New 'depends' command added

Basic command to display where a given package is referenced.
This helps in figuring out where a package is used, and even
can help in troubleshooting dependency issues.

src/Composer/Command/DependsCommand.php

@@ -0,0 +1,98 @@
+<?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\Command;
+
+use Composer\Composer;
+use Composer\Package\PackageInterface;
+use Symfony\Component\Console\Input\InputInterface;
+use Symfony\Component\Console\Input\InputArgument;
+use Symfony\Component\Console\Output\OutputInterface;
+
+/**
+ * @author Justin Rainbow <justin.rainbow@gmail.com>
+ */
+class DependsCommand extends Command
+{
+    protected function configure()
+    {
+        $this
+            ->setName('depends')
+            ->setDescription('Where is a package used?')
+            ->setDefinition(array(
+                new InputArgument('package', InputArgument::REQUIRED, 'the package to inspect')
+            ))
+            ->setHelp(<<<EOT
+The depends command displays detailed information about where a
+package is referenced.
+<info>php composer.phar depends composer/composer</info>
+
+EOT
+            )
+        ;
+    }
+
+    protected function execute(InputInterface $input, OutputInterface $output)
+    {
+        $composer = $this->getComposer();
+        $references = $this->getReferences($input, $output, $composer);
+
+        $this->printLinks($input, $output, $references);
+    }
+
+    /**
+     * finds a list of packages which depend on another package
+     *
+     * @param InputInterface $input
+     * @param OutputInterface $output
+     * @param Composer $composer
+     * @return array
+     * @throws \InvalidArgumentException
+     */
+    protected function getReferences(InputInterface $input, OutputInterface $output, Composer $composer)
+    {
+        $needle = $input->getArgument('package');
+
+        $references = array();
+        $recommendedDependencies = array();
+
+        // check if we have a local installation so we can grab the right package/version
+        $repos = array_merge(
+            array($composer->getRepositoryManager()->getLocalRepository()),
+            $composer->getRepositoryManager()->getRepositories()
+        );
+        foreach ($repos as $repository) {
+            foreach ($repository->getPackages() as $package) {
+                foreach ($package->getRequires() as $link) {
+                    if ($link->getTarget() === $needle) {
+                        $references[] = $link;
+                    }
+                }
+
+                foreach ($package->getSuggests() as $link) {
+                    if ($link->getTarget() === $needle) {
+                        $references[] = $link;
+                    }
+                }
+            }
+        }
+
+        return $references;
+    }
+
+    protected function printLinks(InputInterface $input, OutputInterface $output, array $links)
+    {
+        foreach ($links as $link) {
+            $output->writeln($link->getSource() . ' <comment>' . $link->getPrettyConstraint() . '</comment>');
+        }
+    }
+}

src/Composer/Console/Application.php

@@ -171,6 +171,7 @@ class Application extends BaseApplication
     protected function registerCommands()
     {
         $this->add(new Command\AboutCommand());
+        $this->add(new Command\DependsCommand());
         $this->add(new Command\InstallCommand());
         $this->add(new Command\UpdateCommand());
         $this->add(new Command\DebugPackagesCommand());