ckeditor/ckeditor5-utils

/**
 * @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 utils/difftochanges
 */

/**
 * Creates a set of changes which need to be applied to the input in order to transform
 * it into the output. This function can be used with strings or arrays.
 *
 *        const input = Array.from( 'abc' );
 *        const output = Array.from( 'xaby' );
 *        const changes = diffToChanges( diff( input, output ), output );
 *
 *        changes.forEach( change => {
 *            if ( change.type == 'insert' ) {
 *                input.splice( change.index, 0, ...change.values );
 *            } else if ( change.type == 'delete' ) {
 *                input.splice( change.index, change.howMany );
 *            }
 *        } );
 *
 *        input.join( '' ) == output.join( '' ); // -> true
 *
 * @param {Array.<'equal'|'insert'|'delete'>} diff Result of {@link module:utils/diff~diff}.
 * @param {String|Array} output The string or array which was passed as diff's output.
 * @returns {Array.<Object>} Set of changes (insert or delete) which need to be applied to the input
 * in order to transform it into the output.
 */
export default function diffToChanges( diff, output ) {
    const changes = [];
    let index = 0;
    let lastOperation;

    diff.forEach( change => {
        if ( change == 'equal' ) {
            pushLast();

            index++;
        } else if ( change == 'insert' ) {
            if ( isContinuationOf( 'insert' ) ) {
                lastOperation.values.push( output[ index ] );
            } else {
                pushLast();

                lastOperation = {
                    type: 'insert',
                    index,
                    values: [ output[ index ] ]
                };
            }

            index++;
        } else /* if ( change == 'delete' ) */ {
            if ( isContinuationOf( 'delete' ) ) {
                lastOperation.howMany++;
            } else {
                pushLast();

                lastOperation = {
                    type: 'delete',
                    index,
                    howMany: 1
                };
            }
        }
    } );

    pushLast();

    return changes;

    function pushLast() {
        if ( lastOperation ) {
            changes.push( lastOperation );
            lastOperation = null;
        }
    }

    function isContinuationOf( expected ) {
        return lastOperation && lastOperation.type == expected;
    }
}