New function: Return true if field has subfield

* src/HAB/Pica/Record/Field.php (hasSubfield): New function. Return true
  if field has subfield.

src/HAB/Pica/Record/Field.php

@@ -262,6 +262,21 @@ class Field
         return null;
     }
 
+    /**
+     * Return true if a subfield with given code exists.
+     *
+     * If optional argument $n return true if the field has subfield
+     * at the specified position.
+     *
+     * @param  string $code Subfield code
+     * @param  integer $n Optional zero-based subfield index
+     * @return boolean
+     */
+    public function hasSubfield ($code, $n = 0)
+    {
+        return (boolean)$this->getNthSubfield($code, $n);
+    }
+
     /**
      * Return all subfields with the specified code.
      *

tests/unit-tests/src/HAB/Pica/Record/FieldTest.php

@@ -130,6 +130,17 @@ class FieldTest extends PHPUnit_FrameWork_TestCase
         $this->assertNull($s);
     }
 
+    public function testHasSubfield ()
+    {
+        $f = new Field('003@', 0, array(new Subfield('a', 'first a'),
+                                        new Subfield('b', 'first b'),
+                                        new Subfield('a', 'second a')));
+        $this->assertTrue($f->hasSubfield('a'));
+        $this->assertTrue($f->hasSubfield('a', 1));
+        $this->assertFalse($f->hasSubfield('a', 2));
+        $this->assertFalse($f->hasSubfield('c'));
+    }
+
     /**
      * @depends testSetSubfields
      */