add --prefer-lowest-stable to update command

src/Composer/Command/UpdateCommand.php

@@ -47,6 +47,7 @@ class UpdateCommand extends Command
                 new InputOption('verbose', 'v|vv|vvv', InputOption::VALUE_NONE, 'Shows more details including new commits pulled in when updating packages.'),
                 new InputOption('optimize-autoloader', 'o', InputOption::VALUE_NONE, 'Optimize autoloader during autoloader dump.'),
                 new InputOption('ignore-platform-reqs', null, InputOption::VALUE_NONE, 'Ignore platform requirements (php & ext- packages).'),
+                new InputOption('prefer-lowest-stable', null, InputOption::VALUE_NONE, 'Forces all packages to their lowest stable version.'),
             ))
             ->setHelp(<<<EOT
 The <info>update</info> command reads the composer.json file from the
@@ -121,6 +122,7 @@ EOT
             ->setUpdateWhitelist($input->getOption('lock') ? array('lock') : $input->getArgument('packages'))
             ->setWhitelistDependencies($input->getOption('with-dependencies'))
             ->setIgnorePlatformRequirements($input->getOption('ignore-platform-reqs'))
+            ->setPreferLowestStable($input->getOption('prefer-lowest-stable'))
         ;
 
         if ($input->getOption('no-plugins')) {

src/Composer/DependencyResolver/DefaultPolicy.php

@@ -24,10 +24,12 @@ use Composer\Package\LinkConstraint\VersionConstraint;
 class DefaultPolicy implements PolicyInterface
 {
     private $preferStable;
+    private $preferLowest;
 
-    public function __construct($preferStable = false)
+    public function __construct($preferStable = false, $preferLowest = false)
     {
         $this->preferStable = $preferStable;
+        $this->preferLowest = $preferLowest;
     }
 
     public function versionCompare(PackageInterface $a, PackageInterface $b, $operator)
@@ -195,6 +197,7 @@ class DefaultPolicy implements PolicyInterface
 
     protected function pruneToBestVersion(Pool $pool, $literals)
     {
+        $operator = $this->preferLowest ? '<' : '>';
         $bestLiterals = array($literals[0]);
         $bestPackage = $pool->literalToPackage($literals[0]);
         foreach ($literals as $i => $literal) {
@@ -204,7 +207,7 @@ class DefaultPolicy implements PolicyInterface
 
             $package = $pool->literalToPackage($literal);
 
-            if ($this->versionCompare($package, $bestPackage, '>')) {
+            if ($this->versionCompare($package, $bestPackage, $operator)) {
                 $bestPackage = $package;
                 $bestLiterals = array($literal);
             } elseif ($this->versionCompare($package, $bestPackage, '==')) {

src/Composer/Installer.php

@@ -107,6 +107,7 @@ class Installer
     protected $update = false;
     protected $runScripts = true;
     protected $ignorePlatformReqs = false;
+    protected $preferLowestStable = false;
     /**
      * Array of package names/globs flagged for update
      *
@@ -693,6 +694,7 @@ class Installer
 
     private function createPolicy()
     {
+        $preferLowest = false;
         $preferStable = null;
         if (!$this->update && $this->locker->isLocked()) {
             $preferStable = $this->locker->getPreferStable();
@@ -700,10 +702,14 @@ class Installer
         // old lock file without prefer stable will return null
         // so in this case we use the composer.json info
         if (null === $preferStable) {
-            $preferStable = $this->package->getPreferStable();
+            if ($this->preferLowestStable) {
+                $preferStable = $preferLowest = true;
+            } else {
+                $preferStable = $this->package->getPreferStable();
+            }
         }
 
-        return new DefaultPolicy($preferStable);
+        return new DefaultPolicy($preferStable, $preferLowest);
     }
 
     private function createRequest(Pool $pool, RootPackageInterface $rootPackage, PlatformRepository $platformRepo)
@@ -1244,6 +1250,19 @@ class Installer
         return $this;
     }
 
+    /**
+     * Should packages be forced to their lowest stable version when updating?
+     *
+     * @param  boolean   $preferLowestStable
+     * @return Installer
+     */
+    public function setPreferLowestStable($preferLowestStable = true)
+    {
+        $this->preferLowestStable = (boolean) $preferLowestStable;
+
+        return $this;
+    }
+
     /**
      * Disables plugins.
      *

tests/Composer/Test/DependencyResolver/DefaultPolicyTest.php

@@ -247,4 +247,20 @@ class DefaultPolicyTest extends TestCase
 
         return $map;
     }
+
+    public function testSelectLowest()
+    {
+        $policy = new DefaultPolicy(false, true);
+
+        $this->repo->addPackage($packageA1 = $this->getPackage('A', '1.0'));
+        $this->repo->addPackage($packageA2 = $this->getPackage('A', '2.0'));
+        $this->pool->addRepository($this->repo);
+
+        $literals = array($packageA1->getId(), $packageA2->getId());
+        $expected = array($packageA1->getId());
+
+        $selected = $policy->selectPreferedPackages($this->pool, array(), $literals);
+
+        $this->assertEquals($expected, $selected);
+    }
 }

tests/Composer/Test/Fixtures/installer/update-prefer-lowest-stable.test

@@ -0,0 +1,40 @@
+--TEST--
+Updates packages to their lowest stable version
+--COMPOSER--
+{
+    "repositories": [
+        {
+            "type": "package",
+            "package": [
+                { "name": "a/a", "version": "1.0.0-rc1" },
+                { "name": "a/a", "version": "1.0.1" },
+                { "name": "a/a", "version": "1.1.0" },
+
+                { "name": "a/b", "version": "1.0.0" },
+                { "name": "a/b", "version": "1.0.1" },
+                { "name": "a/b", "version": "2.0.0" },
+
+                { "name": "a/c", "version": "1.0.0" },
+                { "name": "a/c", "version": "2.0.0" }
+            ]
+        }
+    ],
+    "require": {
+        "a/a": "~1.0@dev",
+        "a/c": "2.*"
+    },
+    "require-dev": {
+        "a/b": "*"
+    }
+}
+--INSTALLED--
+[
+    { "name": "a/a", "version": "1.0.0-rc1" },
+    { "name": "a/c", "version": "2.0.0" },
+    { "name": "a/b", "version": "1.0.1" }
+]
+--RUN--
+update --prefer-lowest-stable
+--EXPECT--
+Updating a/a (1.0.0-rc1) to a/a (1.0.1)
+Updating a/b (1.0.1) to a/b (1.0.0)

tests/Composer/Test/InstallerTest.php

@@ -217,6 +217,7 @@ class InstallerTest extends TestCase
                 ->setDryRun($input->getOption('dry-run'))
                 ->setUpdateWhitelist($input->getArgument('packages'))
                 ->setWhitelistDependencies($input->getOption('with-dependencies'))
+                ->setPreferLowestStable($input->getOption('prefer-lowest-stable'))
                 ->setIgnorePlatformRequirements($input->getOption('ignore-platform-reqs'));
 
             return $installer->run();