text: /**
   * @license Copyright (c) 2003-2020, CKSource - Frederico Knabben. All rights reserved.
   * For licensing, see LICENSE.md or https://ckeditor.com/legal/ckeditor-oss-license
   */
  
  /**
   * @module engine/model/selection
   */
  
  import Position from './position';
  import Element from './element';
  import Node from './node';
  import Range from './range';
  import EmitterMixin from '@ckeditor/ckeditor5-utils/src/emittermixin';
  import CKEditorError from '@ckeditor/ckeditor5-utils/src/ckeditorerror';
  import mix from '@ckeditor/ckeditor5-utils/src/mix';
  import isIterable from '@ckeditor/ckeditor5-utils/src/isiterable';
  
  /**
   * Selection is a set of {@link module:engine/model/range~Range ranges}. It has a direction specified by its
   * {@link module:engine/model/selection~Selection#anchor anchor} and {@link module:engine/model/selection~Selection#focus focus}
   * (it can be {@link module:engine/model/selection~Selection#isBackward forward or backward}).
   * Additionally, selection may have its own attributes (think – whether text typed in in this selection
   * should have those attributes – e.g. whether you type a bolded text).
   *
   * @mixes module:utils/emittermixin~EmitterMixin
   */
  export default class Selection {
  	/**
  	 * Creates a new selection instance based on the given {@link module:engine/model/selection~Selectable selectable}
  	 * or creates an empty selection if no arguments were passed.
  	 *
  	 *		// Creates empty selection without ranges.
  	 *		const selection = writer.createSelection();
  	 *
  	 *		// Creates selection at the given range.
  	 *		const range = writer.createRange( start, end );
  	 *		const selection = writer.createSelection( range );
  	 *
  	 *		// Creates selection at the given ranges
  	 *		const ranges = [ writer.createRange( start1, end2 ), writer.createRange( star2, end2 ) ];
  	 *		const selection = writer.createSelection( ranges );
  	 *
  	 *		// Creates selection from the other selection.
  	 *		// Note: It doesn't copies selection attributes.
  	 *		const otherSelection = writer.createSelection();
  	 *		const selection = writer.createSelection( otherSelection );
  	 *
  	 *		// Creates selection from the given document selection.
  	 *		// Note: It doesn't copies selection attributes.
  	 *		const documentSelection = model.document.selection;
  	 *		const selection = writer.createSelection( documentSelection );
  	 *
  	 *		// Creates selection at the given position.
  	 *		const position = writer.createPositionFromPath( root, path );
  	 *		const selection = writer.createSelection( position );
  	 *
  	 *		// Creates selection at the given offset in the given element.
  	 *		const paragraph = writer.createElement( 'paragraph' );
  	 *		const selection = writer.createSelection( paragraph, offset );
  	 *
  	 *		// Creates a range inside an {@link module:engine/model/element~Element element} which starts before the
  	 *		// first child of that element and ends after the last child of that element.
  	 *		const selection = writer.createSelection( paragraph, 'in' );
  	 *
  	 *		// Creates a range on an {@link module:engine/model/item~Item item} which starts before the item and ends
  	 *		// just after the item.
  	 *		const selection = writer.createSelection( paragraph, 'on' );
  	 *
  	 * Selection's constructor allow passing additional options (`'backward'`) as the last argument.
  	 *
  	 *		// Creates backward selection.
  	 *		const selection = writer.createSelection( range, { backward: true } );
  	 *
  	 * @param {module:engine/model/selection~Selectable} selectable
  	 * @param {Number|'before'|'end'|'after'|'on'|'in'} [placeOrOffset] Sets place or offset of the selection.
  	 * @param {Object} [options]
  	 * @param {Boolean} [options.backward] Sets this selection instance to be backward.
  	 */
  	constructor( selectable, placeOrOffset, options ) {
  		/**
  		 * Specifies whether the last added range was added as a backward or forward range.
  		 *
  		 * @private
  		 * @type {Boolean}
  		 */
  		this._lastRangeBackward = false;
  
  		/**
  		 * Stores selection ranges.
  		 *
  		 * @protected
  		 * @type {Array.<module:engine/model/range~Range>}
  		 */
  		this._ranges = [];
  
