composer/src/Composer/DependencyResolver/DefaultPolicy.php

<?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\DependencyResolver;

use Composer\Repository\RepositoryInterface;
use Composer\Package\PackageInterface;

/**
 * @author Nils Adermann <naderman@naderman.de>
 */
class DefaultPolicy implements PolicyInterface
{
    public function allowUninstall()
    {
        return false;
    }

    public function allowDowngrade()
    {
        return false;
    }

    public function versionCompare(PackageInterface $a, PackageInterface $b, $operator)
    {
        return version_compare($a->getVersion(), $b->getVersion(), $operator);
    }

    public function findUpdatePackages(Solver $solver, Pool $pool, RepositoryInterface $repo, PackageInterface $package, $allowAll = false)
    {
        $packages = array();

        foreach ($pool->whatProvides($package->getName()) as $candidate) {
            // skip old packages unless downgrades are an option
            if (!$allowAll && !$this->allowDowngrade() && $this->versionCompare($package, $candidate, '>')) {
                continue;
            }

            if ($candidate !== $package) {
                $packages[] = $candidate;
            }
        }

        return $packages;
    }

    public function installable(Solver $solver, Pool $pool, RepositoryInterface $repo, PackageInterface $package)
    {
        // todo: package blacklist?
        return true;
    }

    public function selectPreferedPackages(Solver $solver, Pool $pool, RepositoryInterface $installed, array $literals)
    {
        // prefer installed, newest version, recommended, highest priority repository, ...
        $newest = $this->selectNewestPackages($installed, $literals);

        $selected = array();
        foreach ($newest as $literal) {
            if ($literal->getPackage()->getRepository() === $installed) {
                $selected[] = $literal;
            }
        }
        if (count($selected)) {
            return $selected;
        }

        return $newest;
    }

    public function selectNewestPackages(RepositoryInterface $installed, array $literals)
    {
        $maxLiterals = array($literals[0]);
        $maxPackage = $literals[0]->getPackage();
        foreach ($literals as $i => $literal) {
            if (0 === $i) {
                continue;
            }

            if ($this->versionCompare($literal->getPackage(), $maxPackage, '>')) {
                $maxPackage = $literal->getPackage();
                $maxLiterals = array($literal);
            } else if ($this->versionCompare($literal->getPackage(), $maxPackage, '==')) {
                $maxLiterals[] = $literal;
            }
        }

        return $maxLiterals;
    }
}