const { equal } = require('assert')

const add = {
    name: 'ADD',
    fn(...args) {
        return args.map(x => Number(x)).filter(Boolean).reduce((sum, x) => sum + x, 0)
    }
}

const sub = {
    name: 'SUB',
    fn(...args) {
        return args.map(x => Number(x)).filter(Boolean).reduce((sum, x) => sum - x)
    }
}

const FUNCTIONS = [
    add,
    sub
]

const isFn = (input) =>
    /.+\s*\(.+\)/.test(input)


const excel = (input) => {

    const token = /^(\w+)\s*\((.+)\)/.exec(input)

    if (!token) throw new Error('no function to parse')

    const firstFunction = token[1].trim();
    const found = FUNCTIONS.find(({ name }) => name === firstFunction.toUpperCase())
    if (!found) throw new Error(`unknown ${found} function`);

    const fn = found.fn

    const args =
        token[2]
            .split(/,(?![^,]+\))/)
            .map(x => x.trim())
            .map(x => isFn(x) ? excel(x) : x);

    return fn(...args);
}

equal(
    excel('ADD(1, 2)'),
    3
)

equal(
    excel('ADD(1, 2, 3, 4)'),
    10
)

equal(
    excel('SUB(9, 3)'),
    6
)

equal(
    excel('ADD(1, 2, SUB(ADD(5, 4), 3))'),
    9
)