import type { ChangeSpec } from '@codemirror/state';

/**
 *
 * Compares the updated text with the current to determine the transaction to fire
 * in the CodeMirror editor.
 *
 * @param curText string - the current CodeMirror text
 * @param newText string - the new ProseMirror text
 * @returns The changes or undefined if no change
 */

const N_LINE_FEED = '\n'.charCodeAt(0);
const R_LINE_FEED = '\r'.charCodeAt(0);

/**
 * Example of CRLF differences:
 * newText: 'abc\r\ndef\r\nghi'
 * curText: 'abc\ndef\nghi'
 */
export const getCMSelectionChanges = (curText: string, newText: string): ChangeSpec | undefined => {
	if (newText !== curText) {
		let curStart = 0,
			newStart = 0;
		let curEnd = curText.length,
			newEnd = newText.length;

		while (
			curStart < curEnd &&
			(curText.charCodeAt(curStart) === newText.charCodeAt(newStart) ||
				(newText.charCodeAt(newStart) === R_LINE_FEED &&
					curText.charCodeAt(curStart) === N_LINE_FEED))
		) {
			if (
				newText.charCodeAt(newStart) === R_LINE_FEED &&
				curText.charCodeAt(curStart) === N_LINE_FEED &&
				newText.charCodeAt(newStart + 1) === N_LINE_FEED
			) {
				newStart++;
			}
			curStart++;
			newStart++;
		}
		while (
			curEnd > curStart &&
			newEnd > newStart &&
			(curText.charCodeAt(curEnd - 1) === newText.charCodeAt(newEnd - 1) ||
				(newText.charCodeAt(newEnd - 1) === N_LINE_FEED &&
					curText.charCodeAt(curEnd - 1) === N_LINE_FEED))
		) {
			if (
				newText.charCodeAt(newEnd - 1) === N_LINE_FEED &&
				curText.charCodeAt(curEnd - 1) === N_LINE_FEED &&
				newText.charCodeAt(newEnd - 2) === R_LINE_FEED
			) {
				newEnd--;
			}
			curEnd--;
			newEnd--;
		}
		return {
			from: curStart,
			to: curEnd,
			insert: newText.slice(newStart, newEnd),
		};
	}
};