Introduce BaseRepository common ancestor class for all repositories to offload command logic.

src/Composer/Repository/ArrayRepository.php

@@ -24,7 +24,7 @@ use Composer\Semver\Constraint\Constraint;
  *
  * @author Nils Adermann <naderman@naderman.de>
  */
-class ArrayRepository implements RepositoryInterface
+class ArrayRepository extends BaseRepository
 {
     /** @var PackageInterface[] */
     protected $packages;

src/Composer/Repository/BaseRepository.php

@@ -0,0 +1,66 @@
+<?php
+
+/*
+ * This file is part of Composer.
+ *
+ * (c) Nils Adermann <naderman@naderman.de>
+ *     Jordi Boggiano <j.boggiano@seld.be>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Composer\Repository;
+
+use Composer\Package\RootPackageInterface;
+use Composer\Semver\Constraint\ConstraintInterface;
+
+/**
+ * Common ancestor class for generic repository functionality.
+ *
+ * @author Niels Keurentjes <niels.keurentjes@omines.com>
+ */
+abstract class BaseRepository implements RepositoryInterface
+{
+    /**
+     * Returns a list of links causing the requested needle packages to be installed, as an associative array with the
+     * dependent's name as key, and an array containing in order the PackageInterface and Link describing the relationship
+     * as values. If recursive lookup was requested a third value is returned containing an identically formed array up
+     * to the root package.
+     *
+     * @param string|string[] $needle The package name(s) to inspect.
+     * @param ConstraintInterface|null $constraint Optional constraint to filter by.
+     * @param bool $invert Whether to invert matches to discover reasons for the package *NOT* to be installed.
+     * @param bool $recurse Whether to recursively expand the requirement tree up to the root package.
+     * @return array An associative array of arrays as described above.
+     */
+    private function getDependents($needle, $constraint = null, $invert = false, $recurse = true)
+    {
+        $needles = is_array($needle) ? $needle : array($needle);
+        $results = array();
+
+        // Loop over all currently installed packages.
+        foreach ($this->getPackages() as $package) {
+            // Requirements and replaces are both considered valid reasons for a package to be installed
+            $links = $package->getRequires() + $package->getReplaces();
+
+            // Require-dev is only relevant for the root package
+            if ($package instanceof RootPackageInterface) {
+                $links += $package->getDevRequires();
+            }
+
+            // Cross-reference all discovered links to the needles
+            foreach ($links as $link) {
+                foreach ($needles as $needle) {
+                    if ($link->getTarget() === $needle) {
+                        if (is_null($constraint) || (($link->getConstraint()->matches($constraint) === !$invert))) {
+                            $results[$link->getSource()] = array($package, $link, $recurse ? $this->getDependents($link->getSource(), null, false, true) : array());
+                        }
+                    }
+                }
+            }
+        }
+        ksort($results);
+        return $results;
+    }
+}

src/Composer/Repository/CompositeRepository.php

@@ -19,7 +19,7 @@ use Composer\Package\PackageInterface;
  *
  * @author Beau Simensen <beau@dflydev.com>
  */
-class CompositeRepository implements RepositoryInterface
+class CompositeRepository extends BaseRepository
 {
     /**
      * List of repositories