Make previous languages have most recent in the beginning

This is already expected by the callers. Added addPreviousLanguage
to make updating the list easier.

Change-Id: Ie4f888235ea4de4a2ce6bf5a9f24955e423e8c48

resources/js/ext.uls.compactlinks.js

@@ -121,13 +121,8 @@
 				 * @param {string} language language code
 				 */
 				onSelect: function ( language ) {
-					var previousLanguages = mw.uls.getPreviousLanguages();
-
 					self.$trigger.removeClass( 'selector-open' );
-
-					previousLanguages.push( language );
-					previousLanguages = unique( previousLanguages );
-					mw.uls.setPreviousLanguages( previousLanguages );
+					mw.uls.addPreviousLanguage( language );
 					location.href = self.interlanguageList[ language ].href;
 				},
 				onVisible: function () {

resources/js/ext.uls.init.js

@@ -72,7 +72,7 @@
 		try {
 			localStorage.setItem(
 				mw.uls.previousLanguagesStorageKey,
-				JSON.stringify( previousLanguages.slice( -5 ) )
+				JSON.stringify( previousLanguages.slice( 0, 5 ) )
 			);
 		} catch ( e ) {}
 	};
@@ -87,7 +87,24 @@
 			);
 		} catch ( e ) {}
 
-		return previousLanguages.slice( -5 );
+		return previousLanguages.slice( 0, 5 );
+	};
+
+	/**
+	 * Add a selected language to the list of previously selected languages.
+	 *
+	 * @param {string} language Language code.
+	 * @since 2016.05
+	 */
+	mw.uls.addPreviousLanguage = function ( language ) {
+		var languages = mw.uls.getPreviousLanguages();
+
+		// Avoid duplicates
+		languages = $.map( languages, function ( element ) {
+			return element === language ? undefined : element;
+		} );
+		languages.unshift( language );
+		mw.uls.setPreviousLanguages( languages );
 	};
 
 	/**

resources/js/ext.uls.interface.js

@@ -203,14 +203,14 @@
 	}
 
 	function userHasChangedLanguage() {
-		var previousLang = mw.uls.getPreviousLanguages().slice( -1 )[ 0 ],
+		var previousLang = mw.uls.getPreviousLanguages()[ 0 ],
 			currentLang = mw.config.get( 'wgUserLanguage' );
 
 		// Changed language is saved in showUndoTooltip, which is never
 		// called if previousLang is not defined, which will never be
 		// defined unless we do it now.
 		if ( previousLang === undefined ) {
-			mw.uls.setPreviousLanguages( [ currentLang ] );
+			mw.uls.addPreviousLanguage( currentLang );
 		}
 
 		return previousLang && previousLang !== currentLang;
@@ -275,10 +275,9 @@
 		}
 
 		previousLanguages = mw.uls.getPreviousLanguages();
-		previousLang = previousLanguages.slice( -1 )[ 0 ];
+		previousLang = previousLanguages[ 0 ];
 
-		previousLanguages.push( currentLang );
-		mw.uls.setPreviousLanguages( previousLanguages );
+		mw.uls.addPreviousLanguage( currentLang );
 
 		getUndoAutonym( previousLang ).done( function ( autonym ) {
 			// Attach a tipsy tooltip to the trigger