Add a weak stack.

A weak stack is a stack where null elements are not iterated. Example:

    $stack = new WeakStack();
    $stack->push(2);
    $stack->push(null);
    $stack->push(4);
    $stack->push(null);
    $stack->push(null);
    $stack->pop();

    foreach($stack as $element)
        var_dump($element);

    /**
     * Will output:
     *     int(4)
     *     int(2)
     */

1 files changed, 67 insertions, 0 deletions

diff --git a/Iterator/WeakStack.php b/Iterator/WeakStack.php
new file mode 100644
index 0000000..df51ae5
--- /dev/null
+++ b/Iterator/WeakStack.php
@@ -0,0 +1,67 @@
+<?php
+
+/**
+ * Hoa
+ *
+ *
+ * @license
+ *
+ * New BSD License
+ *
+ * Copyright © 2007-2013, Ivan Enderlin. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *     * Redistributions of source code must retain the above copyright
+ *       notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above copyright
+ *       notice, this list of conditions and the following disclaimer in the
+ *       documentation and/or other materials provided with the distribution.
+ *     * Neither the name of the Hoa nor the names of its contributors may be
+ *       used to endorse or promote products derived from this software without
+ *       specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDERS AND CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+
+namespace Hoa\Praspel\Iterator {
+
+/**
+ * Class \Hoa\Praspel\Iterator\WeakStack.
+ *
+ * A weak stack is a SplStack with null element that are not iterated.
+ *
+ * @author     Ivan Enderlin <ivan.enderlin@hoa-project.net>
+ * @copyright  Copyright © 2007-2013 Ivan Enderlin.
+ * @license    New BSD License
+ */
+
+class WeakStack extends \SplStack {
+
+    /**
+     * Skip null elements while validated the current one.
+     *
+     * @access  public
+     * @return  bool
+     */
+    public function valid ( ) {
+
+        while(   true === parent::valid()
+              && null === $this->current()
+              && null === parent::next());
+
+        return parent::valid();
+    }
+}
+
+}