src/lerobot/datasets/streaming_dataset.py

#!/usr/bin/env python

# Copyright 2025 The HuggingFace Inc. team. All rights reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
from collections import deque
from collections.abc import Callable, Generator, Iterable, Iterator
from pathlib import Path

import datasets
import numpy as np
import torch
from datasets import load_dataset

from lerobot.configs import DEFAULT_DEPTH_UNIT, DEPTH_METER_UNIT, DepthEncoderConfig
from lerobot.utils.constants import HF_LEROBOT_HOME, LOOKAHEAD_BACKTRACKTABLE, LOOKBACK_BACKTRACKTABLE

from .dataset_metadata import CODEBASE_VERSION, LeRobotDatasetMetadata
from .depth_utils import MM_PER_METRE, dequantize_depth
from .feature_utils import get_delta_indices
from .io_utils import item_to_torch
from .utils import (
    check_version_compatibility,
    find_float_index,
    is_float_in_list,
    safe_shard,
)
from .video_utils import (
    VideoDecoderCache,
    decode_video_frames,
    decode_video_frames_torchcodec,
)


class LookBackError(Exception):
    """
    Exception raised when trying to look back in the history of a Backtrackable object.
    """

    pass


class LookAheadError(Exception):
    """
    Exception raised when trying to look ahead in the future of a Backtrackable object.
    """

    pass


class Backtrackable[T]:
    """
    Wrap any iterator/iterable so you can step back up to `history` items
    and look ahead up to `lookahead` items.

    This is useful for streaming datasets where you need to access previous and future items
    but can't load the entire dataset into memory.

    Example:
    -------
    ```python
    ds = load_dataset("c4", "en", streaming=True, split="train")
    rev = Backtrackable(ds, history=3, lookahead=2)

    x0 = next(rev)  # forward
    x1 = next(rev)
    x2 = next(rev)

    # Look ahead
    x3_peek = rev.peek_ahead(1)  # next item without moving cursor
    x4_peek = rev.peek_ahead(2)  # two items ahead

    # Look back
    x1_again = rev.peek_back(1)  # previous item without moving cursor
    x0_again = rev.peek_back(2)  # two items back

    # Move backward
    x1_back = rev.prev()  # back one step
    next(rev)  # returns x2, continues forward from where we were
    ```
    """

    __slots__ = ("_source", "_back_buf", "_ahead_buf", "_cursor", "_history", "_lookahead")

    def __init__(self, iterable: Iterable[T], *, history: int = 1, lookahead: int = 0):
        if history < 1:
            raise ValueError("history must be >= 1")
        if lookahead <= 0:
            raise ValueError("lookahead must be > 0")

        self._source: Iterator[T] = iter(iterable)
        self._back_buf: deque[T] = deque(maxlen=history)
        self._ahead_buf: deque[T] = deque(maxlen=lookahead) if lookahead > 0 else deque()
        self._cursor: int = 0
        self._history = history
        self._lookahead = lookahead

    def __iter__(self) -> "Backtrackable[T]":
        return self

    def __next__(self) -> T:
        # If we've stepped back, consume from back buffer first
        if self._cursor < 0:  # -1 means "last item", etc.
            self._cursor += 1
            return self._back_buf[self._cursor]

        # If we have items in the ahead buffer, use them first
        item = self._ahead_buf.popleft() if self._ahead_buf else next(self._source)

        # Add current item to back buffer and reset cursor
        self._back_buf.append(item)
        self._cursor = 0
        return item

    def prev(self) -> T:
        """
        Step one item back in history and return it.
        Raises IndexError if already at the oldest buffered item.
        """
        if len(self._back_buf) + self._cursor <= 1:
            raise LookBackError("At start of history")

        self._cursor -= 1
        return self._back_buf[self._cursor]

    def peek_back(self, n: int = 1) -> T:
        """
        Look `n` items back (n=1 == previous item) without moving the cursor.
        """
        if n < 0 or n + 1 > len(self._back_buf) + self._cursor:
            raise LookBackError("peek_back distance out of range")

        return self._back_buf[self._cursor - (n + 1)]

    def peek_ahead(self, n: int = 1) -> T:
        """
        Look `n` items ahead (n=1 == next item) without moving the cursor.
        Fills the ahead buffer if necessary.
        """
        if n < 1:
            raise LookAheadError("peek_ahead distance must be 1 or more")
        elif n > self._lookahead:
            raise LookAheadError("peek_ahead distance exceeds lookahead limit")

