Add counter for multiple occurences

index.d.ts

@@ -141,3 +141,10 @@ declare function slugify(
 ): string;
 
 export = slugify;
+
+declare function counter(
+	string: string,
+	options?: slugify.Options
+): string;
+
+declare function reset(): void;

index.js

@@ -21,7 +21,7 @@ const removeMootSeparators = (string, separator) => {
 		.replace(new RegExp(`^${escapedSeparator}|${escapedSeparator}$`, 'g'), '');
 };
 
-module.exports = (string, options) => {
+const slugify = (string, options) => {
 	if (typeof string !== 'string') {
 		throw new TypeError(`Expected a string, got \`${typeof string}\``);
 	}
@@ -65,3 +65,30 @@ module.exports = (string, options) => {
 
 	return string;
 };
+
+const counter = () => {
+	const occurrences = new Map();
+
+	const countable = (string, options) => {
+		string = slugify(string, options);
+		const stringLower = string.toLowerCase();
+		const numberless = occurrences.get(stringLower.replace(/(?:-\d+?)+?$/, '')) || 0;
+		const counter = occurrences.get(stringLower);
+		occurrences.set(stringLower, typeof counter === 'number' ? counter + 1 : 1);
+		const newCounter = occurrences.get(stringLower) || 2;
+		if (newCounter >= 2 || numberless > 2) {
+			string = `${string}-${newCounter}`;
+		}
+
+		return string;
+	};
+
+	countable.reset = () => {
+		occurrences.clear();
+	};
+
+	return countable;
+};
+
+module.exports = slugify;
+module.exports.counter = counter;

test.js

@@ -131,3 +131,31 @@ test('leading underscore', t => {
 	t.is(slugify('__foo__bar', {preserveLeadingUnderscore: true}), '_foo-bar');
 	t.is(slugify('____-___foo__bar', {preserveLeadingUnderscore: true}), '_foo-bar');
 });
+
+test('counter', t => {
+	const countableSlugify = slugify.counter();
+	t.is(countableSlugify('foo bar'), 'foo-bar');
+	t.is(countableSlugify('foo bar'), 'foo-bar-2');
+	countableSlugify.reset();
+	t.is(countableSlugify('foo'), 'foo');
+	t.is(countableSlugify('foo'), 'foo-2');
+	t.is(countableSlugify('foo 1'), 'foo-1');
+	t.is(countableSlugify('foo-1'), 'foo-1-2');
+	t.is(countableSlugify('foo-1'), 'foo-1-3');
+	t.is(countableSlugify('foo'), 'foo-3');
+	t.is(countableSlugify('foo'), 'foo-4');
+	t.is(countableSlugify('foo-1'), 'foo-1-4');
+	t.is(countableSlugify('foo-2'), 'foo-2-1'); // Or foo-2-2 ??
+	t.is(countableSlugify('foo-2'), 'foo-2-2');
+	t.is(countableSlugify('foo-2-1'), 'foo-2-1-1');
+	t.is(countableSlugify('foo-2-1'), 'foo-2-1-2');
+	t.is(countableSlugify('foo-11'), 'foo-11-1');
+	t.is(countableSlugify('foo-111'), 'foo-111-1');
+	t.is(countableSlugify('foo-111-1'), 'foo-111-1-1');
+	t.is(countableSlugify('fooCamelCase', {lowercase: false, decamelize: false}), 'fooCamelCase');
+	t.is(countableSlugify('fooCamelCase', {decamelize: false}), 'foocamelcase-2');
+
+	const countableSlugify2 = slugify.counter();
+	t.is(countableSlugify2('foo'), 'foo');
+	t.is(countableSlugify2('foo'), 'foo-2');
+});