[format] Add 'x' format specifier and field width

Sometimes it is useful to print numbers in hex format - add the
appropriate specifier. Also support a minimum field width to
pad the formatted string with spaces (or zeros if the width is
prefixed with '0').

https://bugzilla.gnome.org/show_bug.cgi?id=622597

js/misc/format.js

@@ -6,8 +6,10 @@
  * 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
+ * It supports %s, %d, %x and %f, for %f it also support precisions like
- * "%.2f".format(1.526)
+ * "%.2f".format(1.526). All specifiers can be prefixed with a minimum
+ * field width, e.g. "%5s".format("foo"). Unless the width is prefixed
+ * with '0', the formatted string will be padded with spaces.
  */
 
 function format() {
@@ -15,30 +17,44 @@ function format() {
     let i = 0;
     let args = arguments;
 
-    return str.replace(/%(?:\.([0-9]+))?(.)/g, function (str, precisionGroup, genericGroup) {
+    return str.replace(/%([0-9]+)?(?:\.([0-9]+))?(.)/g, function (str, widthGroup, precisionGroup, genericGroup) {
 
                     if (precisionGroup != '' && genericGroup != 'f')
                         throw new Error("Precision can only be specified for 'f'");
 
+                    let fillChar = (widthGroup[0] == '0') ? '0' : ' ';
+                    let width = parseInt(widthGroup, 10) || 0;
+
+                    function fillWidth(s, c, w) {
+                        let fill = '';
+                        for (let i = 0; i < w; i++)
+                            fill += c;
+                        return fill.substr(s.length) + s;
+                    }
+
+                    let s = '';
                     switch (genericGroup) {
                         case '%':
                             return '%';
                             break;
                         case 's':
-                            return args[i++].toString();
+                            s = args[i++].toString();
                             break;
                         case 'd':
-                            return parseInt(args[i++]);
+                            s = parseInt(args[i++]).toString();
+                            break;
+                        case 'x':
+                            s = parseInt(args[i++]).toString(16);
                             break;
                         case 'f':
                             if (precisionGroup == '')
-                                return parseFloat(args[i++]);
+                                s = parseFloat(args[i++]).toString();
                             else
-                                return parseFloat(args[i++]).toFixed(parseInt(precisionGroup));
+                                s = parseFloat(args[i++]).toFixed(parseInt(precisionGroup));
                             break;
                         default:
                             throw new Error('Unsupported conversion character %' + genericGroup);
                     }
-                    return ""; // Suppress warning
+                    return fillWidth(s, fillChar, width);
                 });
 }

tests/unit/format.js

@@ -20,8 +20,15 @@ assertEquals("%s", "%%s".format('foo'));
 assertEquals("%%s", "%%%%s".format('foo'));
 assertEquals("foo 5", "%s %d".format('foo', 5));
 assertEquals("8", "%d".format(8));
+assertEquals("f", "%x".format(15));
 assertEquals("2.58 6.96", "%f %.2f".format(2.58, 6.958));
 
+// Test field width
+assertEquals("007  foo", "%03d %4s".format(7, 'foo'));
+assertEquals(" 2.58 06.96", "%5f %05.2f".format(2.58, 6.958));
+assertEquals("cafe", "%2x".format(0xcafe));
+assertEquals("foo", "%0s".format('foo'));
+
 // Precision is only allowed for %f
 assertRaises(function() { "%.2d".format(5.21) });