        # Fill ahead buffer if we don't have enough items
        while len(self._ahead_buf) < n:
            try:
                item = next(self._source)
                self._ahead_buf.append(item)

            except StopIteration as err:
                raise LookAheadError("peek_ahead: not enough items in source") from err

        return self._ahead_buf[n - 1]

    def history(self) -> list[T]:
        """
        Return a copy of the buffered history (most recent last).
        The list length ≤ `history` argument passed at construction.
        """
        if self._cursor == 0:
            return list(self._back_buf)

        # When cursor<0, slice so the order remains chronological
        return list(self._back_buf)[: self._cursor or None]

    def can_peek_back(self, steps: int = 1) -> bool:
        """
        Check if we can go back `steps` items without raising an IndexError.
        """
        return steps <= len(self._back_buf) + self._cursor

    def can_peek_ahead(self, steps: int = 1) -> bool:
        """
        Check if we can peek ahead `steps` items.
        This may involve trying to fill the ahead buffer.
        """
        if self._lookahead > 0 and steps > self._lookahead:
            return False

        # Try to fill ahead buffer to check if we can peek that far
        try:
            while len(self._ahead_buf) < steps:
                if self._lookahead > 0 and len(self._ahead_buf) >= self._lookahead:
                    return False
                item = next(self._source)
                self._ahead_buf.append(item)
            return True
        except StopIteration:
            return False


class StreamingLeRobotDataset(torch.utils.data.IterableDataset):
    """LeRobotDataset with streaming capabilities.

    This class extends LeRobotDataset to add streaming functionality, allowing data to be streamed
    rather than loaded entirely into memory. This is especially useful for large datasets that may
    not fit in memory or when you want to quickly explore a dataset without downloading it completely.

    The key innovation is using a Backtrackable iterator that maintains a bounded buffer of recent
    items, allowing us to access previous frames for delta timestamps without loading the entire
    dataset into memory.

    Example:
        Basic usage:
        ```python
        from lerobot.common.datasets.streaming_dataset import StreamingLeRobotDataset

        # Create a streaming dataset with delta timestamps
        delta_timestamps = {
            "observation.image": [-1.0, -0.5, 0.0],  # 1 sec ago, 0.5 sec ago, current
            "action": [0.0, 0.1, 0.2],  # current, 0.1 sec future, 0.2 sec future
        }

        dataset = StreamingLeRobotDataset(
            repo_id="your-dataset-repo-id",
            delta_timestamps=delta_timestamps,
            streaming=True,
            buffer_size=1000,
        )

        # Iterate over the dataset
        for i, item in enumerate(dataset):
            print(f"Sample {i}: Episode {item['episode_index']} Frame {item['frame_index']}")
            # item will contain stacked frames according to delta_timestamps
            if i >= 10:
                break
        ```
    """

    def __init__(
        self,
        repo_id: str,
        root: str | Path | None = None,
        episodes: list[int] | None = None,
        image_transforms: Callable | None = None,
        delta_timestamps: dict[list[float]] | None = None,
        tolerance_s: float = 1e-4,
        revision: str | None = None,
        force_cache_sync: bool = False,
        streaming: bool = True,
        buffer_size: int = 1000,
        max_num_shards: int = 16,
        seed: int = 42,
        rng: np.random.Generator | None = None,
        shuffle: bool = True,
        return_uint8: bool = False,
        depth_output_unit: str = DEFAULT_DEPTH_UNIT,
    ):
        """Initialize a StreamingLeRobotDataset.

        Args:
            repo_id (str): This is the repo id that will be used to fetch the dataset.
            root (Path | None, optional): Local directory to use for local datasets. When omitted, Hub
                metadata is resolved through a revision-safe snapshot cache under
                ``$HF_LEROBOT_HOME/hub``.
            episodes (list[int] | None, optional): If specified, this will only load episodes specified by
                their episode_index in this list.
            image_transforms (Callable | None, optional): Transform to apply to image data.
            tolerance_s (float, optional): Tolerance in seconds for timestamp matching.
            revision (str, optional): Git revision id (branch name, tag, or commit hash).
            force_cache_sync (bool, optional): Flag to sync and refresh local files first.
            streaming (bool, optional): Whether to stream the dataset or load it all. Defaults to True.
            buffer_size (int, optional): Buffer size for shuffling when streaming. Defaults to 1000.
            max_num_shards (int, optional): Number of shards to re-shard the input dataset into. Defaults to 16.
            seed (int, optional): Reproducibility random seed.
            rng (np.random.Generator | None, optional): Random number generator.
            shuffle (bool, optional): Whether to shuffle the dataset across exhaustions. Defaults to True.
            depth_output_unit (str, optional): Physical unit depth maps are dequantized to ("m" or "mm").
                Defaults to "mm".
        """
        super().__init__()
        self.repo_id = repo_id
        self._requested_root = Path(root) if root else None
        self.root = self._requested_root if self._requested_root is not None else HF_LEROBOT_HOME / repo_id
        self.streaming_from_local = root is not None

