export type PdfThumbnail = {
	pageNumber: number;
	imageUrl: string;
	width: number;
	height: number;
};

export type PdfDocumentHandle = {
	pageCount: number;
	thumbnails: PdfThumbnail[];
	destroy: () => void;
};

/** Pixel scale used for on-demand page previews. */
export const PDF_PREVIEW_SCALE = 1.25;

export type PdfPagePreview = {
	pageNumber: number;
	imageUrl: string;
	width: number;
	height: number;
};

/**
 * Renders one page at a time. Used by tools that operate on a whole document
 * (pagination) where rasterising every page up front would exhaust memory.
 */
export type PdfPageRenderer = {
	pageCount: number;
	render: (pageNumber: number) => Promise<PdfPagePreview>;
	destroy: () => void;
};

const loadPdfJs = async () => {
	const pdfjsLib = await import('pdfjs-dist');
	const { getDocument, GlobalWorkerOptions } = pdfjsLib;

	GlobalWorkerOptions.workerSrc = await import('pdfjs-dist/build/pdf.worker.mjs?url').then(
		(module) => module.default
	);

	return getDocument;
};

const canvasToPngBlob = (canvas: HTMLCanvasElement) =>
	new Promise<Blob>((resolve, reject) => {
		canvas.toBlob(
			(result) => (result ? resolve(result) : reject(new Error('Could not encode PDF thumbnail.'))),
			'image/png'
		);
	});

export async function loadPdfDocument(data: Uint8Array): Promise<PdfDocumentHandle> {
	const getDocument = await loadPdfJs();

	const loadingTask = getDocument({ data: new Uint8Array(data) });
	const thumbnails: PdfThumbnail[] = [];

	try {
		const pdfDocument = await loadingTask.promise;

		for (let pageNumber = 1; pageNumber <= pdfDocument.numPages; pageNumber += 1) {
			const page = await pdfDocument.getPage(pageNumber);
			const viewport = page.getViewport({ scale: 0.5 });
			const canvas = document.createElement('canvas');
			canvas.width = Math.ceil(viewport.width);
			canvas.height = Math.ceil(viewport.height);
			const canvasContext = canvas.getContext('2d');

			if (!canvasContext) throw new Error('Could not create thumbnail canvas context.');

			await page.render({ canvas, canvasContext, viewport }).promise;

			thumbnails.push({
				pageNumber,
				imageUrl: URL.createObjectURL(await canvasToPngBlob(canvas)),
				width: viewport.width,
				height: viewport.height
			});
		}

		let destroyed = false;
		return {
			pageCount: pdfDocument.numPages,
			thumbnails,
			destroy: () => {
				if (destroyed) return;
				destroyed = true;
				for (const thumbnail of thumbnails) URL.revokeObjectURL(thumbnail.imageUrl);
				void loadingTask.destroy();
			}
		};
	} catch (error) {
		for (const thumbnail of thumbnails) URL.revokeObjectURL(thumbnail.imageUrl);
		await loadingTask.destroy();
		throw error;
	}
}

export async function loadPdfPageRenderer(data: Uint8Array): Promise<PdfPageRenderer> {
	const getDocument = await loadPdfJs();

	const loadingTask = getDocument({ data: new Uint8Array(data) });
	const pdfDocument = await loadingTask.promise;

	let destroyed = false;
	let renderToken = 0;
	let currentUrl: string | null = null;

	const releaseUrl = () => {
		if (currentUrl) URL.revokeObjectURL(currentUrl);
		currentUrl = null;
	};

	return {
		pageCount: pdfDocument.numPages,
		render: async (pageNumber) => {
			if (destroyed) throw new Error('Die PDF-Vorschau wurde bereits geschlossen.');

			const token = ++renderToken;
			const clampedPage = Math.min(
				pdfDocument.numPages,
				Math.max(1, Math.round(Number(pageNumber) || 1))
			);
			const page = await pdfDocument.getPage(clampedPage);
			const viewport = page.getViewport({ scale: PDF_PREVIEW_SCALE });
			const canvas = document.createElement('canvas');
			canvas.width = Math.ceil(viewport.width);
			canvas.height = Math.ceil(viewport.height);
			const canvasContext = canvas.getContext('2d');

			if (!canvasContext) throw new Error('Could not create preview canvas context.');

			await page.render({ canvas, canvasContext, viewport }).promise;
			const imageUrl = URL.createObjectURL(await canvasToPngBlob(canvas));

			// A newer render (or unmount) superseded this one: drop the bitmap
			// instead of leaving an orphaned object URL behind.
			if (destroyed || token !== renderToken) {
				URL.revokeObjectURL(imageUrl);
				throw new Error('Die PDF-Vorschau wurde zwischenzeitlich geändert.');
			}

			releaseUrl();
			currentUrl = imageUrl;

			return {
				pageNumber: clampedPage,
				imageUrl,
				width: viewport.width,
				height: viewport.height
			};
		},
		destroy: () => {
			if (destroyed) return;
			destroyed = true;
			renderToken += 1;
			releaseUrl();
			void loadingTask.destroy();
		}
	};
}