"""CSV codec — bytes → ParsedFile (SD-0002 §6.5.1). File-level gates only (PUC-5a);
row semantics live in validate.py. Dialect detection is the INV-17 seam (SLICE-8
adds Shopify)."""
from __future__ import annotations

import csv
import io

from .errors import FileRejected
from .models import KNOWN_COLUMNS, MAX_DATA_ROWS, MAX_FILE_BYTES, ParsedFile, Row

_REQUIRED_HEADER_COLUMNS = ("Handle", "Title")


def detect_dialect(header: list[str]) -> str:
    """The INV-17 seam: SLICE-8 recognizes Shopify's exact header set here."""
    return "canonical"


def parse_csv(data: bytes) -> ParsedFile:
    if len(data) > MAX_FILE_BYTES:
        raise FileRejected("file_too_large", "This file is larger than 10 MB.")
    try:
        text = data.decode("utf-8-sig")
    except UnicodeDecodeError:
        raise FileRejected("not_csv", "This file isn't readable as CSV.") from None
    reader = csv.reader(io.StringIO(text))
    try:
        try:
            raw_header = next(reader)
        except StopIteration:
            raise FileRejected("not_csv", "This file isn't readable as CSV.") from None
        header = [h.strip() for h in raw_header]
        for col in _REQUIRED_HEADER_COLUMNS:
            if col not in header:
                raise FileRejected(
                    "missing_required_column",
                    f"This file is missing the required column '{col}'.",
                )
        # First occurrence of a duplicated column wins.
        col_index: dict[str, int] = {}
        for i, name in enumerate(header):
            if name and name not in col_index:
                col_index[name] = i
        known_present = [c for c in col_index if c in KNOWN_COLUMNS]
        unknown = [c for c in col_index if c not in KNOWN_COLUMNS]
        rows: list[Row] = []
        for raw in reader:
            if not any(cell.strip() for cell in raw):
                continue
            if len(rows) >= MAX_DATA_ROWS:
                raise FileRejected(
                    "too_many_rows",
                    f"This file has more than {MAX_DATA_ROWS:,} rows — split it and import in parts.",
                )
            cells = {
                c: (raw[col_index[c]].strip() if col_index[c] < len(raw) else "")
                for c in known_present
            }
            rows.append(Row(line_number=reader.line_num, cells=cells))
    except csv.Error:
        raise FileRejected("not_csv", "This file isn't readable as CSV.") from None
    return ParsedFile(
        dialect=detect_dialect(header), header=header, unknown_columns=unknown, rows=rows
    )