data/advanced_datasets.py

import torch
from torch.utils.data import IterableDataset, get_worker_info
import threading
from queue import Queue
from typing import Iterator
import itertools
import random

random.seed(42)  # Set the random seed to the meaning of life for good luck

class ConstantLengthDataset(IterableDataset):
    def __init__(
        self,
        dataset,
        infinite: bool = False,
        max_sample_length: int = 1024,
        seq_length: int = 1024,
        num_of_sequences: int = 1024,
        queue_size: int = 2,
        max_images_per_example: int = 4,
        max_images_per_knapsack: int = 18,
    ):
        self.dataset = dataset
        self.max_sample_length = max_sample_length
        self.seq_length = seq_length
        self.max_length = seq_length * num_of_sequences
        self.epoch = 0  # only advanced when infinite=True
        self.infinite = infinite
        self.queue_size = max(queue_size, 1)
        self.max_images_per_example = max_images_per_example
        self.max_images_per_knapsack = max_images_per_knapsack
        self._sentinel = object()

    def __iter__(self) -> Iterator[dict]:
        """
        Returns an iterator over the dataset that yields fixed-length sequences for training.

        The iterator uses a producer-consumer pattern with a background thread to efficiently
        pre-fetch and buffer samples. The producer thread continuously reads from the base
        dataset and fills a queue, while the main thread consumes from the queue.

        The dataset is automatically sharded across workers when using num_workers > 1.

        Returns:
            Iterator[dict]: An iterator that yields training samples with the following structure:
                - input_ids: Tensor of token ids of shape (seq_length,)
                - labels: Tensor of labels of shape (seq_length,)
                - attention_mask: Tensor of attention mask of shape (seq_length,)
                - images: List of processed image tensors
                - model_patch_positions: List of per-image tensors of original 2D positions of flattened model-size patches
        """
        worker_info = get_worker_info()
        worker_id = worker_info.id if worker_info else 0
        num_workers = worker_info.num_workers if worker_info else 1

        def make_base_iterator():
            """Return a (sharded) iterator over the underlying dataset."""
            if not hasattr(self.dataset.dataset, "__len__"):
                return self.dataset.iter_for_worker()  # with iterable datasets, each worker gets different shards
            
            all_indices = range(len(self.dataset))

            # Shard the *indices* first, before any data is fetched.
            if num_workers > 1:
                worker_indices = itertools.islice(
                    all_indices, worker_id, None, num_workers
                )
            else:
                worker_indices = all_indices

            # Create an iterator that only calls __getitem__ for the assigned indices.
            def sharded_item_iterator():
                for idx in worker_indices:
                    yield self.dataset[idx]

            return sharded_item_iterator()

        queue: Queue = Queue(maxsize=self.queue_size)

        producer = threading.Thread(
            target=self._producer, args=(make_base_iterator, queue), daemon=True
        )
        producer.start()

        while True:
            batch_of_batches = queue.get()
            if batch_of_batches is self._sentinel:
                break
            for batch in batch_of_batches:
                yield batch

    def _producer(
        self,
        make_iterator,  # a zero-arg lambda that returns a fresh (possibly sharded) iterator
        queue: Queue,
    ):
        """Runs in a separate daemon thread and keeps `queue` full."""
        iterator = make_iterator()
        more_examples = True

        while more_examples:
            # ------------- 1) pull raw samples until we have enough -------- #
            buffer, buffer_len = [], 0
            while buffer_len < self.max_length:
                try:
                    sample = next(iterator)
                except StopIteration:
                    if self.infinite:
                        iterator = make_iterator()
                        self.epoch += 1
                        print(f"Epoch {self.epoch} finished, restarting iterator")
                        continue
                    else:
                        more_examples = False
                        break

                if sample is None:  # Ratings filtered out the sample
                    continue

                if len(sample["input_ids"]) >= self.max_sample_length:
                    continue  # skip overly long samples
                if len(sample["images"]) > self.max_images_per_example:
                    continue  # skip samples that exceed the image constraint

                sample["input_ids"] = torch.cat(
                    [
                        sample["input_ids"],
                        torch.tensor([self.dataset.tokenizer.pad_token_id]),
                    ]
                )
                sample["attention_mask"] = torch.cat(
                    [sample["attention_mask"], torch.tensor([0])]
                )
                sample["labels"] = torch.cat([sample["labels"], torch.tensor([-100])])
                
