OSCAR-2109 datasets are misaligned and truncated
Author: adrianeboyd | Pull request: no | State: closed

## Describe the bug

The `oscar-corpus/OSCAR-2109` data appears to be misaligned and truncated by the dataset builder for subsets that contain more than one part and for cases where the texts contain non-unix newlines.

## Steps to reproduce the bug

A few examples, although I'm not sure how deterministic the particular (mis)alignment is in various configurations:

```python
from datasets import load_dataset
dataset = load_dataset("oscar-corpus/OSCAR-2109", "deduplicated_fi", split="train", use_auth_token=True)
entry = dataset[0]
# entry["text"] is from fi_part_3.txt.gz
# entry["meta"] is from fi_meta_part_2.jsonl.gz

dataset = load_dataset("oscar-corpus/OSCAR-2109", "deduplicated_no", split="train", use_auth_token=True)
entry = dataset[900000]
# entry["text"] is from no_part_3.txt.gz and contains a blank line
# entry["meta"] is from no_meta_part_1.jsonl.gz

dataset = load_dataset("oscar-corpus/OSCAR-2109", "deduplicated_mk", split="train", streaming=True, use_auth_token=True)
# 9088 texts in the dataset are empty
```

For `deduplicated_fi`, all exported raw texts from the dataset are 17GB rather than 20GB as reported in the data splits overview table. The token count with `wc -w` for the raw texts is 2,067,556,874 rather than the expected 2,357,264,196 from the data splits table.

For `deduplicated_no` all exported raw texts contain 624,040,887 rather than the expected 776,354,517 tokens.

For `deduplicated_mk` it is 122,236,936 rather than 134,544,934 tokens. 

I'm not expecting the `wc -w` counts to line up exactly with the data splits table, but for comparison the `wc -w` count for `deduplicated_mk` on the raw texts is 134,545,424.

## Issues

* The meta / text files are not paired correctly when loading, so the extracted texts do not have the right offsets, the metadata is not associated with the correct text, and the text files may not be processed to the end or may be processed beyond the end (empty texts).
* The line count offset is not reset per file so the texts aren't aligned to the right offsets in any parts beyond the first part, leading to truncation when in effect blank lines are not skipped.
* Non-unix newline characters are treated as newlines when reading the text files while the metadata only counts unix newlines for its line offsets, leading to further misalignments between the metadata and the extracted texts, and which also results in truncation.

## Expected results

All texts from the OSCAR release are extracted according to the metadata and aligned with the correct metadata.

## Fixes

Not necessarily the exact fixes/checks you may want to use (I didn't test all languages or do any cross-platform testing, I'm not sure all the details are compatible with streaming), however to highlight the issues:

```diff
diff --git a/OSCAR-2109.py b/OSCAR-2109.py
index bbac1076..5eee8de7 100644
--- a/OSCAR-2109.py
+++ b/OSCAR-2109.py
@@ -20,6 +20,7 @@
 import collections
 import gzip
 import json
+import os
 
 import datasets
 
@@ -387,9 +388,20 @@ class Oscar2109(datasets.GeneratorBasedBuilder):
         with open(checksum_file, encoding="utf-8") as f:
             data_filenames = [line.split()[1] for line in f if line]
             data_urls = [self.config.base_data_path + data_filename for data_filename in data_filenames]
-        text_files = dl_manager.download([url for url in data_urls if url.endswith(".txt.gz")])
-        metadata_files = dl_manager.download([url for url in data_urls if url.endswith(".jsonl.gz")])
+        # sort filenames so corresponding parts are aligned
+        text_files = sorted(dl_manager.download([url for url in data_urls if url.endswith(".txt.gz")]))
+        metadata_files = sorted(dl_manager.download([url for url in data_urls if url.endswith(".jsonl.gz")]))
+        assert len(text_files) == len(metadata_files)
         metadata_and_text_files = list(zip(metadata_files, text_files))
+        for meta_path, text_path in metadata_and_text_files:
+            # check that meta/text part numbers are the same
+            if "part" in os.path.basename(text_path):
+                assert (
+                    os.path.basename(text_path).replace(".txt.gz", "").split("_")[-1]
+                    == os.path.basename(meta_path).replace(".jsonl.gz", "").split("_")[-1]
+                )
+            else:
+                assert len(metadata_and_text_files) == 1
         return [
             datasets.SplitGenerator(name=datasets.Split.TRAIN, gen_kwargs={"metadata_and_text_files": metadata_and_text_files}),
         ]
@@ -397,10 +409,14 @@ class Oscar2109(datasets.GeneratorBasedBuilder):
     def _generate_examples(self, metadata_and_text_files):
         """This function returns the examples in the raw (text) form by iterating on all the files."""
         id_ = 0
-        offset = 0
         for meta_path, text_path in metadata_and_text_files:
+            # line offsets are per text file
+            offset = 0
             logger.info("generating examples from = %s", text_path)
-            with gzip.open(open(text_path, "rb"), "rt", encoding="utf-8") as text_f:
+            # some texts contain non-Unix newlines that should not be
+            # interpreted as line breaks for the line counts in the metadata
+            # with readline()
+            with gzip.open(open(text_path, "rb"), "rt", encoding="utf-8", newline="\n") as text_f:
                 with gzip.open(open(meta_path, "rb"), "rt", encoding="utf-8") as meta_f:
                     for line in meta_f:
                         # read meta
@@ -411,7 +427,12 @@ class Oscar2109(datasets.GeneratorBasedBuilder):
                             offset += 1
                             text_f.readline()
                         # read text
-                        text = "".join([text_f.readline() for _ in range(meta["nb_sentences"])]).rstrip()
+                        text_lines = [text_f.readline() for _ in range(meta["nb_sentences"])]
+                        # all lines contain text (no blank lines or EOF)
+                        assert all(text_lines)
+                        assert "\n" not in text_lines
                         offset += meta["nb_sentences"]
+                        # only strip the trailing newline
+                        text = "".join(text_lines).rstrip("\n")
                         yield id_, {"id": id_, "text": text, "meta": meta}
                         id_ += 1
```

I've tested this with a number of smaller deduplicated languages with 1-20 parts and the resulting datasets looked correct in terms of word count and size when compared to the data splits table and raw texts, and the text/metadata alignments were correct in all my spot checks. However, there are many many languages I didn't test and I'm not sure that there aren't any texts containing blank lines in the corpus, for instance. For the cases I tested, the assertions related to blank lines and EOF made it easier to verify that the text and metadata were aligned as intended, since there would be little chance of spurious alignments of variable-length texts across so much data.