from __future__ import annotations
from dataclasses import dataclass, field
from pathlib import Path
from typing import Literal
import yaml


@dataclass
class ColumnMapping:
    source: str
    target: str
    dtype: str | None = None  # override detected type, e.g. "VARCHAR(100)", "NUMBER"
    skip: bool = False


@dataclass
class SheetConfig:
    sheet: str | int = 0          # sheet name or index
    header_row: int = 0           # 0-based row index of the header
    skip_rows: int = 0            # rows to skip before header
    target_table: str = ""
    columns: list[ColumnMapping] = field(default_factory=list)
    mode: Literal["append", "replace", "upsert"] = "append"
    upsert_keys: list[str] = field(default_factory=list)  # column names for upsert PK


@dataclass
class ImportConfig:
    dsn: str                        # SQLAlchemy DSN
    sheets: list[SheetConfig] = field(default_factory=list)
    default_varchar_length: int = 255

    @classmethod
    def from_yaml(cls, path: str | Path) -> "ImportConfig":
        with open(path) as f:
            raw = yaml.safe_load(f)

        sheets = []
        for s in raw.get("sheets", []):
            columns = [ColumnMapping(**c) for c in s.pop("columns", [])]
            upsert_keys = s.pop("upsert_keys", [])
            sheets.append(SheetConfig(**s, columns=columns, upsert_keys=upsert_keys))

        return cls(
            dsn=raw["dsn"],
            default_varchar_length=raw.get("default_varchar_length", 255),
            sheets=sheets,
        )