import csv
from io import StringIO

from infrastructure.s3_client import S3Client
from utils.s3 import parse_s3_uri


class CsvS3Client(S3Client):
    """:class:`S3Client` subclass that round-trips CSV row dictionaries.

    Rows are represented as ``list[dict[str, str]]`` — the same shape used by
    :func:`csv.DictReader`/``DictWriter`` — which keeps the API trivially
    compatible with existing CSV helpers in ``utils/s3.py``.
    """

    def read_rows(self, s3_uri: str) -> list[dict[str, str]]:
        """Fetch the object at ``s3_uri`` and decode it as a CSV.

        The bucket portion of the URI is validated against this client's
        configured bucket so cross-bucket reads fail loudly rather than
        silently fetching from the wrong place.
        """
        bucket, key = parse_s3_uri(s3_uri)
        if bucket != self.bucket:
            raise ValueError(
                f"s3_uri bucket {bucket!r} does not match client bucket {self.bucket!r}"
            )
        raw = self.get_object(key)
        text = raw.decode("utf-8-sig")
        reader = csv.DictReader(StringIO(text))
        return [dict(row) for row in reader]

    def save_rows(self, rows: list[dict[str, str]], key: str) -> str:
        """Serialise ``rows`` to CSV under ``key`` and return the ``s3://`` URI.

        An empty ``rows`` list is rejected because we cannot otherwise infer
        a header row.
        """
        if not rows:
            raise ValueError("Cannot save an empty rows list: header is unknown")
        buffer = StringIO()
        fieldnames = list(rows[0].keys())
        writer = csv.DictWriter(buffer, fieldnames=fieldnames)
        writer.writeheader()
        writer.writerows(rows)
        return self.put_object(key, buffer.getvalue().encode("utf-8"))