  		/**
  		 * List of attributes set on current selection.
  		 *
  		 * @protected
  		 * @type {Map.<String,*>}
  		 */
  		this._attrs = new Map();
  
  		if ( selectable ) {
  			this.setTo( selectable, placeOrOffset, options );
  		}
  	}
  
  	/**
  	 * Selection anchor. Anchor is the position from which the selection was started. If a user is making a selection
  	 * by dragging the mouse, the anchor is where the user pressed the mouse button (the beggining of the selection).
  	 *
  	 * Anchor and {@link #focus} define the direction of the selection, which is important
  	 * when expanding/shrinking selection. The focus moves, while the anchor should remain in the same place.
  	 *
  	 * Anchor is always set to the {@link module:engine/model/range~Range#start start} or
  	 * {@link module:engine/model/range~Range#end end} position of the last of selection's ranges. Whether it is
  	 * the `start` or `end` depends on the specified `options.backward`. See the {@link #setTo `setTo()`} method.
  	 *
  	 * May be set to `null` if there are no ranges in the selection.
  	 *
  	 * @see #focus
  	 * @readonly
  	 * @type {module:engine/model/position~Position|null}
  	 */
  	get anchor() {
  		if ( this._ranges.length > 0 ) {
  			const range = this._ranges[ this._ranges.length - 1 ];
  
  			return this._lastRangeBackward ? range.end : range.start;
  		}
  
  		return null;
  	}
  
  	/**
  	 * Selection focus. Focus is the position where the selection ends. If a user is making a selection
  	 * by dragging the mouse, the focus is where the mouse cursor is.
  	 *
  	 * May be set to `null` if there are no ranges in the selection.
  	 *
  	 * @see #anchor
  	 * @readonly
  	 * @type {module:engine/model/position~Position|null}
  	 */
  	get focus() {
  		if ( this._ranges.length > 0 ) {
  			const range = this._ranges[ this._ranges.length - 1 ];
  
  			return this._lastRangeBackward ? range.start : range.end;
  		}
  
  		return null;
  	}
  
  	/**
  	 * Whether the selection is collapsed. Selection is collapsed when there is exactly one range in it
  	 * and it is collapsed.
  	 *
  	 * @readonly
  	 * @type {Boolean}
  	 */
  	get isCollapsed() {
  		const length = this._ranges.length;
  
  		if ( length === 1 ) {
  			return this._ranges[ 0 ].isCollapsed;
  		} else {
  			return false;
  		}
  	}
  
  	/**
  	 * Returns the number of ranges in the selection.
  	 *
  	 * @readonly
  	 * @type {Number}
  	 */
  	get rangeCount() {
  		return this._ranges.length;
  	}
  
  	/**
  	 * Specifies whether the selection's {@link #focus} precedes the selection's {@link #anchor}.
  	 *
  	 * @readonly
  	 * @type {Boolean}
  	 */
  	get isBackward() {
  		return !this.isCollapsed && this._lastRangeBackward;
  	}
  
  	/**
  	 * Checks whether this selection is equal to the given selection. Selections are equal if they have the same directions,
  	 * the same number of ranges and all ranges from one selection equal to ranges from the another selection.
  	 *
  	 * @param {module:engine/model/selection~Selection|module:engine/model/documentselection~DocumentSelection} otherSelection
  	 * Selection to compare with.
  	 * @returns {Boolean} `true` if selections are equal, `false` otherwise.
  	 */
  	isEqual( otherSelection ) {
  		if ( this.rangeCount != otherSelection.rangeCount ) {
  			return false;
  		} else if ( this.rangeCount === 0 ) {
  			return true;
  		}
  
  		if ( !this.anchor.isEqual( otherSelection.anchor ) || !this.focus.isEqual( otherSelection.focus ) ) {
  			return false;
  		}
  
  		for ( const thisRange of this._ranges ) {
  			let found = false;
  
  			for ( const otherRange of otherSelection._ranges ) {
  				if ( thisRange.isEqual( otherRange ) ) {
  					found = true;
  					break;
  				}
  			}
  
  			if ( !found ) {
  				return false;
  			}
  		}
  
  		return true;
  	}
  
  	/**
  	 * Returns an iterable object that iterates over copies of selection ranges.
  	 *
  	 * @returns {Iterable.<module:engine/model/range~Range>}
  	 */
  	* getRanges() {
  		for ( const range of this._ranges ) {
  			yield new Range( range.start, range.end );
  		}
  	}
  
