Introduced view.Node#getCommonAncestor().

src/view/node.js

@@ -138,6 +138,26 @@ export default class Node {
 		return ancestors;
 	}
 
+	/**
+	 * Returns a {@link module:engine/view/element~Element} or {@link module:engine/view/documentfragment~DocumentFragment}
+	 * which is a common ancestor of both nodes.
+	 *
+	 * @param {module:engine/view/node~Node} node The second node.
+	 * @returns {module:engine/view/element~Element|module:engine/view/documentfragment~DocumentFragment|null}
+	 */
+	getCommonAncestor( node ) {
+		const ancestorsA = this.getAncestors();
+		const ancestorsB = node.getAncestors();
+
+		let i = 0;
+
+		while ( ancestorsA[ i ] == ancestorsB[ i ] && ancestorsA[ i ] ) {
+			i++;
+		}
+
+		return i === 0 ? null : ancestorsA[ i - 1 ];
+	}
+
 	/**
 	 * Removes node from parent.
 	 */

tests/view/node.js

@@ -109,6 +109,57 @@ describe( 'Node', () => {
 		} );
 	} );
 
+	describe( 'getCommonAncestor()', () => {
+		it( 'should return the parent element for the same node', () => {
+			expect( img.getCommonAncestor( img ) ).to.equal( two );
+		} );
+
+		it( 'should return null for detached subtrees', () => {
+			const detached = new Element( 'foo' );
+
+			expect( img.getCommonAncestor( detached ) ).to.be.null;
+			expect( detached.getCommonAncestor( img ) ).to.be.null;
+		} );
+
+		it( 'should return null when one of the nodes is a tree root itself', () => {
+			expect( root.getCommonAncestor( img ) ).to.be.null;
+			expect( img.getCommonAncestor( root ) ).to.be.null;
+			expect( root.getCommonAncestor( root ) ).to.be.null;
+		} );
+
+		it( 'should return parent of the nodes at the same level', () => {
+			expect( img.getCommonAncestor( charA ) ).to.equal( two );
+			expect( charB.getCommonAncestor( charA ) ).to.equal( two );
+		} );
+
+		it( 'should return proper element for nodes in different branches and on different levels', () => {
+			const foo = new Text( 'foo' );
+			const bar = new Text( 'bar' );
+			const bom = new Text( 'bom' );
+			const d = new Element( 'd', null, [ bar ] );
+			const c = new Element( 'c', null, [ foo, d ] );
+			const b = new Element( 'b', null, [ c ] );
+			const e = new Element( 'e', null, [ bom ] );
+			const a = new Element( 'a', null, [ b, e ] );
+
+			// <a><b><c>foo<d>bar</d></c></b><e>bom</e></a>
+
+			expect( bar.getCommonAncestor( foo ), 1 ).to.equal( c );
+			expect( foo.getCommonAncestor( d ), 2 ).to.equal( c );
+			expect( c.getCommonAncestor( b ), 3 ).to.equal( a );
+			expect( bom.getCommonAncestor( d ), 4 ).to.equal( a );
+			expect( b.getCommonAncestor( bom ), 5 ).to.equal( a );
+		} );
+
+		it( 'should return document fragment', () => {
+			const foo = new Text( 'foo' );
+			const bar = new Text( 'bar' );
+			const df = new DocumentFragment( [ foo, bar ] );
+
+			expect( foo.getCommonAncestor( bar ) ).to.equal( df );
+		} );
+	} );
+
 	describe( 'getIndex()', () => {
 		it( 'should return null if the parent is null', () => {
 			expect( root.index ).to.be.null;