Implement the Text importer.

spec/drupol/phptree/Importer/TextSpec.php

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+<?php
+
+declare(strict_types = 1);
+
+namespace spec\drupol\phptree\Importer;
+
+use drupol\phptree\Importer\Text;
+use drupol\phptree\Node\ValueNode;
+use drupol\phptree\Node\ValueNodeInterface;
+use PhpSpec\ObjectBehavior;
+
+class TextSpec extends ObjectBehavior
+{
+    public function it_is_initializable()
+    {
+        $this->shouldHaveType(Text::class);
+    }
+
+    public function it_can_import()
+    {
+        $string = '[root [A [C [G] [H]] [D [I] [J]]] [B [E] [F]]]';
+
+        $tree = new ValueNode('root', 2);
+
+        $nodes = [];
+        foreach (\range('A', 'J') as $value) {
+            $nodes[$value] = new ValueNode($value);
+        }
+
+        $tree->add(...\array_values($nodes));
+
+        $this
+            ->import($string)
+            ->shouldImplement(ValueNodeInterface::class);
+
+        $this
+            ->import($string)
+            ->count()
+            ->shouldReturn(10);
+
+        $this
+            ->import($string)
+            ->isRoot()
+            ->shouldReturn(TRUE);
+    }
+}

src/Importer/ImporterInterface.php

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+<?php
+
+declare(strict_types = 1);
+
+namespace drupol\phptree\Importer;
+
+use drupol\phptree\Node\NodeInterface;
+
+/**
+ * Interface ImporterInterface
+ */
+interface ImporterInterface
+{
+    /**
+     * Import data into a node.
+     *
+     * @param mixed $data
+     *   The data to import.
+     *
+     * @return NodeInterface
+     *   The new node.
+     */
+    public function import($data): NodeInterface;
+}

src/Importer/Text.php

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+<?php
+
+declare(strict_types = 1);
+
+namespace drupol\phptree\Importer;
+
+use drupol\phptree\Node\NodeInterface;
+use drupol\phptree\Node\ValueNode;
+use drupol\phptree\Node\ValueNodeInterface;
+
+/**
+ * Class Text
+ */
+class Text implements ImporterInterface
+{
+    /**
+     * {@inheritdoc}
+     */
+    public function import($data): NodeInterface
+    {
+        $parsed = $this->parse($data);
+
+        return $this->arrayToTree($parsed[0]);
+    }
+
+    /**
+     * Convert an array into a tree.
+     *
+     * @param array $data
+     *
+     * @return \drupol\phptree\Node\ValueNodeInterface
+     *   The tree.
+     */
+    private function arrayToTree(array $data): ValueNodeInterface
+    {
+        $data += [
+            'children' => [],
+        ];
+
+        $node = new ValueNode($data['value']);
+
+        foreach ($data['children'] as $key => $child) {
+            $node->add($this->arrayToTree($child));
+        }
+
+        return $node;
+    }
+
+    /**
+     * Parse a string into an array.
+     *
+     * @param string $subject
+     *   The subject string.
+     *
+     * @return array|bool
+     *   The array.
+     */
+    private function parse(string $subject)
+    {
+        $result = false;
+
+        \preg_match_all('~[^\[\]]+|\[(?<nested>(?R)*)\]~', $subject, $matches);
+
+        foreach (\array_filter($matches['nested']) as $match) {
+            $item = [];
+            $position = \strpos($match, '[');
+
+            if (false !== $position) {
+                $item['value'] = \substr($match, 0, $position);
+            } else {
+                $item['value'] = $match;
+            }
+
+            if (false !== $children = $this->parse($match)) {
+                $item['children'] = $children;
+            }
+
+            $result[] = $item;
+        }
+
+        return $result;
+    }
+}