        self.image_transforms = image_transforms
        self.episodes = episodes
        self.tolerance_s = tolerance_s
        self.revision = revision if revision else CODEBASE_VERSION
        self.seed = seed
        self.rng = rng if rng is not None else np.random.default_rng(seed)
        self.shuffle = shuffle

        self.streaming = streaming
        self.buffer_size = buffer_size
        self._return_uint8 = return_uint8
        self._depth_output_unit = depth_output_unit

        # We cache the video decoders to avoid re-initializing them at each frame (avoiding a ~10x slowdown)
        self.video_decoder_cache = None

        if self._requested_root is not None:
            self.root.mkdir(exist_ok=True, parents=True)

        # Load metadata
        self.meta = LeRobotDatasetMetadata(
            self.repo_id, self._requested_root, self.revision, force_cache_sync=force_cache_sync
        )
        self.root = self.meta.root
        self.revision = self.meta.revision
        self.meta.rescale_depth_stats(self._depth_output_unit)
        # Check version
        check_version_compatibility(self.repo_id, self.meta._version, CODEBASE_VERSION)

        self._depth_encoder_configs: dict[str, DepthEncoderConfig] = {
            vid_key: DepthEncoderConfig.from_video_info(self.meta.features[vid_key].get("info"))
            for vid_key in self.meta.depth_keys
        }

        # Input unit of each depth feature stored as raw images (dequantized separately from videos).
        self._image_depth_units: dict[str, str | None] = {
            key: (self.meta.features[key].get("info") or {}).get("depth_unit")
            for key in self.meta.depth_keys
            if key in self.meta.image_keys
        }

        self.delta_timestamps = None
        self.delta_indices = None

        if delta_timestamps is not None:
            self._validate_delta_timestamp_keys(delta_timestamps)  # raises ValueError if invalid
            self.delta_timestamps = delta_timestamps
            self.delta_indices = get_delta_indices(self.delta_timestamps, self.fps)

        self.hf_dataset: datasets.IterableDataset = load_dataset(
            self.repo_id if not self.streaming_from_local else str(self.root),
            split="train",
            streaming=self.streaming,
            data_files="data/*/*.parquet",
            revision=self.revision,
        )

        self.num_shards = min(self.hf_dataset.num_shards, max_num_shards)

    @property
    def num_frames(self):
        return self.meta.total_frames

    @property
    def num_episodes(self):
        return self.meta.total_episodes

    @property
    def fps(self):
        return self.meta.fps

    @property
    def depth_output_unit(self) -> str:
        """Physical unit (``"m"`` or ``"mm"``) depth maps are returned in on read."""
        return self._depth_output_unit

    @staticmethod
    def _iter_random_indices(
        rng: np.random.Generator, buffer_size: int, random_batch_size=100
    ) -> Iterator[int]:
        while True:
            yield from (int(i) for i in rng.integers(0, buffer_size, size=random_batch_size))

    @staticmethod
    def _infinite_generator_over_elements(rng: np.random.Generator, elements: list[int]) -> Iterator[int]:
        while True:
            yield rng.choice(elements)

    # TODO(fracapuano): Implement multi-threaded prefetching to accelerate data loading.
    # The current sequential iteration is a bottleneck. A producer-consumer pattern
    # could be used with a ThreadPoolExecutor to run `make_frame` (especially video decoding)
    # in parallel, feeding a queue from which this iterator will yield processed items.
    def __iter__(self) -> Iterator[dict[str, torch.Tensor]]:
        if self.video_decoder_cache is None:
            self.video_decoder_cache = VideoDecoderCache()

        # keep the same seed across exhaustions if shuffle is False, otherwise shuffle data across exhaustions
        rng = np.random.default_rng(self.seed) if not self.shuffle else self.rng

        buffer_indices_generator = self._iter_random_indices(rng, self.buffer_size)

        idx_to_backtrack_dataset = {
            idx: self._make_backtrackable_dataset(safe_shard(self.hf_dataset, idx, self.num_shards))
            for idx in range(self.num_shards)
        }