  	/**
  	 * Returns a copy of the first range in the selection.
  	 * First range is the one which {@link module:engine/model/range~Range#start start} position
  	 * {@link module:engine/model/position~Position#isBefore is before} start position of all other ranges
  	 * (not to confuse with the first range added to the selection).
  	 *
  	 * Returns `null` if there are no ranges in selection.
  	 *
  	 * @returns {module:engine/model/range~Range|null}
  	 */
  	getFirstRange() {
  		let first = null;
  
  		for ( const range of this._ranges ) {
  			if ( !first || range.start.isBefore( first.start ) ) {
  				first = range;
  			}
  		}
  
  		return first ? new Range( first.start, first.end ) : null;
  	}
  
  	/**
  	 * Returns a copy of the last range in the selection.
  	 * Last range is the one which {@link module:engine/model/range~Range#end end} position
  	 * {@link module:engine/model/position~Position#isAfter is after} end position of all other ranges (not to confuse with the range most
  	 * recently added to the selection).
  	 *
  	 * Returns `null` if there are no ranges in selection.
  	 *
  	 * @returns {module:engine/model/range~Range|null}
  	 */
  	getLastRange() {
  		let last = null;
  
  		for ( const range of this._ranges ) {
  			if ( !last || range.end.isAfter( last.end ) ) {
  				last = range;
  			}
  		}
  
  		return last ? new Range( last.start, last.end ) : null;
  	}
  
  	/**
  	 * Returns the first position in the selection.
  	 * First position is the position that {@link module:engine/model/position~Position#isBefore is before}
  	 * any other position in the selection.
  	 *
  	 * Returns `null` if there are no ranges in selection.
  	 *
  	 * @returns {module:engine/model/position~Position|null}
  	 */
  	getFirstPosition() {
  		const first = this.getFirstRange();
  
  		return first ? first.start.clone() : null;
  	}
  
  	/**
  	 * Returns the last position in the selection.
  	 * Last position is the position that {@link module:engine/model/position~Position#isAfter is after}
  	 * any other position in the selection.
  	 *
  	 * Returns `null` if there are no ranges in selection.
  	 *
  	 * @returns {module:engine/model/position~Position|null}
  	 */
  	getLastPosition() {
  		const lastRange = this.getLastRange();
  
  		return lastRange ? lastRange.end.clone() : null;
  	}
  
  	/**
  	 * Sets this selection's ranges and direction to the specified location based on the given
  	 * {@link module:engine/model/selection~Selectable selectable}.
  	 *
  	 *		// Removes all selection's ranges.
  	 *		selection.setTo( null );
  	 *
  	 *		// Sets selection to the given range.
  	 *		const range = writer.createRange( start, end );
  	 *		selection.setTo( range );
  	 *
  	 *		// Sets selection to given ranges.
  	 *		const ranges = [ writer.createRange( start1, end2 ), writer.createRange( star2, end2 ) ];
  	 *		selection.setTo( ranges );
  	 *
  	 *		// Sets selection to other selection.
  	 *		// Note: It doesn't copies selection attributes.
  	 *		const otherSelection = writer.createSelection();
  	 *		selection.setTo( otherSelection );
  	 *
  	 *		// Sets selection to the given document selection.
  	 *		// Note: It doesn't copies selection attributes.
  	 *		const documentSelection = new DocumentSelection( doc );
  	 *		selection.setTo( documentSelection );
  	 *
  	 *		// Sets collapsed selection at the given position.
  	 *		const position = writer.createPositionFromPath( root, path );
  	 *		selection.setTo( position );
  	 *
  	 *		// Sets collapsed selection at the position of the given node and an offset.
  	 *		selection.setTo( paragraph, offset );
  	 *
  	 * Creates a range inside an {@link module:engine/model/element~Element element} which starts before the first child of
   	 * that element and ends after the last child of that element.
  	 *
  	 *		selection.setTo( paragraph, 'in' );
  	 *
  	 * Creates a range on an {@link module:engine/model/item~Item item} which starts before the item and ends just after the item.
  	 *
  	 *		selection.setTo( paragraph, 'on' );
  	 *
  	 * `Selection#setTo()`' method allow passing additional options (`backward`) as the last argument.
  	 *
  	 *		// Sets backward selection.
  	 *		const selection = writer.createSelection( range, { backward: true } );
  	 *
  	 * @param {module:engine/model/selection~Selectable} selectable
  	 * @param {Number|'before'|'end'|'after'|'on'|'in'} [placeOrOffset] Sets place or offset of the selection.
  	 * @param {Object} [options]
  	 * @param {Boolean} [options.backward] Sets this selection instance to be backward.
  	 */
  	setTo( selectable, placeOrOffset, options ) {
  		if ( selectable === null ) {
  			this._setRanges( [] );
  		} else if ( selectable instanceof Selection ) {
  			this._setRanges( selectable.getRanges(), selectable.isBackward );
  		} else if ( selectable && typeof selectable.getRanges == 'function' ) {
  			// We assume that the selectable is a DocumentSelection.
  			// It can't be imported here, because it would lead to circular imports.
  			this._setRanges( selectable.getRanges(), selectable.isBackward );
  		} else if ( selectable instanceof Range ) {
  			this._setRanges( [ selectable ], !!placeOrOffset && !!placeOrOffset.backward );
  		} else if ( selectable instanceof Position ) {
  			this._setRanges( [ new Range( selectable ) ] );
  		} else if ( selectable instanceof Node ) {
  			const backward = !!options && !!options.backward;
  			let range;
  
