Support offset when numbering

lib/numbering.js

@@ -103,8 +103,8 @@ function getNumberingRanges(str) {
  * @return {String}
  */
 function replaceNumberingRanges(str, ranges, value) {
-    const replaced = replaceRanges(str, ranges, token => {
-        let _value = String(value);
+    const replaced = replaceRanges(str, ranges, (token, offset) => {
+        let _value = String(value + offset);
         // pad values for multiple numbering tokens, e.g. 3 for $$$ becomes 003
         while (_value.length < token.length) {
             _value = '0' + _value;

lib/utils.js

@@ -45,9 +45,19 @@ export function replaceRanges(str, ranges, value) {
 	for (let i = ranges.length - 1; i >= 0; i--) {
 		const r = ranges[i];
 
+        let offset = 0;
+        let offsetLength = 0;
+        if (str.substr(r[0] + r[1], 1) === '@'){
+            const matches = str.substr(r[0] + r[1] + 1).match(/^(\d+)/);
+            if (matches) {
+                offsetLength = matches[1].length + 1;
+                offset = parseInt(matches[1]) - 1;
+            }
+        }
+
 		str = str.substring(0, r[0])
-			+ (typeof value === 'function' ? value(str.substr(r[0], r[1])) : value)
-			+ str.substring(r[0] + r[1]);
+			+ (typeof value === 'function' ? value(str.substr(r[0], r[1]), offset) : value)
+			+ str.substring(r[0] + r[1] + offsetLength);
 	}
 
 	return str;

test/numbering.js

@@ -41,4 +41,9 @@ describe('Item numbering', () => {
 		const tree = numbering(parse('[!foo="bar$"]*2'));
 		assert(tree.firstChild.getAttribute('foo').options.implied);
 	});
+
+	it('add offset to numbering', () => {
+		assert.equal(expand('span.item$@3*2'), '<span class="item3"></span><span class="item4"></span>');
+		assert.equal(expand('span.item$$@3*2'), '<span class="item03"></span><span class="item04"></span>');
+	});
 });