import csv
import io
from datetime import date

import openpyxl
from fastapi import HTTPException
from fastapi.responses import StreamingResponse


def resolve_period(
    period: str,
    date_from: date | None,
    date_to: date | None,
) -> tuple[date, date]:
    """Convert a named period to a (start, end) date tuple.

    Raises HTTP 422 for ``custom`` when either date is missing.
    """
    today = date.today()

    if period == "today":
        return today, today
    elif period == "last_7_days":
        from datetime import timedelta
        return today - timedelta(days=7), today
    elif period == "mtd":
        return today.replace(day=1), today
    elif period == "ytd":
        return today.replace(month=1, day=1), today
    elif period == "custom":
        if date_from is None or date_to is None:
            raise HTTPException(
                status_code=422,
                detail="date_from and date_to are required when period is 'custom'.",
            )
        return date_from, date_to
    else:
        raise HTTPException(
            status_code=422,
            detail=f"Invalid period '{period}'.",
        )


def apply_projection(
    rows: list[dict],
    projection_list: list[str] | None,
) -> list[dict]:
    """Filter dict keys to those in *projection_list*.

    ``_id`` is always excluded.  Returns *rows* unchanged when
    *projection_list* is ``None`` or empty (except ``_id`` is still stripped).
    """
    if not projection_list:
        # Still strip _id even when no explicit projection is requested
        return [{k: v for k, v in row.items() if k != "_id"} for row in rows]
    allowed = set(projection_list) - {"_id"}
    return [{k: v for k, v in row.items() if k in allowed} for row in rows]


async def stream_export(
    rows: list[dict],
    slug: str,
    export_format: str,
) -> StreamingResponse:
    """Build a CSV or XLSX file from *rows* and return a :class:`StreamingResponse`.

    Raises HTTP 400 when ``len(rows) > 100_000``.
    """
    if len(rows) > 100_000:
        raise HTTPException(
            status_code=400,
            detail="Export limit exceeded. Apply filters to reduce the result set.",
        )

    today_str = date.today().strftime("%Y%m%d")

    if export_format == "csv":
        buffer = io.StringIO()
        fieldnames = list(rows[0].keys()) if rows else []
        writer = csv.DictWriter(buffer, fieldnames=fieldnames)
        writer.writeheader()
        writer.writerows(rows)
        content = buffer.getvalue().encode("utf-8")

        filename = f"{slug}_{today_str}.csv"
        return StreamingResponse(
            iter([content]),
            media_type="text/csv; charset=utf-8",
            headers={"Content-Disposition": f'attachment; filename="{filename}"'},
        )

    elif export_format == "xlsx":
        wb = openpyxl.Workbook()
        ws = wb.active
        ws.title = slug

        fieldnames = list(rows[0].keys()) if rows else []
        ws.append(fieldnames)
        for row in rows:
            ws.append([row.get(f) for f in fieldnames])

        buffer = io.BytesIO()
        wb.save(buffer)
        buffer.seek(0)
        content = buffer.read()

        filename = f"{slug}_{today_str}.xlsx"
        return StreamingResponse(
            iter([content]),
            media_type="application/vnd.openxmlformats-officedocument.spreadsheetml.sheet",
            headers={"Content-Disposition": f'attachment; filename="{filename}"'},
        )

    else:
        raise HTTPException(
            status_code=400,
            detail="Invalid export format. Must be csv or xlsx.",
        )