  			if ( placeOrOffset == 'in' ) {
  				range = Range._createIn( selectable );
  			} else if ( placeOrOffset == 'on' ) {
  				range = Range._createOn( selectable );
  			} else if ( placeOrOffset !== undefined ) {
  				range = new Range( Position._createAt( selectable, placeOrOffset ) );
  			} else {
  				/**
  				 * selection.setTo requires the second parameter when the first parameter is a node.
  				 *
  				 * @error model-selection-setTo-required-second-parameter
  				 */
  				throw new CKEditorError(
  					'model-selection-setTo-required-second-parameter: ' +
  					'selection.setTo requires the second parameter when the first parameter is a node.',
  					[ this, selectable ]
  				);
  			}
  
  			this._setRanges( [ range ], backward );
  		} else if ( isIterable( selectable ) ) {
  			// We assume that the selectable is an iterable of ranges.
  			this._setRanges( selectable, placeOrOffset && !!placeOrOffset.backward );
  		} else {
  			/**
  			 * Cannot set the selection to the given place.
  			 *
  			 * Invalid parameters were specified when setting the selection. Common issues:
  			 *
  			 * * A {@link module:engine/model/textproxy~TextProxy} instance was passed instead of
  			 * a real {@link module:engine/model/text~Text}.
  			 * * View nodes were passed instead of model nodes.
  			 * * `null`/`undefined` was passed.
  			 *
  			 * @error model-selection-setTo-not-selectable
  			 */
  			throw new CKEditorError(
  				'model-selection-setTo-not-selectable: Cannot set the selection to the given place.',
  				[ this, selectable ]
  			);
  		}
  	}
  
  	/**
  	 * Replaces all ranges that were added to the selection with given array of ranges. Last range of the array
  	 * is treated like the last added range and is used to set {@link module:engine/model/selection~Selection#anchor} and
  	 * {@link module:engine/model/selection~Selection#focus}. Accepts a flag describing in which direction the selection is made.
  	 *
  	 * @protected
  	 * @fires change:range
  	 * @param {Iterable.<module:engine/model/range~Range>} newRanges Ranges to set.
  	 * @param {Boolean} [isLastBackward=false] Flag describing if last added range was selected forward - from start to end (`false`)
  	 * or backward - from end to start (`true`).
  	 */
  	_setRanges( newRanges, isLastBackward = false ) {
  		newRanges = Array.from( newRanges );
  
