aureooms/js-fingertree

import assert from 'assert';

import {Tree, Empty, Single, Deep} from '../../3-tree/index.js';
import {delay} from '../../4-lazy/index.js';
import {_prepend, _append} from '../_fast/index.js';

export function app3(A, list, B) {
    assert(A instanceof Tree);
    assert(B instanceof Tree);

    A = A.force();
    B = B.force();

    if (A instanceof Empty) return _prepend(B, list);
    if (B instanceof Empty) return _append(A, list);

    if (A instanceof Single) return _prepend(B, list).cons(A.head());
    if (B instanceof Single) return _append(A, list).push(B.last());

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