assert sorted(epoch_1) == sorted(epoch_0)
+    sampler_a.set_epoch(0)
+    assert list(sampler_a) == epoch_0
+    assert list(deterministic_sampler([0], [100], shuffle=True, seed=7)) != epoch_0
+
+
+def test_deterministic_sampler_resume_mid_epoch():
+    reference = deterministic_sampler(*EPISODE_BOUNDS, shuffle=True, seed=42)
+    epoch_0 = list(reference)
+    epoch_1 = list(reference)
+    for start in (0, 1, 4, len(epoch_0)):
+        resumed = deterministic_sampler(*EPISODE_BOUNDS, shuffle=True, seed=42)
+        resumed.load_state_dict({"epoch": 0, "start_index": start})
+        assert list(resumed) == epoch_0[start:]
+        # the resumed sampler continues into the same epoch 1 as the uninterrupted one
+        assert list(resumed) == epoch_1
+
+
+def test_deterministic_sampler_constant_memory():
+    # A trillion-frame dataset must instantiate instantly and seek anywhere in O(1):
+    # only per-episode boundaries are stored, never per-frame indices.
+    num_frames = 10**12
+    sampler = deterministic_sampler([0], [num_frames], shuffle=True, seed=0)
+    assert len(sampler) == num_frames
+    sampler.load_state_dict({"epoch": 3, "start_index": num_frames - 3})
+    # Collect via the iterator: list(sampler) would call PyObject_LengthHint -> sampler.__len__
+    # (the full epoch length, here 10**12) and pre-allocate that many slots before iterating. The
+    # iterator itself exposes no length hint, so this stays O(1) like the resumed epoch it drains.
+    tail = list(iter(sampler))
+    assert len(tail) == 3
+    assert all(0 <= idx < num_frames for idx in tail)
+
+
+def test_deterministic_sampler_validation_matches_episode_aware():
+    with pytest.raises(ValueError, match="drop_n_first_frames must be >= 0"):
+        deterministic_sampler([0], [10], drop_n_first_frames=-1)
+    with pytest.raises(ValueError, match="drop_n_last_frames must be >= 0"):
+        deterministic_sampler([0], [10], drop_n_last_frames=-1)
+    with pytest.raises(ValueError, match="No valid frames remain"):
+        deterministic_sampler([0, 1, 2], [1, 2, 3], drop_n_first_frames=1)
+
+
+def test_deterministic_sampler_partial_episode_drop_warns(caplog):
+    with caplog.at_level(logging.WARNING, logger="lerobot.datasets.sampler"):
+        sampler = deterministic_sampler([0, 1], [1, 6], drop_n_first_frames=1, shuffle=False)
+    assert list(sampler) == [2, 3, 4, 5]
+    assert "Episode 0" in caplog.text
+
+
+def test_compute_sampler_state():
+    # 100 frames, batch 10, 2 ranks -> 10 underlying batches, 5 per rank per epoch.
+    assert compute_sampler_state(step=0, num_frames=100, batch_size=10, num_processes=2) == {
+        "epoch": 0,
+        "start_index": 0,
+    }
+    # step 7 -> epoch 1, 2 per-rank batches in = 2 * 10 * 2 = 40 samples in
+    assert compute_sampler_state(step=7, num_frames=100, batch_size=10, num_processes=2) == {
+        "epoch": 1,
+        "start_index": 40,
+    }
+    # uneven epoch: 95 frames -> 10 underlying batches (last short), still 5 per rank
+    assert compute_sampler_state(step=12, num_frames=95, batch_size=10, num_processes=2) == {
+        "epoch": 2,
+        "start_index": 40,
+    }
+    # uneven sharding: 105 frames -> 11 underlying batches, 6 per rank (even_batches pads)
+    assert compute_sampler_state(step=11, num_frames=105, batch_size=10, num_processes=2) == {
+        "epoch": 1,
+        "start_index": 100,
+    }