  		// Check whether there is any range in new ranges set that is different than all already added ranges.
  		const anyNewRange = newRanges.some( newRange => {
  			if ( !( newRange instanceof Range ) ) {
  				/**
  				 * Selection range set to an object that is not an instance of {@link module:engine/model/range~Range}.
  				 *
  				 * Only {@link module:engine/model/range~Range} instances can be used to set a selection.
  				 * Common mistakes leading to this error are:
  				 *
  				 * * using DOM `Range` object,
  				 * * incorrect CKEditor 5 installation with multiple `ckeditor5-engine` packages having different versions.
  				 *
  				 * @error model-selection-set-ranges-not-range
  				 */
  				throw new CKEditorError(
  					'model-selection-set-ranges-not-range: ' +
  					'Selection range set to an object that is not an instance of model.Range.',
  					[ this, newRanges ]
  				);
  			}
  
  			return this._ranges.every( oldRange => {
  				return !oldRange.isEqual( newRange );
  			} );
  		} );
  
  		// Don't do anything if nothing changed.
  		if ( newRanges.length === this._ranges.length && !anyNewRange ) {
  			return;
  		}
  
  		this._removeAllRanges();
  
  		for ( const range of newRanges ) {
  			this._pushRange( range );
  		}
  
  		this._lastRangeBackward = !!isLastBackward;
  
  		this.fire( 'change:range', { directChange: true } );
  	}
  
  	/**
  	 * Moves {@link module:engine/model/selection~Selection#focus} to the specified location.
  	 *
  	 * The location can be specified in the same form as
  	 * {@link module:engine/model/writer~Writer#createPositionAt writer.createPositionAt()} parameters.
  	 *
  	 * @fires change:range
  	 * @param {module:engine/model/item~Item|module:engine/model/position~Position} itemOrPosition
  	 * @param {Number|'end'|'before'|'after'} [offset] Offset or one of the flags. Used only when
  	 * first parameter is a {@link module:engine/model/item~Item model item}.
  	 */
  	setFocus( itemOrPosition, offset ) {
  		if ( this.anchor === null ) {
  			/**
  			 * Cannot set selection focus if there are no ranges in selection.
  			 *
  			 * @error model-selection-setFocus-no-ranges
  			 */
  			throw new CKEditorError(
  				'model-selection-setFocus-no-ranges: Cannot set selection focus if there are no ranges in selection.',
  				[ this, itemOrPosition ]
  			);
  		}
  
  		const newFocus = Position._createAt( itemOrPosition, offset );
  
  		if ( newFocus.compareWith( this.focus ) == 'same' ) {
  			return;
  		}
  
  		const anchor = this.anchor;
  
  		if ( this._ranges.length ) {
  			this._popRange();
  		}
  
  		if ( newFocus.compareWith( anchor ) == 'before' ) {
  			this._pushRange( new Range( newFocus, anchor ) );
  			this._lastRangeBackward = true;
  		} else {
  			this._pushRange( new Range( anchor, newFocus ) );
  			this._lastRangeBackward = false;
  		}
  
  		this.fire( 'change:range', { directChange: true } );
  	}
  
  	/**
  	 * Gets an attribute value for given key or `undefined` if that attribute is not set on the selection.
  	 *
  	 * @param {String} key Key of attribute to look for.
  	 * @returns {*} Attribute value or `undefined`.
  	 */
  	getAttribute( key ) {
  		return this._attrs.get( key );
  	}
  
  	/**
  	 * Returns iterable that iterates over this selection's attributes.
  	 *
  	 * Attributes are returned as arrays containing two items. First one is attribute key and second is attribute value.
  	 * This format is accepted by native `Map` object and also can be passed in `Node` constructor.
  	 *
  	 * @returns {Iterable.<*>}
  	 */
  	getAttributes() {
  		return this._attrs.entries();
  	}
  
  	/**
  	 * Returns iterable that iterates over this selection's attribute keys.
  	 *
  	 * @returns {Iterable.<String>}
  	 */
  	getAttributeKeys() {
  		return this._attrs.keys();
  	}
  
  	/**
  	 * Checks if the selection has an attribute for given key.
  	 *
  	 * @param {String} key Key of attribute to check.
  	 * @returns {Boolean} `true` if attribute with given key is set on selection, `false` otherwise.
  	 */
  	hasAttribute( key ) {
  		return this._attrs.has( key );
  	}
  
