Cassidoo 2020.10.12

baby_lisp.js

/* 
Given a basic Lisp-like string expression, parse it, 
where the available functions are
add, subtract, multiply, and divide,
and they all take in 2 values.

Examples:

babyLisp('(add 1 2)')
3
babyLisp('(multiply 4 (add 2 3))')
20
babyLisp('(multiply (add 2 (multiply 3 5)) 4)')
68
*/

const babyLisp = (str) => {
  const functions = {
    add: (a, b) => babyLisp(a) + babyLisp(b),
    subtract: (a, b) => babyLisp(a) - babyLisp(b),
    multiply: (a, b) => babyLisp(a) * babyLisp(b),
    divide: (a, b) => babyLisp(a) / babyLisp(b),
  };

  let body = `${str}`;
  if (body[0] === '(' && body[body.length - 1] === ')') {
    body = body.slice(1, -1);
  }
  const [first, ...rest] = body.split(' ');

  if (!isNaN(Number(first)) && rest.length === 0) {
    // Ladies and gentlemen, we got it //
    return Number(first);
  }

  const args = [];
  let index = 0;
  let parenthesisDepth = 0; // We will deal with depth with recursion

  for (const v of body.slice(first.length + 1)) {
    if (v === '(') parenthesisDepth++;
    if (v === ')') parenthesisDepth--;
    if (v === ' ' && parenthesisDepth === 0) {
      index++;
    } else {
      args[index] = (args[index] || '') + v;
    }
  }
  return functions[first](...args);
};