bea-edit/src/lib/pdf-text-overlay.ts

import { degrees, type PDFFont, type PDFPage, type RGB } from 'pdf-lib';
import { getPositionCoords, type OverlayPosition } from './pdf-overlay';

const DEFAULT_MARGIN = 20;
const DEFAULT_PADDING = 8;
const DEFAULT_MIN_SIZE = 4;
const DEFAULT_BORDER_WIDTH = 2;
/** Baseline nudge used by the framed text stamps so glyphs sit inside the border. */
const BASELINE_OFFSET_RATIO = 0.2;
const DEFAULT_TOO_LONG_MESSAGE =
	'Der Text ist für mindestens eine Seite zu lang. Kürzen Sie den Text.';

export type TextOverlayOptions = {
	font: PDFFont;
	text: string;
	size: number;
	color: RGB;
	position: OverlayPosition;
	/** Distance between the mark and the page edge in points. */
	margin?: number;
	/** Inner box padding; use `0` for a frameless, anchor-aligned mark. */
	padding?: number;
	framed?: boolean;
	borderWidth?: number;
	/** Additional counter-clockwise rotation of the mark in degrees. */
	rotation?: number;
	/** Font size floor while shrinking to fit the page. */
	minSize?: number;
	/**
	 * Place the mark on rotated scans (`/Rotate 90`, `180`, `270`) where it is
	 * visible instead of sideways. Opt-in so existing tools keep their output.
	 */
	compensatePageRotation?: boolean;
	tooLongMessage?: string;
};

type TextBox = {
	size: number;
	textHeight: number;
	boxWidth: number;
	boxHeight: number;
};

const measureText = (font: PDFFont, text: string, size: number) => {
	try {
		return { width: font.widthOfTextAtSize(text, size), height: font.heightAtSize(size) };
	} catch {
		throw new Error(
			'Der Text enthält Zeichen, die nicht dargestellt werden können. Entfernen Sie z. B. Emoji.'
		);
	}
};

const fitTextToPage = (
	font: PDFFont,
	text: string,
	requestedSize: number,
	pageWidth: number,
	pageHeight: number,
	margin: number,
	padding: number,
	minSize: number,
	tooLongMessage: string
): TextBox => {
	const availableWidth = pageWidth - margin * 2;
	const availableHeight = pageHeight - margin * 2;
	let size = requestedSize;
	let metrics = measureText(font, text, size);

	while (
		size > minSize &&
		(metrics.width + padding * 2 > availableWidth || metrics.height + padding * 2 > availableHeight)
	) {
		size = Math.max(minSize, size - 0.5);
		metrics = measureText(font, text, size);
	}

	if (
		metrics.width + padding * 2 > availableWidth ||
		metrics.height + padding * 2 > availableHeight
	) {
		throw new Error(tooLongMessage);
	}

	return {
		size,
		textHeight: metrics.height,
		boxWidth: metrics.width + padding * 2,
		boxHeight: metrics.height + padding * 2
	};
};

/**
 * Lower-left corner for a (possibly rotated) box inside the given page size.
 * The bounding box of the rotated mark is aligned to the requested position, so
 * tilted text never crosses the page edge.
 */
export const getRotatedBoxPlacement = (
	pageWidth: number,
	pageHeight: number,
	boxWidth: number,
	boxHeight: number,
	position: OverlayPosition,
	angle: number,
	margin = DEFAULT_MARGIN
) => {
	const radians = (angle * Math.PI) / 180;
	const cosine = Math.cos(radians);
	const sine = Math.sin(radians);
	const rotatedCorners = [
		{ x: 0, y: 0 },
		{ x: boxWidth * cosine, y: boxWidth * sine },
		{ x: -boxHeight * sine, y: boxHeight * cosine },
		{
			x: boxWidth * cosine - boxHeight * sine,
			y: boxWidth * sine + boxHeight * cosine
		}
	];
	const minX = Math.min(...rotatedCorners.map((corner) => corner.x));
	const maxX = Math.max(...rotatedCorners.map((corner) => corner.x));
	const minY = Math.min(...rotatedCorners.map((corner) => corner.y));
	const maxY = Math.max(...rotatedCorners.map((corner) => corner.y));
	const boundingCoordinates = getPositionCoords(
		pageWidth,
		pageHeight,
		maxX - minX,
		maxY - minY,
		position,
		margin
	);