  	/**
  	 * Removes an attribute with given key from the selection.
  	 *
  	 * If given attribute was set on the selection, fires the {@link #event:change:range} event with
  	 * removed attribute key.
  	 *
  	 * @fires change:attribute
  	 * @param {String} key Key of attribute to remove.
  	 */
  	removeAttribute( key ) {
  		if ( this.hasAttribute( key ) ) {
  			this._attrs.delete( key );
  
  			this.fire( 'change:attribute', { attributeKeys: [ key ], directChange: true } );
  		}
  	}
  
  	/**
  	 * Sets attribute on the selection. If attribute with the same key already is set, it's value is overwritten.
  	 *
  	 * If the attribute value has changed, fires the {@link #event:change:range} event with
  	 * the attribute key.
  	 *
  	 * @fires change:attribute
  	 * @param {String} key Key of attribute to set.
  	 * @param {*} value Attribute value.
  	 */
  	setAttribute( key, value ) {
  		if ( this.getAttribute( key ) !== value ) {
  			this._attrs.set( key, value );
  
  			this.fire( 'change:attribute', { attributeKeys: [ key ], directChange: true } );
  		}
  	}
  
  	/**
  	 * Returns the selected element. {@link module:engine/model/element~Element Element} is considered as selected if there is only
  	 * one range in the selection, and that range contains exactly one element.
  	 * Returns `null` if there is no selected element.
  	 *
  	 * @returns {module:engine/model/element~Element|null}
  	 */
  	getSelectedElement() {
  		if ( this.rangeCount !== 1 ) {
  			return null;
  		}
  
  		const range = this.getFirstRange();
  		const nodeAfterStart = range.start.nodeAfter;
  		const nodeBeforeEnd = range.end.nodeBefore;
  
  		return ( nodeAfterStart instanceof Element && nodeAfterStart == nodeBeforeEnd ) ? nodeAfterStart : null;
  	}
  
  	/**
  	 * Checks whether this object is of the given.
  	 *
  	 *		selection.is( 'selection' ); // -> true
  	 *		selection.is( 'model:selection' ); // -> true
  	 *
  	 *		selection.is( 'view:selection' ); // -> false
  	 *		selection.is( 'range' ); // -> false
  	 *
  	 * {@link module:engine/model/node~Node#is Check the entire list of model objects} which implement the `is()` method.
  	 *
  	 * @param {String} type
  	 * @returns {Boolean}
  	 */
  	is( type ) {
  		return type == 'selection' || type == 'model:selection';
  	}
  
  	/**
  	 * Gets elements of type {@link module:engine/model/schema~Schema#isBlock "block"} touched by the selection.
  	 *
  	 * This method's result can be used for example to apply block styling to all blocks covered by this selection.
  	 *
  	 * **Note:** `getSelectedBlocks()` returns blocks that are nested in other non-block elements
  	 * but will not return blocks nested in other blocks.
  	 *
  	 * In this case the function will return exactly all 3 paragraphs (note: `<blockQuote>` is not a block itself):
  	 *
  	 *		<paragraph>[a</paragraph>
  	 *		<blockQuote>
  	 *			<paragraph>b</paragraph>
  	 *		</blockQuote>
  	 *		<paragraph>c]d</paragraph>
  	 *
  	 * In this case the paragraph will also be returned, despite the collapsed selection:
  	 *
  	 *		<paragraph>[]a</paragraph>
  	 *
  	 * In such a scenario, however, only blocks A, B & E will be returned as blocks C & D are nested in block B:
  	 *
  	 *		[<blockA></blockA>
  	 *		<blockB>
  	 *			<blockC></blockC>
  	 *			<blockD></blockD>
  	 *		</blockB>
  	 *		<blockE></blockE>]
  	 *
  	 * If the selection is inside a block all the inner blocks (A & B) are returned:
  	 *
  	 * 		<block>
  	 *			<blockA>[a</blockA>
  	 * 			<blockB>b]</blockB>
  	 * 		</block>
  	 *
  	 * **Special case**: If a selection ends at the beginning of a block, that block is not returned as from user perspective
  	 * this block wasn't selected. See [#984](https://github.com/ckeditor/ckeditor5-engine/issues/984) for more details.
  	 *
  	 *		<paragraph>[a</paragraph>
  	 *		<paragraph>b</paragraph>
  	 *		<paragraph>]c</paragraph> // this block will not be returned
  	 *
  	 * @returns {Iterable.<module:engine/model/element~Element>}
  	 */
  	* getSelectedBlocks() {
  		const visited = new WeakSet();
  
