diff --git a/baukit/tokendataset.py b/baukit/tokendataset.py
new file mode 100644
index 0000000..e2c7e61
--- /dev/null
+++ b/baukit/tokendataset.py
@@ -0,0 +1,102 @@
+import torch
+from torch.utils.data import Dataset
+from torch.nn.utils.rnn import pad_sequence
+
+
+class TokenizedDataset(Dataset):
+    """
+    Converts a dataset of text samples into a dataset of token sequences,
+    as converted by a supplied tokenizer. The tokens come along with position
+    ids and attention masks, they can be supplied direcly to the model.
+    """
+    def __init__(self, text_dataset, tokenizer=None, maxlen=None, field="text"):
+        self.text_dataset = text_dataset
+        self.field = field
+        self.tokenizer = tokenizer
+        self.maxlen = maxlen
+        if hasattr(text_dataset, "info"):
+            self.info = text_dataset.info
+
+    def __len__(self):
+        return len(self.text_dataset)
+
+    def __getitem__(self, i):
+        text = self.text_dataset[i]
+        if self.field is not None:
+            text = text[self.field]
+        token_list = self.tokenizer.encode(
+            text, truncation=True, max_length=self.maxlen
+        )
+        position_ids = list(range(len(token_list)))
+        attention_mask = [1] * len(token_list)
+        return dict(
+            input_ids=torch.tensor(token_list),
+            position_ids=torch.tensor(position_ids),
+            attention_mask=torch.tensor(attention_mask)
+        )
+
+
+def dict_to_(data, device):
+    """
+    Moves a dictionary of tensors to the specified device.
+    """
+    for k in data:
+        data[k] = data[k].to(device)
+    return data
+
+
+def length_collation(token_size):
+    """
+    Sorts a batch of sequences and breaks it up into subbatches
+    of same-sized sequences, padding as needed.  Each batch
+    has no more than token_size total tokens (or a single
+    sequence, if the sequence happens to be larger).
+    """
+
+    def collate_fn(items):
+        items = sorted(items, key=lambda x: -len(x["input_ids"]))
+        batches = []
+        batch = []
+        batch_width = 0
+        for item in items:
+            item_width = len(item["input_ids"])
+            if item_width == 0:
+                break
+            if batch_width * (len(batch) + 1) > token_size:
+                batches.append(make_padded_batch(batch))
+                batch = []
+                batch_width = 0
+            if not batch:
+                batch_width = item_width
+            batch.append(item)
+        if len(batch):
+            batches.append(make_padded_batch(batch))
+        return batches
+
+    return collate_fn
+
+
+def make_padded_batch(items):
+    """
+    Pads sequences in a batch, so they are all the same length as the longest.
+    """
+    max_len = max(len(d["input_ids"]) for d in items)
+    if max_len == 0:
+        return {k: torch.zeros((0, 0), dtype=torch.long) for k in items[0]}
+
+    def join_items(items, k):
+        nonempty_data = [d[k] for d in items if len(d["input_ids"])]
+        if torch.is_tensor(items[0][k]):
+            return pad_sequence(nonempty_data, batch_first=True)
+        return nonempty_data
+
+    return { k: join_items(items, k) for k, v in items[0].items() }
+
+
+def flatten_masked_batch(data, mask):
+    """
+    Flattens feature data, ignoring items that are masked out of attention.
+    """
+    flat_data = data.view(-1, data.size(-1))
+    attended_tokens = mask.view(-1).nonzero()[:, 0]
+    return flat_data[attended_tokens]