Mathematical Expression String Evaluation

math-expression-str-evaluate.js

const calculate = (num1, operator, num2) => {
  switch (operator) {
    case '+': return num1 + num2;
    case '-': return  num1 - num2;
    case '*': return num1 * num2;
    case '/': return  num1 / num2;
  }
}

const isOperator = str => ['+','-', '*', '/'].includes(str);

const isLowerPrecedingOperator = str => ['+','-'].includes(str);

const isHighPrecedingOperator = str => ['*','/'].includes(str);

const isOpeningBrace = str => str === '(';

const isClosingBrace = str => str === ')';

function Calculator(inputs, lowerOperationsOnly) {
  
  let nestedInputs = '';
  let isBraceOpen;
  let evaluated = 0;
  const stack = [];
  
  for (let index = 0; index < inputs.length; index++) {
    
    let input = inputs[index];
    stack.push(input);
    // console.log('stack', stack);
    
    if (isClosingBrace(input)) {
      isBraceOpen = false;
      input = Calculator(nestedInputs);
      let nestedInputsLength = nestedInputs.length + 2;
      while (nestedInputsLength) {
        stack.pop();
        nestedInputsLength--;
      }
      nestedInputs = '';
      stack.push(input);
      const nextInput = inputs[index + 1];
      if (nextInput && !isOperator(nextInput)) {
        stack.push('*');
      }
    }
    
    if (isBraceOpen) {
      nestedInputs += input;
      continue;
    }
    
    if (isOpeningBrace(input)) {
      isBraceOpen = true;
      continue;
    }
    
    const prevInput = stack[stack.length - 2];
    const isLastIndex = index === inputs.length - 1;
    const canCompute = isHighPrecedingOperator(prevInput)
      || isLastIndex
      || (lowerOperationsOnly && isLowerPrecedingOperator(prevInput));
    if (canCompute) {
      const isNegativeNo = stack[stack.length - 4] === '-' ? -1 : 1;
      const prevToPrevInput = (stack[stack.length - 3] || 0)*isNegativeNo;
      evaluated = Math.abs(calculate(parseInt(prevToPrevInput), prevInput, parseInt(input)));
      // console.log(prevToPrevInput, prevInput, input, '=', evaluated)
      stack.pop();
      stack.pop();
      stack.pop();
      stack.push(evaluated);

      if (stack.length > 1) {
        evaluated = Calculator(stack, true);
        while(stack.length) {
          stack.pop();
        }
        stack.push(evaluated);
      }
    }

  }

  return evaluated;
}
   
// keep this function call here 
console.log(Calculator(readline()));