getLocator.js

export default function getLocator ( source ) {
	const originalLines = source.split( '\n' );
	const lineOffsets = [];

	for ( let i = 0, pos = 0; i < originalLines.length; i++ ) {
		lineOffsets.push( pos );
		pos += originalLines[i].length + 1;
	}

	return function locate ( index ) {
		let i = 0;
		let j = lineOffsets.length;
		while ( i < j ) {
			const m = ( i + j ) >> 1;
			if ( index < lineOffsets[m] ) {
				j = m;
			} else {
				i = m + 1;
			}
		}
		const line = i - 1;
		const column = index - lineOffsets[line];
		return { line, column };
	};
}