function calculate(a, b, operation) {
  switch (operation) {
    case 'add':      return { result: a + b };
    case 'subtract': return { result: a - b };
    case 'multiply': return { result: a * b };
    case 'divide':
      if (b === 0) return { error: 'Division by zero is not allowed.' };
      return { result: a / b };
    default:
      return { error: 'Unknown operation: ' + operation };
  }
}

(function () {
  var form = document.getElementById('calculator-form');
  var operationBtns = document.querySelectorAll('button[name="operation"]');
  var resultEl = document.getElementById('result');
  var errorEl = document.getElementById('error-message');
  var selectedOperation = null;

  operationBtns.forEach(function (btn) {
    btn.addEventListener('click', function () {
      operationBtns.forEach(function (b) { b.setAttribute('aria-pressed', 'false'); });
      btn.setAttribute('aria-pressed', 'true');
      selectedOperation = btn.value;
    });
  });

  form.addEventListener('submit', function (e) {
    e.preventDefault();
    resultEl.textContent = '';
    errorEl.textContent = '';

    var a = parseFloat(document.getElementById('operand1').value);
    var b = parseFloat(document.getElementById('operand2').value);

    if (isNaN(a) || isNaN(b)) {
      errorEl.textContent = 'Please enter valid numbers for both operands.';
      return;
    }

    if (!selectedOperation) {
      errorEl.textContent = 'Please select an operation.';
      return;
    }

    var outcome = calculate(a, b, selectedOperation);

    if (outcome.error) {
      errorEl.textContent = outcome.error;
    } else {
      resultEl.textContent = outcome.result;
    }
  });

  form.addEventListener('reset', function () {
    resultEl.textContent = '';
    errorEl.textContent = '';
    operationBtns.forEach(function (btn) { btn.setAttribute('aria-pressed', 'false'); });
    selectedOperation = null;
  });
}());