import { unparse } from "papaparse";

/**
 * Shared CSV serialization for list-view exports (issue #605).
 *
 * Quoting/escaping is delegated to papaparse (`unparse`, RFC 4180); this
 * module only owns the cross-view conventions: column mapping, UTF-8 BOM,
 * and the filename timestamp — so the export format stays identical on every
 * dashboard.
 */

export interface CsvColumn<T> {
  /** Header label — should match the visible column label in the UI. */
  header: string;
  /** Cell value extractor. `null`/`undefined` render as an empty field. */
  value: (row: T) => string | number | null | undefined;
}

/** Serialize rows to a CSV string with a header row (CRLF line endings). */
export function toCsv<T>(rows: readonly T[], columns: readonly CsvColumn<T>[]): string {
  return unparse(
    {
      fields: columns.map((c) => c.header),
      data: rows.map((row) => columns.map((c) => c.value(row) ?? "")),
    },
    { newline: "\r\n" },
  );
}

/**
 * `YYYY-MM-DD-HHMM` local-time stamp for export filenames
 * (e.g. `purchase-orders-2026-06-08-1430.csv`).
 */
export function csvTimestamp(now: Date = new Date()): string {
  const pad = (n: number) => String(n).padStart(2, "0");
  return (
    `${now.getFullYear()}-${pad(now.getMonth() + 1)}-${pad(now.getDate())}` +
    `-${pad(now.getHours())}${pad(now.getMinutes())}`
  );
}

/**
 * Trigger a browser download of a CSV string. Prepends a UTF-8 BOM (U+FEFF) so
 * Excel detects the encoding and renders non-ASCII characters without mojibake.
 */
export function downloadCsv(csv: string, filename: string): void {
  const blob = new Blob(["\uFEFF" + csv], { type: "text/csv;charset=utf-8;" });
  const url = URL.createObjectURL(blob);
  const anchor = document.createElement("a");
  anchor.href = url;
  anchor.download = filename;
  document.body.appendChild(anchor);
  anchor.click();
  document.body.removeChild(anchor);
  URL.revokeObjectURL(url);
}