// Pure text transformations for the minimal Markdown editor. They take the
// textarea state and return the new content plus the selection to restore,
// which keeps them testable without a DOM.

export interface EditState {
	content: string;
	selectionStart: number;
	selectionEnd: number;
}

export interface LinePrefix {
	add: string;
	detect: RegExp;
}

const lineBounds = (content: string, start: number, end: number) => {
	const lineStart = content.lastIndexOf('\n', start - 1) + 1;
	let lineEnd = content.indexOf('\n', end);
	if (lineEnd < 0) lineEnd = content.length;
	return { start: lineStart, end: lineEnd };
};

export const insertTextAtSelection = (state: EditState, text: string): EditState => {
	const cursor = state.selectionStart + text.length;
	return {
		content: state.content.substring(0, state.selectionStart) + text + state.content.substring(state.selectionEnd),
		selectionStart: cursor,
		selectionEnd: cursor,
	};
};

export const toggleWrap = (state: EditState, marker: string, placeholder: string): EditState => {
	const { content, selectionStart, selectionEnd } = state;
	const selected = content.substring(selectionStart, selectionEnd);
	const before = content.substring(0, selectionStart);
	const after = content.substring(selectionEnd);

	if (before.endsWith(marker) && after.startsWith(marker)) {
		return {
			content: before.substring(0, before.length - marker.length) + selected + after.substring(marker.length),
			selectionStart: selectionStart - marker.length,
			selectionEnd: selectionEnd - marker.length,
		};
	}

	if (selected.length >= marker.length * 2 && selected.startsWith(marker) && selected.endsWith(marker)) {
		const inner = selected.substring(marker.length, selected.length - marker.length);
		return {
			content: before + inner + after,
			selectionStart,
			selectionEnd: selectionStart + inner.length,
		};
	}

	const text = selected.length > 0 ? selected : placeholder;
	return {
		content: `${before}${marker}${text}${marker}${after}`,
		selectionStart: selectionStart + marker.length,
		selectionEnd: selectionStart + marker.length + text.length,
	};
};

export const toggleLinePrefix = (state: EditState, prefix: LinePrefix): EditState => {
	const { start, end } = lineBounds(state.content, state.selectionStart, state.selectionEnd);
	const lines = state.content.substring(start, end).split('\n');
	const nonEmpty = lines.filter(line => line.trim().length > 0);
	const remove = nonEmpty.length > 0 && nonEmpty.every(line => prefix.detect.test(line));
	const collapsed = state.selectionStart === state.selectionEnd;

	let selectionStart = state.selectionStart;
	let selectionEnd = state.selectionEnd;
	const newLines: string[] = [];
	let offset = start;
	for (const line of lines) {
		const isEmpty = line.trim().length === 0;
		const newLine = isEmpty ? line : (remove ? line.replace(prefix.detect, '') : `${prefix.add}${line}`);
		const delta = newLine.length - line.length;
		// An inserted prefix lands before a cursor sitting at the line start, so
		// the cursor follows it. A removed prefix leaves the line start unchanged.
		if (offset < selectionStart || (offset === selectionStart && !remove)) selectionStart += delta;
		if (offset < selectionEnd || (offset === selectionEnd && collapsed && !remove)) selectionEnd += delta;
		offset += line.length + 1;
		newLines.push(newLine);
	}

	return {
		content: state.content.substring(0, start) + newLines.join('\n') + state.content.substring(end),
		selectionStart,
		selectionEnd,
	};
};

export const toggleCodeBlock = (state: EditState): EditState => {
	const { content, selectionStart, selectionEnd } = state;
	const selected = content.substring(selectionStart, selectionEnd);
	const before = content.substring(0, selectionStart);
	const after = content.substring(selectionEnd);
	const fence = '```';

	const fenced = new RegExp(`^\\s*${fence}\\n([\\s\\S]*)\\n${fence}\\s*$`);
	const fencedSelection = selected.match(fenced);
	if (fencedSelection) {
		const inner = fencedSelection[1];
		return {
			content: before + inner + after,
			selectionStart,
			selectionEnd: selectionStart + inner.length,
		};
	}

	if (selected.length > 0) {
		return {
			content: `${before}${fence}\n${selected}\n${fence}${after}`,
			selectionStart: selectionStart + fence.length + 1,
			selectionEnd: selectionStart + fence.length + 1 + selected.length,
		};
	}

	const cursor = selectionStart + fence.length + 1;
	return {
		content: `${before}${fence}\n\n${fence}${after}`,
		selectionStart: cursor,
		selectionEnd: cursor,
	};
};

export const wrapWithLink = (state: EditState, url: string): EditState => {
	const { content, selectionStart, selectionEnd } = state;
	const selected = content.substring(selectionStart, selectionEnd);
	const link = `[${selected}](${url})`;
	return {
		content: content.substring(0, selectionStart) + link + content.substring(selectionEnd),
		selectionStart: selectionStart + 1,
		selectionEnd: selectionStart + 1 + selected.length,
	};
};