  		for ( const range of this.getRanges() ) {
  			// Get start block of range in case of a collapsed range.
  			const startBlock = getParentBlock( range.start, visited );
  
  			if ( startBlock && isTopBlockInRange( startBlock, range ) ) {
  				yield startBlock;
  			}
  
  			for ( const value of range.getWalker() ) {
  				const block = value.item;
  
  				if ( value.type == 'elementEnd' && isUnvisitedTopBlock( block, visited, range ) ) {
  					yield block;
  				}
  			}
  
  			const endBlock = getParentBlock( range.end, visited );
  
  			// #984. Don't return the end block if the range ends right at its beginning.
  			if ( endBlock && !range.end.isTouching( Position._createAt( endBlock, 0 ) ) && isTopBlockInRange( endBlock, range ) ) {
  				yield endBlock;
  			}
  		}
  	}
  
  	/**
  	 * Checks whether the selection contains the entire content of the given element. This means that selection must start
  	 * at a position {@link module:engine/model/position~Position#isTouching touching} the element's start and ends at position
  	 * touching the element's end.
  	 *
  	 * By default, this method will check whether the entire content of the selection's current root is selected.
  	 * Useful to check if e.g. the user has just pressed <kbd>Ctrl</kbd> + <kbd>A</kbd>.
  	 *
  	 * @param {module:engine/model/element~Element} [element=this.anchor.root]
  	 * @returns {Boolean}
  	 */
  	containsEntireContent( element = this.anchor.root ) {
  		const limitStartPosition = Position._createAt( element, 0 );
  		const limitEndPosition = Position._createAt( element, 'end' );
  
  		return limitStartPosition.isTouching( this.getFirstPosition() ) &&
  			limitEndPosition.isTouching( this.getLastPosition() );
  	}
  
  	/**
  	 * Adds given range to internal {@link #_ranges ranges array}. Throws an error
  	 * if given range is intersecting with any range that is already stored in this selection.
  	 *
  	 * @protected
  	 * @param {module:engine/model/range~Range} range Range to add.
  	 */
  	_pushRange( range ) {
  		this._checkRange( range );
  		this._ranges.push( new Range( range.start, range.end ) );
  	}
  
  	/**
  	 * Checks if given range intersects with ranges that are already in the selection. Throws an error if it does.
  	 *
  	 * @protected
  	 * @param {module:engine/model/range~Range} range Range to check.
  	 */
  	_checkRange( range ) {
  		for ( let i = 0; i < this._ranges.length; i++ ) {
  			if ( range.isIntersecting( this._ranges[ i ] ) ) {
  				/**
  				 * Trying to add a range that intersects with another range in the selection.
  				 *
  				 * @error model-selection-range-intersects
  				 * @param {module:engine/model/range~Range} addedRange Range that was added to the selection.
  				 * @param {module:engine/model/range~Range} intersectingRange Range in the selection that intersects with `addedRange`.
  				 */
  				throw new CKEditorError(
  					'model-selection-range-intersects: Trying to add a range that intersects with another range in the selection.',
  					[ this, range ],
  					{ addedRange: range, intersectingRange: this._ranges[ i ] }
  				);
  			}
  		}
  	}
  
  	/**
  	 * Deletes ranges from internal range array. Uses {@link #_popRange _popRange} to
  	 * ensure proper ranges removal.
  	 *
  	 * @protected
  	 */
  	_removeAllRanges() {
  		while ( this._ranges.length > 0 ) {
  			this._popRange();
  		}
  	}
  
  	/**
  	 * Removes most recently added range from the selection.
  	 *
  	 * @protected
  	 */
  	_popRange() {
  		this._ranges.pop();
  	}
  
