( function ( mw ) {

	// Sugar for the mw.popups.reducers.eventLogging reducer.
	var counts = mw.popups.counts;

	mw.popups.reducers = {};

	/**
	 * Creates the next state tree from the current state tree and some updates.
	 *
	 * N.B. OO.copy doesn't copy Element instances, whereas $.extend does.
	 * However, OO.copy does copy properties whose values are undefined or null,
	 * whereas $.extend doesn't. Since the state tree contains an Element instance
	 * - the preview.activeLink property - and we want to copy undefined/null into
	 * the state we need to manually iterate over updates and check with
	 * hasOwnProperty to copy over to the new state.
	 *
	 * In [change listeners](/doc/change_listeners.md), for example, we talk about
	 * the previous state and the current state (the `prevState` and `state`
	 * parameters, respectively). Since
	 * [reducers](http://redux.js.org/docs/basics/Reducers.html) take the current
	 * state and an action and make updates, "next state" seems appropriate.
	 *
	 * @param {Object} state
	 * @param {Object} updates
	 * @return {Object}
	 */
	function nextState( state, updates ) {
		var result = {},
			key;

		for ( key in state ) {
			if ( state.hasOwnProperty( key ) && !updates.hasOwnProperty( key ) ) {
				result[key] = state[key];
			}
		}

		for ( key in updates ) {
			if ( updates.hasOwnProperty( key ) ) {
				result[key] = updates[key];
			}
		}

		return result;
	}

	/**
	 * Reducer for actions that modify the state of the preview model
	 *
	 * @param {Object} state before action
	 * @param {Object} action Redux action that modified state.
	 *  Must have `type` property.
	 * @return {Object} state after action
	 */
	mw.popups.reducers.preview = function ( state, action ) {
		if ( state === undefined ) {
			state = {
				enabled: undefined,
				activeLink: undefined,
				activeEvent: undefined,
				shouldShow: false,
				isUserDwelling: false
			};
		}

		switch ( action.type ) {
			case mw.popups.actionTypes.BOOT:
				return nextState( state, {
					enabled: action.user.isInCondition
				} );
			case mw.popups.actionTypes.LINK_DWELL:
				return nextState( state, {
					activeLink: action.el,
					activeEvent: action.event,

					// When the user dwells on a link with their keyboard, a preview is
					// renderered, and then dwells on another link, the LINK_ABANDON_END
					// action will be ignored.
					//
					// Ensure that all the preview is hidden.
					shouldShow: false
				} );
			case mw.popups.actionTypes.LINK_ABANDON_END:
				if ( action.el !== state.activeLink ) {
					return state;
				}

				/* falls through */
			case mw.popups.actionTypes.PREVIEW_ABANDON_END:
				if ( !state.isUserDwelling ) {
					return nextState( state, {
						activeLink: undefined,
						activeEvent: undefined,
						fetchResponse: undefined,
						shouldShow: false
					} );
				}

				return state;

			case mw.popups.actionTypes.PREVIEW_DWELL:
				return nextState( state, {
					isUserDwelling: true
				} );
			case mw.popups.actionTypes.PREVIEW_ABANDON_START:
				return nextState( state, {
					isUserDwelling: false
				} );
			case mw.popups.actionTypes.FETCH_START:
				return nextState( state, {
					fetchResponse: undefined
				} );
			case mw.popups.actionTypes.FETCH_END:
				if ( action.el === state.activeLink ) {
					return nextState( state, {
						fetchResponse: action.result,
						shouldShow: true
					} );
				}

				/* falls through */
			default:
				return state;
		}
	};

	/**
	 * Reducer for actions that may result in an event being logged via Event
	 * Logging.
	 *
	 * The base data represents data that's shared between all events logged with
	 * the Popups schema ("Popups events"). Very nearly all of it is initialized
	 * during the BOOT action and doesn't change between link interactions, e.g.
	 * the user being an anon or the number of edits they've made.
	 *
	 * The user's number of previews, however, does change between link
	 * interactions and the associated bucket (a computed property) is what is
	 * logged. This is reflected in the state tree: the `previewCount` property is
	 * used to store the user's number of previews and the
	 * `baseData.previewCountBucket` property is used to store the associated
	 * bucket.
	 *
	 * @param {Object} state
	 * @param {Object} action
	 * @return {Object} The state as a result of processing the action
	 */
	mw.popups.reducers.eventLogging = function ( state, action ) {
		var nextCount;

		if ( state === undefined ) {
			state = {
				previewCount: undefined,
				baseData: {},
				interaction: undefined,
				event: undefined
			};
		}

		switch ( action.type ) {
			case mw.popups.actionTypes.BOOT:
				return nextState( state, {
					previewCount: action.user.previewCount,
					baseData: {
						pageTitleSource: action.page.title,
						namespaceIdSource: action.page.namespaceID,
						pageIdSource: action.page.id,
						isAnon: action.user.isAnon,
						popupEnabled: action.user.isInCondition,
						pageToken: action.pageToken,
						sessionToken: action.sessionToken,
						editCountBucket: counts.getEditCountBucket( action.user.editCount ),
						previewCountBucket: counts.getPreviewCountBucket( action.user.previewCount )
					},
					event: {
						action: 'pageLoaded'
					}
				} );

			case mw.popups.actionTypes.EVENT_LOGGED:
				return nextState( state, {
					event: undefined
				} );

			case mw.popups.actionTypes.PREVIEW_SHOW:
				nextCount = state.previewCount + 1;

				return nextState( state, {
					previewCount: nextCount,
					baseData: nextState( state.baseData, {
						previewCountBucket: counts.getPreviewCountBucket( nextCount )
					} ),
					interaction: nextState( state.interaction, {
						timeToPreviewShow: action.timestamp - state.interaction.started
					} )
				} );

			case mw.popups.actionTypes.LINK_DWELL:
				return nextState( state, {
					interaction: {
						token: action.interactionToken,
						started: action.timestamp
					}
				} );

			case mw.popups.actionTypes.LINK_CLICK:
				return nextState( state, {
					event: {
						action: 'opened',
						linkInteractionToken: state.interaction.token,
						totalInteractionTime: Math.round( action.timestamp - state.interaction.started )
					}
				} );

			case mw.popups.actionTypes.LINK_ABANDON_START:
			case mw.popups.actionTypes.PREVIEW_ABANDON_START:
				return nextState( state, {
					interaction: nextState( state.interaction, {
						finished: action.timestamp
					} )
				} );

			case mw.popups.actionTypes.LINK_ABANDON_END:
			case mw.popups.actionTypes.PREVIEW_ABANDON_END:
				return nextState( state, {
					event: {
						action: state.interaction.timeToPreviewShow ? 'dismissed' : 'dwelledButAbandoned',
						linkInteractionToken: state.interaction.token,
						totalInteractionTime: Math.round( state.interaction.finished - state.interaction.started )
					}
				} );

			default:
				return state;
		}
	};

	/**
	 * Reducer for actions that modify the state of the settings
	 *
	 * @param {Object} state
	 * @param {Object} action
	 * @return {Object} state after action
	 */
	mw.popups.reducers.settings = function ( state, action ) {
		if ( state === undefined ) {
			state = {
				shouldShow: false
			};
		}

		switch ( action.type ) {
			case mw.popups.actionTypes.SETTINGS_SHOW:
				return nextState( state, {
					shouldShow: true
				} );
			case mw.popups.actionTypes.SETTINGS_HIDE:
				return nextState( state, {
					shouldShow: false
				} );
			default:
				return state;
		}
	};
}( mediaWiki ) );