Fix ReDoS vulnerability for the .end() method

The other methods do not have this vulnerability, but I’m refactoring them while I’m at it.

index.js

@@ -1,6 +1,35 @@
 export default function trimNewlines(string) {
-	return string.replace(/^[\r\n]+|[\r\n]+$/g, '');
+	let start = 0;
+	let end = string.length;
+
+	while (start < end && (string[start] === '\r' || string[start] === '\n')) {
+		start++;
+	}
+
+	while (end > 0 && (string[end - 1] === '\r' || string[end - 1] === '\n')) {
+		end--;
+	}
+
+	return (start > 0 || end < string.length) ? string.slice(start, end) : string;
 }
 
-trimNewlines.start = string => string.replace(/^[\r\n]+/, '');
-trimNewlines.end = string => string.replace(/[\r\n]+$/, '');
+trimNewlines.start = string => {
+	const end = string.length;
+	let start = 0;
+
+	while (start < end && (string[start] === '\r' || string[start] === '\n')) {
+		start++;
+	}
+
+	return start > 0 ? string.slice(start, end) : string;
+};
+
+trimNewlines.end = string => {
+	let end = string.length;
+
+	while (end > 0 && (string[end - 1] === '\r' || string[end - 1] === '\n')) {
+		end--;
+	}
+
+	return end < string.length ? string.slice(0, end) : string;
+};

test.js

@@ -2,13 +2,19 @@ import test from 'ava';
 import trimNewlines from './index.js';
 
 test('main', t => {
+	t.is(trimNewlines(''), '');
+	t.is(trimNewlines('  '), '  ');
+	t.is(trimNewlines('\n\n\r'), '');
 	t.is(trimNewlines('\nx\n'), 'x');
 	t.is(trimNewlines('\n\n\nx\n\n\n'), 'x');
 	t.is(trimNewlines('\r\nx\r\n'), 'x');
 	t.is(trimNewlines('\n\r\n\nx\n\r\n\n'), 'x');
 });
 
 test('start', t => {
+	t.is(trimNewlines.start(''), '');
+	t.is(trimNewlines.start('  '), '  ');
+	t.is(trimNewlines.start('\n\n\r'), '');
 	t.is(trimNewlines.start('\nx'), 'x');
 	t.is(trimNewlines.start('\r\nx'), 'x');
 	t.is(trimNewlines.start('\n\n\n\nx'), 'x');
@@ -17,9 +23,42 @@ test('start', t => {
 });
 
 test('end', t => {
+	t.is(trimNewlines.end(''), '');
+	t.is(trimNewlines.end('  '), '  ');
+	t.is(trimNewlines.end('\n\n\r'), '');
 	t.is(trimNewlines.end('x\n'), 'x');
 	t.is(trimNewlines.end('x\r\n'), 'x');
 	t.is(trimNewlines.end('x\n\n\n\n'), 'x');
 	t.is(trimNewlines.end('x\n\n\r\n\n'), 'x');
 	t.is(trimNewlines.end('\n\n\r\n\nx'), '\n\n\r\n\nx');
 });
+
+test('main - does not have exponential performance', t => {
+	for (let index = 0; index < 45000; index += 1000) {
+		const string = String(Array.from({length: index}).fill('\n').join('')) + 'a' + String(Array.from({length: index}).fill('\n').join(''));
+		const start = Date.now();
+		trimNewlines(string);
+		const difference = Date.now() - start;
+		t.true(difference < 10, `Execution time: ${difference}`);
+	}
+});
+
+test('start - does not have exponential performance', t => {
+	for (let index = 0; index < 45000; index += 1000) {
+		const string = String(Array.from({length: index}).fill('\n').join('')) + 'a';
+		const start = Date.now();
+		trimNewlines.start(string);
+		const difference = Date.now() - start;
+		t.true(difference < 10, `Execution time: ${difference}`);
+	}
+});
+
+test('end - does not have exponential performance', t => {
+	for (let index = 0; index < 45000; index += 1000) {
+		const string = 'a' + String(Array.from({length: index}).fill('\n').join(''));
+		const start = Date.now();
+		trimNewlines.end(string);
+		const difference = Date.now() - start;
+		t.true(difference < 10, `Execution time: ${difference}`);
+	}
+});