"""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 .dialect_shopify import is_shopify_header, map_shopify_header
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: recognize Shopify's header set, else canonical (§6.5.1)."""
    return "shopify" if is_shopify_header(header) else "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]
        dialect = detect_dialect(header)

        # INV-17: normalize the header to canonical names at the boundary. mapped[i]
        # is the canonical name for header[i] (or None when that column has no
        # canonical home); unknown is the not-imported warning list. Canonical files
        # map to themselves; unknown columns are warned exactly as before.
        if dialect == "shopify":
            mapped, unknown = map_shopify_header(header)
        else:
            mapped = [c if c in KNOWN_COLUMNS else None for c in header]
            unknown = [c for c in header if c and c not in KNOWN_COLUMNS]

        for col in _REQUIRED_HEADER_COLUMNS:
            if col not in mapped:
                raise FileRejected(
                    "missing_required_column",
                    f"This file is missing the required column '{col}'.",
                )

        # First occurrence of a duplicated canonical column wins.
        col_index: dict[str, int] = {}
        for i, name in enumerate(mapped):
            if name and name not in col_index:
                col_index[name] = i
        # De-dup the warning list, order-preserving.
        seen: set[str] = set()
        unknown = [c for c in unknown if not (c in seen or seen.add(c))]

        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 col_index
            }
            # Shopify grams carry an implicit unit; canonical needs it explicit (§6.5.1).
            if dialect == "shopify" and cells.get("Variant Weight"):
                cells["Variant Weight Unit"] = "g"
            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=dialect, header=header, unknown_columns=unknown, rows=rows)