import { jsPDF } from 'jspdf';
import html2canvas from 'html2canvas';

const PAGE_WIDTH_MM = 210;
const PAGE_HEIGHT_MM = 297;
const MARGIN_MM = 10;
const CONTENT_WIDTH_MM = PAGE_WIDTH_MM - MARGIN_MM * 2;
const USABLE_HEIGHT_MM = PAGE_HEIGHT_MM - MARGIN_MM * 2;

export const usableHeightCssPx = (widthPx: number): number =>
  (USABLE_HEIGHT_MM / CONTENT_WIDTH_MM) * widthPx;

/**
 * Safe page-break offsets for `data-pdf-block` nodes.
 *
 * A block's start is only a break point when the whole block fits on one page.
 * Oversized tables (e.g. a 49-day report) must start under the header instead
 * of being deferred to page 2.
 */
export function pdfBlockBreakOffsets(
  blocks: Array<{ top: number; bottom: number }>,
  usableHeightPx: number
): number[] {
  const breaks: number[] = [];
  for (const block of blocks) {
    const height = block.bottom - block.top;
    if (height <= usableHeightPx && block.top > 0) {
      breaks.push(block.top);
    }
    if (block.bottom > 0) {
      breaks.push(block.bottom);
    }
  }
  return [...new Set(breaks.map((value) => Math.round(value)))].sort((a, b) => a - b);
}

export function choosePdfSliceHeight(
  cursor: number,
  canvasHeight: number,
  usableHeightPx: number,
  breakPoints: number[]
): number {
  const remaining = canvasHeight - cursor;
  if (remaining <= usableHeightPx) return remaining;
  const limit = cursor + usableHeightPx;
  const candidate = [...breakPoints].reverse().find((point) => point > cursor && point <= limit);
  return candidate ? candidate - cursor : usableHeightPx;
}

/**
 * Render an element to a multi-page A4 PDF.
 *
 * html2canvas photographs the whole report once, then the canvas is sliced at
 * `data-pdf-block` boundaries (so a page break never cuts through a table) and
 * each slice is written as a compressed JPEG page.
 */
export async function exportElementToPdf(
  element: HTMLElement,
  filename: string,
  options?: { widthPx?: number }
): Promise<void> {
  const widthPx = options?.widthPx ?? 800;
  const scale = 1.5;
  const jpegQuality = 0.82;
  const blockRects: Array<{ top: number; bottom: number }> = [];

  const canvas = await html2canvas(element, {
    scale,
    useCORS: true,
    backgroundColor: '#ffffff',
    windowWidth: widthPx,
    onclone: (_clonedDoc, clonedElement) => {
      clonedElement.style.width = `${widthPx}px`;
      clonedElement.style.maxWidth = `${widthPx}px`;
      clonedElement.style.margin = '0';

      const top = clonedElement.getBoundingClientRect().top;
      clonedElement.querySelectorAll<HTMLElement>('[data-pdf-block]').forEach((node) => {
        const rect = node.getBoundingClientRect();
        blockRects.push({ top: rect.top - top, bottom: rect.bottom - top });
      });
    },
  });

  const doc = new jsPDF({ orientation: 'portrait', unit: 'mm', format: 'a4', compress: true });
  const pageWidthMm = doc.internal.pageSize.getWidth();
  const pageHeightMm = doc.internal.pageSize.getHeight();
  const contentWidthMm = pageWidthMm - MARGIN_MM * 2;
  const usableHeightMm = pageHeightMm - MARGIN_MM * 2;

  const mmPerPx = contentWidthMm / canvas.width;
  const usableHeightPx = usableHeightMm / mmPerPx;
  const safeBreaks = pdfBlockBreakOffsets(blockRects, usableHeightCssPx(widthPx));

  // Clone-CSS px -> canvas px, de-duplicated and ordered.
  const breakPoints = [...new Set(safeBreaks.map((value) => Math.round(value * scale)))]
    .filter((value) => value > 0 && value < canvas.height)
    .sort((a, b) => a - b);

  let cursor = 0;
  let firstPage = true;

  while (cursor < canvas.height) {
    const sliceHeight = choosePdfSliceHeight(cursor, canvas.height, usableHeightPx, breakPoints);
    if (sliceHeight <= 0) break;

    const slice = document.createElement('canvas');
    slice.width = canvas.width;
    slice.height = Math.max(1, Math.round(sliceHeight));
    const ctx = slice.getContext('2d');
    if (!ctx) break;
    ctx.fillStyle = '#ffffff';
    ctx.fillRect(0, 0, slice.width, slice.height);
    ctx.drawImage(canvas, 0, cursor, canvas.width, slice.height, 0, 0, canvas.width, slice.height);

    if (!firstPage) doc.addPage();
    doc.addImage(
      slice.toDataURL('image/jpeg', jpegQuality),
      'JPEG',
      MARGIN_MM,
      MARGIN_MM,
      contentWidthMm,
      slice.height * mmPerPx
    );

    firstPage = false;
    cursor += slice.height;
  }

  doc.save(filename);
}