functional-data-structure/finger-tree

import assert from 'assert';

import {One, Two, Three, Four} from '../../1-digit/index.js';
import {Empty, Single, Deep} from '../../3-tree/index.js';
import {cache} from '../measure/index.js';

export function _from_digit(M, digit) {
    if (digit instanceof One) return new Single(M, digit.a);
    assert(
        digit instanceof Two || digit instanceof Three || digit instanceof Four,
    );
    return new Deep(M, digit.init(), new Empty(cache(M)), new One(digit.last()));
}