        # This buffer is populated while iterating on the dataset's shards
        # the logic is to add 2 levels of randomness:
        # (1) sample one shard at random from the ones available, and
        # (2) sample one frame from the shard sampled at (1)
        frames_buffer = []
        while available_shards := list(idx_to_backtrack_dataset.keys()):
            shard_key = next(self._infinite_generator_over_elements(rng, available_shards))
            backtrack_dataset = idx_to_backtrack_dataset[shard_key]  # selects which shard to iterate on

            try:
                for frame in self.make_frame(backtrack_dataset):
                    if len(frames_buffer) == self.buffer_size:
                        i = next(buffer_indices_generator)  # samples a element from the buffer
                        yield frames_buffer[i]
                        frames_buffer[i] = frame
                    else:
                        frames_buffer.append(frame)
                    break  # random shard sampled, switch shard
            except (
                RuntimeError,
                StopIteration,
            ):  # NOTE: StopIteration inside a generator throws a RuntimeError since python 3.7
                del idx_to_backtrack_dataset[shard_key]  # Remove exhausted shard, onto another shard

        # Once shards are all exhausted, shuffle the buffer and yield the remaining frames
        rng.shuffle(frames_buffer)
        yield from frames_buffer

    def _get_window_steps(
        self, delta_timestamps: dict[str, list[float]] | None = None, dynamic_bounds: bool = False
    ) -> tuple[int, int]:
        if delta_timestamps is None:
            return 1, 1

        if not dynamic_bounds:
            # Fix the windows
            lookback = LOOKBACK_BACKTRACKTABLE
            lookahead = LOOKAHEAD_BACKTRACKTABLE
        else:
            # Dynamically adjust the windows based on the given delta_timesteps
            all_timestamps = sum(delta_timestamps.values(), [])
            lookback = min(all_timestamps) * self.fps
            lookahead = max(all_timestamps) * self.fps

            # When lookback is >=0 it means no negative timesteps have been provided
            lookback = 0 if lookback >= 0 else (lookback * -1)

        return lookback, lookahead

    def _make_backtrackable_dataset(self, dataset: datasets.IterableDataset) -> Backtrackable:
        lookback, lookahead = self._get_window_steps(self.delta_timestamps)
        return Backtrackable(dataset, history=lookback, lookahead=lookahead)

    def _make_timestamps_from_indices(
        self, start_ts: float, indices: dict[str, list[int]] | None = None
    ) -> dict[str, list[float]]:
        if indices is not None:
            return {
                key: (
                    start_ts + torch.tensor(indices[key]) / self.fps
                ).tolist()  # NOTE: why not delta_timestamps directly?
                for key in self.delta_timestamps
            }
        else:
            return dict.fromkeys(self.meta.video_keys, [start_ts])

    def _make_padding_camera_frame(self, camera_key: str):
        """Variable-shape padding frame for given camera keys, given in (H, W, C)"""
        return torch.zeros(self.meta.info.features[camera_key]["shape"]).permute(-1, 0, 1)

    def _get_video_frame_padding_mask(
        self,
        video_frames: dict[str, torch.Tensor],
        query_timestamps: dict[str, list[float]],
        original_timestamps: dict[str, list[float]],
    ) -> dict[str, torch.BoolTensor]:
        padding_mask = {}

        for video_key, timestamps in original_timestamps.items():
            if video_key not in video_frames:
                continue  # only padding on video keys that are available
            frames = []
            mask = []
            padding_frame = self._make_padding_camera_frame(video_key)
            for ts in timestamps:
                if is_float_in_list(ts, query_timestamps[video_key]):
                    idx = find_float_index(ts, query_timestamps[video_key])
                    frames.append(video_frames[video_key][idx, :])
                    mask.append(False)
                else:
                    frames.append(padding_frame)
                    mask.append(True)

            padding_mask[f"{video_key}_is_pad"] = torch.BoolTensor(mask)

        return padding_mask

    def make_frame(self, dataset_iterator: Backtrackable) -> Generator:
        """Makes a frame starting from a dataset iterator"""
        item = next(dataset_iterator)
        item = item_to_torch(item)

        updates = []  # list of "updates" to apply to the item retrieved from hf_dataset (w/o camera features)

