Add two more url-safe characters to make the distribution even.

index.js

@@ -5,10 +5,10 @@
 var crypto = require('crypto');
 
 /**
- * 62 characters in the ascii range that can be used in URLs without special
+ * 64 characters in the ascii range that can be used in URLs without special
  * encoding.
  */
-var UIDCHARS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
+var UIDCHARS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_';
 
 /**
  * Make a Buffer into a string ready for use in URLs

package.json

@@ -3,6 +3,9 @@
   "description": "strong uid",
   "tags": ["uid"],
   "version": "0.0.3",
+  "scripts": {
+    "test": "node test.js"
+  },
   "dependencies": {
   }
 }

test.js

@@ -0,0 +1,36 @@
+'use strict';
+var uid = require('./index');
+var assert = require('assert');
+
+var freqs = {};
+var ITERATIONS = 1000000;
+var LENGTH = 64;
+var chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_';
+var i;
+
+for (i = 0; i < chars.length; i++) {
+  freqs[chars[i]] = 0;
+}
+
+console.log("Running", ITERATIONS, "iterations...");
+
+while(ITERATIONS--) {
+  var str = uid(LENGTH);
+  for (var i = 0; i < str.length; i++) {
+    freqs[str[i]]++;
+  }
+}
+
+console.log("Done. Distribution:");
+console.log(JSON.stringify(freqs, undefined, 2));
+
+var vals = Object.keys(freqs).map(function(key) {
+  return freqs[key];
+});
+var min = Math.min.apply(null, vals);
+var max = Math.max.apply(null, vals);
+
+console.log("Min freq:", min, "Max freq:", max);
+var diffPcnt = Math.abs(min / max - 1) * 100;
+console.log("Min and max frequencies are " + diffPcnt + "% apart.");
+assert(diffPcnt < 1);