  	/**
  	 * Fired when selection range(s) changed.
  	 *
  	 * @event change:range
  	 * @param {Boolean} directChange In case of {@link module:engine/model/selection~Selection} class it is always set
  	 * to `true` which indicates that the selection change was caused by a direct use of selection's API.
  	 * The {@link module:engine/model/documentselection~DocumentSelection}, however, may change because its position
  	 * was directly changed through the {@link module:engine/model/writer~Writer writer} or because its position was
  	 * changed because the structure of the model has been changed (which means an indirect change).
  	 * The indirect change does not occur in case of normal (detached) selections because they are "static" (as "not live")
  	 * which mean that they are not updated once the document changes.
  	 */
  
  	/**
  	 * Fired when selection attribute changed.
  	 *
  	 * @event change:attribute
  	 * @param {Boolean} directChange In case of {@link module:engine/model/selection~Selection} class it is always set
  	 * to `true` which indicates that the selection change was caused by a direct use of selection's API.
  	 * The {@link module:engine/model/documentselection~DocumentSelection}, however, may change because its attributes
  	 * were directly changed through the {@link module:engine/model/writer~Writer writer} or because its position was
  	 * changed in the model and its attributes were refreshed (which means an indirect change).
  	 * The indirect change does not occur in case of normal (detached) selections because they are "static" (as "not live")
  	 * which mean that they are not updated once the document changes.
  	 * @param {Array.<String>} attributeKeys Array containing keys of attributes that changed.
  	 */
  }
  
  mix( Selection, EmitterMixin );
  
  // Checks whether the given element extends $block in the schema and has a parent (is not a root).
  // Marks it as already visited.
  function isUnvisitedBlock( element, visited ) {
  	if ( visited.has( element ) ) {
  		return false;
  	}
  
  	visited.add( element );
  
  	return element.document.model.schema.isBlock( element ) && element.parent;
  }
  
  // Checks if the given element is a $block was not previously visited and is a top block in a range.
  function isUnvisitedTopBlock( element, visited, range ) {
  	return isUnvisitedBlock( element, visited ) && isTopBlockInRange( element, range );
  }
  
  // Finds the lowest element in position's ancestors which is a block.
  // It will search until first ancestor that is a limit element.
  // Marks all ancestors as already visited to not include any of them later on.
  function getParentBlock( position, visited ) {
  	const schema = position.parent.document.model.schema;
  
  	const ancestors = position.parent.getAncestors( { parentFirst: true, includeSelf: true } );
  
  	let hasParentLimit = false;
  
  	const block = ancestors.find( element => {
  		// Stop searching after first parent node that is limit element.
  		if ( hasParentLimit ) {
  			return false;
  		}
  
  		hasParentLimit = schema.isLimit( element );
  
  		return !hasParentLimit && isUnvisitedBlock( element, visited );
  	} );
  
  	// Mark all ancestors of this position's parent, because find() might've stopped early and
  	// the found block may be a child of another block.
  	ancestors.forEach( element => visited.add( element ) );
  
  	return block;
  }
  
  // Checks if the blocks is not nested in other block inside a range.
  //
  // @param {module:engine/model/elmenent~Element} block Block to check.
  // @param {module:engine/model/range~Range} range Range to check.
  function isTopBlockInRange( block, range ) {
  	const parentBlock = findAncestorBlock( block );
  
  	if ( !parentBlock ) {
  		return true;
  	}
  
  	// Add loose flag to check as parentRange can be equal to range.
  	const isParentInRange = range.containsRange( Range._createOn( parentBlock ), true );
  
  	return !isParentInRange;
  }
  
  // Returns first ancestor block of a node.
  //
  // @param {module:engine/model/node~Node} node
  // @returns {module:engine/model/node~Node|undefined}
  function findAncestorBlock( node ) {
  	const schema = node.document.model.schema;
  
  	let parent = node.parent;
  
  	while ( parent ) {
  		if ( schema.isBlock( parent ) ) {
  			return parent;
  		}
  
  		parent = parent.parent;
  	}
  }
  
  /**
   * An entity that is used to set selection.
   *
   * See also {@link module:engine/model/selection~Selection#setTo}
   *
   * @typedef {
   *     module:engine/model/selection~Selection|
   *     module:engine/model/documentselection~DocumentSelection|
   *     module:engine/model/position~Position|
   *     module:engine/model/range~Range|
   *     module:engine/model/node~Node|
   *     Iterable.<module:engine/model/range~Range>|
   *     null
   * } module:engine/model/selection~Selectable
   */
  