        # Get episode index from the item
        ep_idx = item["episode_index"]

        # "timestamp" restarts from 0 for each episode, whereas we need a global timestep within the single .mp4 file (given by index/fps)
        current_ts = item["index"] / self.fps

        episode_boundaries_ts = {
            key: (
                self.meta.episodes[ep_idx][f"videos/{key}/from_timestamp"],
                self.meta.episodes[ep_idx][f"videos/{key}/to_timestamp"],
            )
            for key in self.meta.video_keys
        }

        # Apply delta querying logic if necessary
        if self.delta_indices is not None:
            query_result, padding = self._get_delta_frames(dataset_iterator, item)
            updates.append(query_result)
            updates.append(padding)

        # Load video frames, when needed
        if len(self.meta.video_keys) > 0:
            original_timestamps = self._make_timestamps_from_indices(current_ts, self.delta_indices)

            # Some timestamps might not result available considering the episode's boundaries
            query_timestamps = self._get_query_timestamps(
                current_ts, self.delta_indices, episode_boundaries_ts
            )
            video_frames = self._query_videos(query_timestamps, ep_idx)

            if self.image_transforms is not None:
                image_keys = self.meta.camera_keys
                for cam in image_keys:
                    video_frames[cam] = self.image_transforms(video_frames[cam])

            updates.append(video_frames)

            if self.delta_indices is not None:
                # We always return the same number of frames. Unavailable frames are padded.
                padding_mask = self._get_video_frame_padding_mask(
                    video_frames, query_timestamps, original_timestamps
                )
                updates.append(padding_mask)

        result = item.copy()
        for update in updates:
            result.update(update)

        # Convert raw-image depth features to the output unit (video depth is already converted).
        for key, stored_unit in self._image_depth_units.items():
            if key in result and stored_unit is not None and stored_unit != self._depth_output_unit:
                result[key] = (
                    result[key] * MM_PER_METRE
                    if stored_unit == DEPTH_METER_UNIT
                    else result[key] / MM_PER_METRE
                )

        result["task"] = self.meta.tasks.iloc[item["task_index"]].name

        yield result

    def _get_query_timestamps(
        self,
        current_ts: float,
        query_indices: dict[str, list[int]] | None = None,
        episode_boundaries_ts: dict[str, tuple[float, float]] | None = None,
    ) -> dict[str, list[float]]:
        query_timestamps = {}
        keys_to_timestamps = self._make_timestamps_from_indices(current_ts, query_indices)
        for key in self.meta.video_keys:
            if query_indices is not None and key in query_indices:
                timestamps = keys_to_timestamps[key]
                # Clamp out timesteps outside of episode boundaries
                query_timestamps[key] = torch.clamp(
                    torch.tensor(timestamps), *episode_boundaries_ts[key]
                ).tolist()

            else:
                query_timestamps[key] = [current_ts]

        return query_timestamps

    def _query_videos(self, query_timestamps: dict[str, list[float]], ep_idx: int) -> dict:
        """Note: When using data workers (e.g. DataLoader with num_workers>0), do not call this function
        in the main process (e.g. by using a second Dataloader with num_workers=0). It will result in a
        Segmentation Fault. This probably happens because a memory reference to the video loader is created in
        the main process and a subprocess fails to access it.
        """

        item = {}
        for video_key, query_ts in query_timestamps.items():
            root = self.meta.url_root if self.streaming and not self.streaming_from_local else self.root
            video_path = f"{root}/{self.meta.get_video_file_path(ep_idx, video_key)}"
            if video_key in self.meta.depth_keys:
                # Depth maps are 12-bit quantized and only decodable via pyav; dequantize back
                # to physical units to match the non-streaming reader.
                frames = decode_video_frames(
                    video_path,
                    query_ts,
                    self.tolerance_s,
                    backend="pyav",
                    return_uint8=False,
                    is_depth=True,
                )
                depth_encoder = self._depth_encoder_configs[video_key]
                frames = dequantize_depth(
                    frames,
                    depth_min=depth_encoder.depth_min,
                    depth_max=depth_encoder.depth_max,
                    shift=depth_encoder.shift,
                    use_log=depth_encoder.use_log,
                    output_unit=self._depth_output_unit,
                )
            else:
                frames = decode_video_frames_torchcodec(
                    video_path,
                    query_ts,
                    self.tolerance_s,
                    decoder_cache=self.video_decoder_cache,
                    return_uint8=self._return_uint8,
                )

            item[video_key] = frames.squeeze(0) if len(query_ts) == 1 else frames

        return item

    def _get_delta_frames(self, dataset_iterator: Backtrackable, current_item: dict):
        # TODO(fracapuano): Modularize this function, refactor the code
        """Get frames with delta offsets using the backtrackable iterator.

