Add String formatting

Add String formatting by extending the String object with a
format method.

Now we can do stuff like "Text: %s, %d".format(somevar, 5)

This is required for proper translation of some strings.

https://bugzilla.gnome.org/show_bug.cgi?id=595661

js/misc/Makefile.am

@@ -1,4 +1,5 @@
 jsmiscdir = $(pkgdatadir)/js/misc
 
 dist_jsmisc_DATA =		\
-	docInfo.js
+	docInfo.js		\
+	format.js

js/misc/format.js

@@ -0,0 +1,44 @@
+/* -*- mode: js2; js2-basic-offset: 4; indent-tabs-mode: nil -*- */
+
+/*
+ * This function is intended to extend the String object and provide
+ * an String.format API for string formatting.
+ * It has to be set up using String.prototype.format = Format.format;
+ * Usage:
+ * "somestring %s %d".format('hello', 5);
+ * It supports %s, %d and %f, for %f it also support precisions like
+ * "%.2f".format(1.526)
+ */
+
+function format() {
+    let str = this;
+    let i = 0;
+    let args = arguments;
+
+    return str.replace(/%(?:\.([0-9]+))?(.)/g, function (str, precisionGroup, genericGroup) {
+
+                    if (precisionGroup != '' && genericGroup != 'f')
+                        throw new Error("Precision can only be specified for 'f'");
+
+                    switch (genericGroup) {
+                        case '%':
+                            return '%';
+                            break;
+                        case 's':
+                            return args[i++].toString();
+                            break;
+                        case 'd':
+                            return parseInt(args[i++]);
+                            break;
+                        case 'f':
+                            if (precisionGroup == '')
+                                return parseFloat(args[i++]);
+                            else
+                                return parseFloat(args[i++]).toFixed(parseInt(precisionGroup));
+                            break;
+                        default:
+                            throw new Error('Unsupported conversion character %' + genericGroup);
+                    }
+
+                });
+}

js/ui/environment.js

@@ -4,6 +4,8 @@ const St = imports.gi.St;
 
 const Tweener = imports.ui.tweener;
 
+const Format = imports.misc.format;
+
 // "monkey patch" in some varargs ClutterContainer methods; we need
 // to do this per-container class since there is no representation
 // of interfaces in Javascript
@@ -30,4 +32,5 @@ _patchContainerClass(St.Table);
 
 function init() {
     Tweener.init();
+    String.prototype.format = Format.format;
 }