export interface ExportColumn<T> {
  header: string
  value: (row: T) => string | number | null | undefined
  /** Right-align and format as a number in Excel. */
  numeric?: boolean
}

const MAX_PAGE_SIZE = 200

/** Walks every page of a server-paged endpoint and returns all rows. */
export async function fetchAllPages<T>(
  fetchPage: (page: number, pageSize: number) => Promise<{ items: T[]; total: number }>,
): Promise<T[]> {
  const rows: T[] = []
  for (let page = 1; ; page++) {
    const res = await fetchPage(page, MAX_PAGE_SIZE)
    rows.push(...res.items)
    if (res.items.length === 0 || rows.length >= res.total) return rows
  }
}

function stamp() {
  return new Date().toISOString().slice(0, 10)
}

function download(blob: Blob, filename: string) {
  const url = URL.createObjectURL(blob)
  const a = document.createElement('a')
  a.href = url
  a.download = filename
  a.click()
  URL.revokeObjectURL(url)
}

export async function exportExcel<T>(name: string, columns: ExportColumn<T>[], rows: T[]) {
  const { Workbook } = await import('exceljs')
  const wb = new Workbook()
  const ws = wb.addWorksheet(name.slice(0, 31))

  ws.columns = columns.map(c => ({ header: c.header, width: Math.max(c.header.length + 4, 14) }))
  ws.getRow(1).font = { bold: true }
  ws.views = [{ state: 'frozen', ySplit: 1 }]

  rows.forEach(r => ws.addRow(columns.map(c => c.value(r) ?? '')))
  columns.forEach((c, i) => {
    if (c.numeric) {
      const col = ws.getColumn(i + 1)
      col.numFmt = '#,##0.##'
      col.alignment = { horizontal: 'right' }
    }
  })

  const buffer = await wb.xlsx.writeBuffer()
  download(
    new Blob([buffer], { type: 'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet' }),
    `${name}-${stamp()}.xlsx`,
  )
}

export async function exportPdf<T>(name: string, title: string, columns: ExportColumn<T>[], rows: T[], subtitle?: string) {
  const [{ jsPDF }, { default: autoTable }] = await Promise.all([import('jspdf'), import('jspdf-autotable')])
  const doc = new jsPDF({ orientation: columns.length > 6 ? 'landscape' : 'portrait', unit: 'pt', format: 'a4' })

  doc.setFontSize(14)
  doc.text(title, 40, 40)
  doc.setFontSize(9)
  doc.text(subtitle ? `${subtitle}  ·  Generated ${stamp()}` : `Generated ${stamp()}`, 40, 56)

  autoTable(doc, {
    startY: 68,
    head: [columns.map(c => c.header)],
    body: rows.map(r => columns.map(c => String(c.value(r) ?? ''))),
    styles: { fontSize: 8 },
    headStyles: { fillColor: [55, 65, 81] },
    columnStyles: Object.fromEntries(
      columns.map((c, i) => [i, c.numeric ? { halign: 'right' as const } : {}]),
    ),
  })

  doc.save(`${name}-${stamp()}.pdf`)
}