        Args:
            current_item (dict): Current item from the iterator.
            ep_idx (int): Episode index.

        Returns:
            tuple: (query_result, padding) - frames at delta offsets and padding info.
        """
        current_episode_idx = current_item["episode_index"]

        # Prepare results
        query_result = {}
        padding = {}

        for key, delta_indices in self.delta_indices.items():
            if key in self.meta.video_keys:
                continue  # visual frames are decoded separately

            target_frames = []
            is_pad = []

            # Create a results dictionary to store frames in processing order, then reconstruct original order for stacking
            delta_results = {}

            # Separate and sort deltas by difficulty (easier operations first)
            negative_deltas = sorted([d for d in delta_indices if d < 0], reverse=True)  # [-1, -2, -3, ...]
            positive_deltas = sorted([d for d in delta_indices if d > 0])  # [1, 2, 3, ...]
            zero_deltas = [d for d in delta_indices if d == 0]

            # Process zero deltas (current frame)
            for delta in zero_deltas:
                delta_results[delta] = (
                    current_item[key],
                    False,
                )

            # Process negative deltas in order of increasing difficulty
            lookback_failed = False

            last_successful_frame = current_item[key]

            for delta in negative_deltas:
                if lookback_failed:
                    delta_results[delta] = (last_successful_frame, True)
                    continue

                try:
                    steps_back = abs(delta)
                    if dataset_iterator.can_peek_back(steps_back):
                        past_item = dataset_iterator.peek_back(steps_back)
                        past_item = item_to_torch(past_item)

                        if past_item["episode_index"] == current_episode_idx:
                            delta_results[delta] = (past_item[key], False)
                            last_successful_frame = past_item[key]

                        else:
                            raise LookBackError("Retrieved frame is from different episode!")
                    else:
                        raise LookBackError("Cannot go back further than the history buffer!")

                except LookBackError:
                    delta_results[delta] = (last_successful_frame, True)
                    lookback_failed = True  # All subsequent negative deltas will also fail

            # Process positive deltas in order of increasing difficulty
            lookahead_failed = False
            last_successful_frame = current_item[key]

            for delta in positive_deltas:
                if lookahead_failed:
                    delta_results[delta] = (last_successful_frame, True)
                    continue

                try:
                    if dataset_iterator.can_peek_ahead(delta):
                        future_item = dataset_iterator.peek_ahead(delta)
                        future_item = item_to_torch(future_item)

                        if future_item["episode_index"] == current_episode_idx:
                            delta_results[delta] = (future_item[key], False)
                            last_successful_frame = future_item[key]

                        else:
                            raise LookAheadError("Retrieved frame is from different episode!")
                    else:
                        raise LookAheadError("Cannot go ahead further than the lookahead buffer!")

                except LookAheadError:
                    delta_results[delta] = (last_successful_frame, True)
                    lookahead_failed = True  # All subsequent positive deltas will also fail

            # Reconstruct original order for stacking
            for delta in delta_indices:
                frame, is_padded = delta_results[delta]

                # add batch dimension for stacking
                target_frames.append(frame)  # frame.unsqueeze(0))
                is_pad.append(is_padded)

            # Stack frames and add to results
            if target_frames:
                query_result[key] = torch.stack(target_frames)
                padding[f"{key}_is_pad"] = torch.BoolTensor(is_pad)

        return query_result, padding

    def _validate_delta_timestamp_keys(self, delta_timestamps: dict[list[float]]) -> None:
        """
        Validate that all keys in delta_timestamps correspond to actual features in the dataset.

        Raises:
            ValueError: If any delta timestamp key doesn't correspond to a dataset feature.
        """
        if delta_timestamps is None:
            return

        # Get all available feature keys from the dataset metadata
        available_features = set(self.meta.features.keys())

        # Get all keys from delta_timestamps
        delta_keys = set(delta_timestamps.keys())

        # Find any keys that don't correspond to features
        invalid_keys = delta_keys - available_features

        if invalid_keys:
            raise ValueError(
                f"The following delta_timestamp keys do not correspond to dataset features: {invalid_keys}. "
                f"Available features are: {sorted(available_features)}"
            )