                # We do not append anything to sample['model_patch_positions'] because 
                # the padding token comes in the very end and doesn't increase the number of
                # image tokens. <image tokens> <text tokens> <padding token>

                buffer.append(sample)
                buffer_len += len(sample["input_ids"])

            if not buffer:
                break  # nothing left and not infinite

            # ------------- 2) run greedy knapsack & pack groups ------------ #
            groups = self._balanced_greedy_knapsack(
                buffer,
                self.seq_length,
                delta=5,
                max_images_per_knapsack=self.max_images_per_knapsack,
            )

            packed_group = []
            for g in groups:
                packed = self._pack_one_group(g, buffer, self.seq_length)
                packed_group.append({
                    "input_ids":                packed[0],
                    "labels":                   packed[1],
                    "attention_mask":           packed[2],
                    "images":                   packed[3],
                    "model_patch_positions":    packed[4],
                })

            if packed_group:
                queue.put(packed_group)

        # finished → unblock consumer
        queue.put(self._sentinel)

    def _balanced_greedy_knapsack(
        self, buffer, L, delta=0, max_images_per_knapsack=None
    ):
        # Extract lengths and image counts from buffer
        lengths = [len(x["input_ids"]) for x in buffer]
        image_counts = [len(x["images"]) for x in buffer]

        # keep the position while sorting
        items = sorted(
            enumerate(zip(lengths, image_counts)), key=lambda x: x[1][0], reverse=True
        )

        min_knapsacks = (sum(lengths) + L - 1) // L + delta
        knapsack_load = [0] * min_knapsacks
        knapsack_image_counts = [0] * min_knapsacks
        knapsack_groups = [[] for _ in range(min_knapsacks)]

        for idx, (item_len, item_image_count) in items:
            # Find a suitable knapsack that satisfies both length and image count constraints
            suitable_knapsack = None

            # First try to find a knapsack that can fit both constraints
            for ks_id in sorted(
                range(len(knapsack_load)), key=knapsack_load.__getitem__
            ):
                length_fits = knapsack_load[ks_id] + item_len <= L
                image_fits = (
                    max_images_per_knapsack is None
                    or knapsack_image_counts[ks_id] + item_image_count
                    <= max_images_per_knapsack
                )

                if length_fits and image_fits:
                    suitable_knapsack = ks_id
                    break

            # If no existing knapsack can fit, create a new one
            if suitable_knapsack is None:
                suitable_knapsack = len(knapsack_load)
                knapsack_load.append(0)
                knapsack_image_counts.append(0)
                knapsack_groups.append([])

            knapsack_groups[suitable_knapsack].append(idx)
            knapsack_load[suitable_knapsack] += item_len
            knapsack_image_counts[suitable_knapsack] += item_image_count

        # remove the completely empty bags that the +delta heuristic created
        random.shuffle(knapsack_groups)  # Knapsacks are semi-ordered after packing, thanks Luis for noticing!
        return [g for g in knapsack_groups if g]

    def _pack_one_group(self, group_indices, batch, max_len):
        ids, lbl, am, ims, mpp = [], [], [], [], []

        for i in group_indices:
            ids.extend(batch[i]["input_ids"])
            lbl.extend(batch[i]["labels"])
            am.extend(batch[i]["attention_mask"])
            ims.extend(batch[i]["images"])
            mpp.extend(batch[i]["model_patch_positions"])

        # safety: assert we never overflow
        if len(ids) > max_len:
            raise ValueError(f"Packed length {len(ids)} > max_len {max_len}")

        return torch.stack(ids), torch.stack(lbl), torch.stack(am), ims, mpp