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diff --git a/www/wiki/includes/registration/VersionChecker.php b/www/wiki/includes/registration/VersionChecker.php
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+++ b/www/wiki/includes/registration/VersionChecker.php
@@ -0,0 +1,236 @@
+<?php
+
+/**
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ * http://www.gnu.org/copyleft/gpl.html
+ *
+ * @author Legoktm
+ * @author Florian Schmidt
+ */
+
+use Composer\Semver\VersionParser;
+use Composer\Semver\Constraint\Constraint;
+
+/**
+ * Provides functions to check a set of extensions with dependencies against
+ * a set of loaded extensions and given version information.
+ *
+ * @since 1.29
+ */
+class VersionChecker {
+	/**
+	 * @var Constraint|bool representing $wgVersion
+	 */
+	private $coreVersion = false;
+
+	/**
+	 * @var array Loaded extensions
+	 */
+	private $loaded = [];
+
+	/**
+	 * @var VersionParser
+	 */
+	private $versionParser;
+
+	/**
+	 * @param string $coreVersion Current version of core
+	 */
+	public function __construct( $coreVersion ) {
+		$this->versionParser = new VersionParser();
+		$this->setCoreVersion( $coreVersion );
+	}
+
+	/**
+	 * Set an array with credits of all loaded extensions and skins.
+	 *
+	 * @param array $credits An array of installed extensions with credits of them
+	 * @return VersionChecker $this
+	 */
+	public function setLoadedExtensionsAndSkins( array $credits ) {
+		$this->loaded = $credits;
+
+		return $this;
+	}
+
+	/**
+	 * Set MediaWiki core version.
+	 *
+	 * @param string $coreVersion Current version of core
+	 */
+	private function setCoreVersion( $coreVersion ) {
+		try {
+			$this->coreVersion = new Constraint(
+				'==',
+				$this->versionParser->normalize( $coreVersion )
+			);
+			$this->coreVersion->setPrettyString( $coreVersion );
+		} catch ( UnexpectedValueException $e ) {
+			// Non-parsable version, don't fatal.
+		}
+	}
+
+	/**
+	 * Check all given dependencies if they are compatible with the named
+	 * installed extensions in the $credits array.
+	 *
+	 * Example $extDependencies:
+	 *     {
+	 *       'FooBar' => {
+	 *         'MediaWiki' => '>= 1.25.0',
+	 *         'extensions' => {
+	 *           'FooBaz' => '>= 1.25.0'
+	 *         },
+	 *         'skins' => {
+	 *           'BazBar' => '>= 1.0.0'
+	 *         }
+	 *       }
+	 *     }
+	 *
+	 * @param array $extDependencies All extensions that depend on other ones
+	 * @return array
+	 */
+	public function checkArray( array $extDependencies ) {
+		$errors = [];
+		foreach ( $extDependencies as $extension => $dependencies ) {
+			foreach ( $dependencies as $dependencyType => $values ) {
+				switch ( $dependencyType ) {
+					case ExtensionRegistry::MEDIAWIKI_CORE:
+						$mwError = $this->handleMediaWikiDependency( $values, $extension );
+						if ( $mwError !== false ) {
+							$errors[] = [
+								'msg' => $mwError,
+								'type' => 'incompatible-core',
+							];
+						}
+						break;
+					case 'extensions':
+					case 'skins':
+						foreach ( $values as $dependency => $constraint ) {
+							$extError = $this->handleExtensionDependency(
+								$dependency, $constraint, $extension, $dependencyType
+							);
+							if ( $extError !== false ) {
+								$errors[] = $extError;
+							}
+						}
+						break;
+					default:
+						throw new UnexpectedValueException( 'Dependency type ' . $dependencyType .
+							' unknown in ' . $extension );
+				}
+			}
+		}
+
+		return $errors;
+	}
+
+	/**
+	 * Handle a dependency to MediaWiki core. It will check, if a MediaWiki version constraint was
+	 * set with self::setCoreVersion before this call (if not, it will return an empty array) and
+	 * checks the version constraint given against it.
+	 *
+	 * @param string $constraint The required version constraint for this dependency
+	 * @param string $checkedExt The Extension, which depends on this dependency
+	 * @return bool|string false if no error, or a string with the message
+	 */
+	private function handleMediaWikiDependency( $constraint, $checkedExt ) {
+		if ( $this->coreVersion === false ) {
+			// Couldn't parse the core version, so we can't check anything
+			return false;
+		}
+
+		// if the installed and required version are compatible, return an empty array
+		if ( $this->versionParser->parseConstraints( $constraint )
+			->matches( $this->coreVersion ) ) {
+			return false;
+		}
+		// otherwise mark this as incompatible.
+		return "{$checkedExt} is not compatible with the current "
+			. "MediaWiki core (version {$this->coreVersion->getPrettyString()}), it requires: "
+			. "$constraint.";
+	}
+
+	/**
+	 * Handle a dependency to another extension.
+	 *
+	 * @param string $dependencyName The name of the dependency
+	 * @param string $constraint The required version constraint for this dependency
+	 * @param string $checkedExt The Extension, which depends on this dependency
+	 * @param string $type Either 'extensions' or 'skins'
+	 * @return bool|array false for no errors, or an array of info
+	 */
+	private function handleExtensionDependency( $dependencyName, $constraint, $checkedExt,
+		$type
+	) {
+		// Check if the dependency is even installed
+		if ( !isset( $this->loaded[$dependencyName] ) ) {
+			return [
+				'msg' => "{$checkedExt} requires {$dependencyName} to be installed.",
+				'type' => "missing-$type",
+				'missing' => $dependencyName,
+			];
+		}
+		// Check if the dependency has specified a version
+		if ( !isset( $this->loaded[$dependencyName]['version'] ) ) {
+			// If we depend upon any version, and none is set, that's fine.
+			if ( $constraint === '*' ) {
+				wfDebug( "{$dependencyName} does not expose its version, but {$checkedExt}"
+					. " mentions it with constraint '*'. Assume it's ok so." );
+				return false;
+			} else {
+				// Otherwise, mark it as incompatible.
+				$msg = "{$dependencyName} does not expose its version, but {$checkedExt}"
+					. " requires: {$constraint}.";
+				return [
+					'msg' => $msg,
+					'type' => "incompatible-$type",
+					'incompatible' => $checkedExt,
+				];
+			}
+		} else {
+			// Try to get a constraint for the dependency version
+			try {
+				$installedVersion = new Constraint(
+					'==',
+					$this->versionParser->normalize( $this->loaded[$dependencyName]['version'] )
+				);
+			} catch ( UnexpectedValueException $e ) {
+				// Non-parsable version, output an error message that the version
+				// string is invalid
+				return [
+					'msg' => "$dependencyName does not have a valid version string.",
+					'type' => 'invalid-version',
+				];
+			}
+			// Check if the constraint actually matches...
+			if (
+				!$this->versionParser->parseConstraints( $constraint )->matches( $installedVersion )
+			) {
+				$msg = "{$checkedExt} is not compatible with the current "
+					. "installed version of {$dependencyName} "
+					. "({$this->loaded[$dependencyName]['version']}), "
+					. "it requires: " . $constraint . '.';
+				return [
+					'msg' => $msg,
+					'type' => "incompatible-$type",
+					'incompatible' => $checkedExt,
+				];
+			}
+		}
+
+		return false;
+	}
+}