functional-data-structure/finger-tree

import assert from 'assert';

import {Empty, Single, Deep} from '../../3-tree/index.js';
import {delay} from '../../4-lazy/index.js';
import {app3} from '../concatenate/index.js';

export function _app3(A, B) {
    assert(A instanceof Deep);

    B = B.force();

    if (B instanceof Empty) return A;

    if (B instanceof Single) return A.push(B.last());

    return new Deep(
        A.M,
        A.left,
        delay(() => app3(A.middle, A.right._nodes(A.M, B.left), B.middle)),
        B.right,
    );
}