import getCaretCoordinates from "textarea-caret";

export interface Anchor {
	line: number;
	top: number;
}

const project = (anchors: Anchor[], value: number, from: keyof Anchor, to: keyof Anchor) => {
	for (let i = 1; i < anchors.length; i++) {
		const prev = anchors[i - 1];
		const next = anchors[i];
		if (value > next[from]) continue;
		const span = next[from] - prev[from];
		if (span <= 0) return prev[to];
		return prev[to] + ((value - prev[from]) / span) * (next[to] - prev[to]);
	}
	return anchors.at(-1)?.[to] ?? 0;
};

export const topForLine = (anchors: Anchor[], line: number) =>
	project(anchors, line, "line", "top");

export const lineForTop = (anchors: Anchor[], top: number) => project(anchors, top, "top", "line");

export const monotonic = (anchors: Anchor[]): Anchor[] =>
	anchors.reduce<Anchor[]>((kept, anchor) => {
		const last = kept.at(-1);
		if (!last || (anchor.line > last.line && anchor.top >= last.top)) kept.push(anchor);
		return kept;
	}, []);

const lineCount = (value: string) => value.split("\n").length;

const previewAnchors = (pane: HTMLElement, lines: number): Anchor[] => {
	const origin = pane.getBoundingClientRect().top - pane.scrollTop;
	const found = Array.from(pane.querySelectorAll<HTMLElement>("[data-line]"), (element) => ({
		line: Number(element.dataset.line),
		top: element.getBoundingClientRect().top - origin
	}));
	return monotonic([{ line: 0, top: 0 }, ...found, { line: lines, top: pane.scrollHeight }]);
};

export const previewLine = (pane: HTMLElement, value: string) =>
	lineForTop(previewAnchors(pane, lineCount(value)), pane.scrollTop);

export const scrollPreviewToLine = (pane: HTMLElement, value: string, line: number) => {
	pane.scrollTop = topForLine(previewAnchors(pane, lineCount(value)), line);
};

const editorGeometry = (textarea: HTMLTextAreaElement) => {
	const starts = [0];
	for (let i = 0; i < textarea.value.length; i++) {
		if (textarea.value[i] === "\n") starts.push(i + 1);
	}
	const origin = getCaretCoordinates(textarea, 0).top;
	const lineTop = (line: number) =>
		line >= starts.length
			? textarea.scrollHeight
			: getCaretCoordinates(textarea, starts[line]).top - origin;
	const around = (line: number): Anchor[] => [
		{ line, top: lineTop(line) },
		{ line: line + 1, top: lineTop(line + 1) }
	];
	return { count: starts.length, lineTop, around };
};

export const editorLine = (textarea: HTMLTextAreaElement) => {
	const { count, lineTop, around } = editorGeometry(textarea);
	const target = textarea.scrollTop;
	let low = 0;
	let high = count - 1;
	while (low < high) {
		const mid = Math.ceil((low + high) / 2);
		if (lineTop(mid) <= target) low = mid;
		else high = mid - 1;
	}
	return lineForTop(around(low), target);
};

export const scrollEditorToLine = (textarea: HTMLTextAreaElement, line: number) => {
	const { count, around } = editorGeometry(textarea);
	const base = Math.min(Math.max(0, Math.floor(line)), count - 1);
	textarea.scrollTop = topForLine(around(base), line);
};