Append identical header ids with counter

This ensures unique HTML id's in line with the HTML spec, and allows anchors
to always reference the intended header.

Fixes GitbookIO#27

lib/renderer.js

@@ -15,6 +15,8 @@ var defaultOptions = {
 
 function Renderer(options) {
   this.options = options || defaultOptions;
+
+  this._idCounter = {}
 }
 
 Renderer.prototype.code = function(code, lang, escaped) {
@@ -56,7 +58,22 @@ Renderer.prototype._createId = function(str) {
   } catch (e) {
     str = str.replace(/[^\w]+/g, '-');
   }
-  return str.replace(/-$/, '');
+  str = str.replace(/-$/, '');
+
+  // make sure internal id has the current id registered. If not, initialize it
+  // at 0
+  if (this._idCounter[str] === undefined) {
+    this._idCounter[str] = 0;
+  }
+
+  // Increment the internal counter for the current id, and save new count
+  var cnt = ++this._idCounter[str];
+
+  if (cnt > 1) {
+    str = str + '-' + cnt
+  }
+
+  return str;
 };
 
 Renderer.prototype.heading = function(text, level, raw) {

test/new/identical_header_1.html

@@ -0,0 +1,2 @@
+<h1 id="identical-ids">Identical IDs</h1>
+<h1 id="identical-ids-2">Identical IDs</h1>

test/new/identical_header_1.text

@@ -0,0 +1,3 @@
+# identical ids
+
+# identical ids

test/new/identical_header_2.html

@@ -0,0 +1,2 @@
+<h1 id="identical-ids">Identical IDs</h1>
+<h1 id="identical-ids-2">Identical IDs</h1>

test/new/identical_header_2.text

@@ -0,0 +1,3 @@
+# identical ids
+
+# identical ids