	return {
		x: boundingCoordinates.x - minX,
		y: boundingCoordinates.y - minY,
		cosine,
		sine
	};
};

/** Normalised page rotation; anything that is not a multiple of 90 is ignored. */
const getPageRotation = (page: PDFPage): number => {
	const angle = Math.round(page.getRotation().angle / 90) * 90;
	return ((angle % 360) + 360) % 360;
};

/** Displayed page size after applying `/Rotate`. */
const getVisualPageSize = (pageWidth: number, pageHeight: number, pageRotation: number) =>
	pageRotation % 180 === 0
		? { width: pageWidth, height: pageHeight }
		: { width: pageHeight, height: pageWidth };

/**
 * Maps a point on the displayed page back into default user space. Viewers
 * rotate page content clockwise by `/Rotate`, so this is the inverse rotation
 * that keeps a mark at the requested visual position.
 */
const toUserSpace = (
	point: { x: number; y: number },
	pageWidth: number,
	pageHeight: number,
	pageRotation: number
) => {
	switch (pageRotation) {
		case 90:
			return { x: pageWidth - point.y, y: point.x };
		case 180:
			return { x: pageWidth - point.x, y: pageHeight - point.y };
		case 270:
			return { x: point.y, y: pageHeight - point.x };
		default:
			return point;
	}
};

/**
 * Draws a single line of text (optionally framed) at one of the standard
 * positions. Shrinks the font to fit the page and throws a German error instead
 * of clipping.
 */
export const drawTextOverlay = (page: PDFPage, options: TextOverlayOptions): void => {
	const {
		font,
		text,
		size,
		color,
		position,
		margin = DEFAULT_MARGIN,
		padding = DEFAULT_PADDING,
		framed = false,
		borderWidth = DEFAULT_BORDER_WIDTH,
		rotation = 0,
		minSize = DEFAULT_MIN_SIZE,
		compensatePageRotation = false,
		tooLongMessage = DEFAULT_TOO_LONG_MESSAGE
	} = options;

	const { width, height } = page.getSize();
	const pageRotation = compensatePageRotation ? getPageRotation(page) : 0;
	const visual = getVisualPageSize(width, height, pageRotation);
	const box = fitTextToPage(
		font,
		text,
		size,
		visual.width,
		visual.height,
		margin,
		padding,
		minSize,
		tooLongMessage
	);
	const placement = getRotatedBoxPlacement(
		visual.width,
		visual.height,
		box.boxWidth,
		box.boxHeight,
		position,
		rotation,
		margin
	);
	const origin = toUserSpace({ x: placement.x, y: placement.y }, width, height, pageRotation);
	const angle = rotation + pageRotation;
	const radians = (angle * Math.PI) / 180;
	const cosine = Math.cos(radians);
	const sine = Math.sin(radians);
	const offsetX = padding;
	const offsetY = padding + box.textHeight * BASELINE_OFFSET_RATIO;

	if (framed) {
		page.drawRectangle({
			x: origin.x,
			y: origin.y,
			width: box.boxWidth,
			height: box.boxHeight,
			borderColor: color,
			borderWidth,
			rotate: degrees(angle)
		});
	}

	page.drawText(text, {
		x: origin.x + offsetX * cosine - offsetY * sine,
		y: origin.y + offsetX * sine + offsetY * cosine,
		size: box.size,
		font,
		color,
		rotate: degrees(angle)
	});
};