// zip two arrays
export function zip(a, b) {
  return a.map((k, i) => [k, b[i]]);
}

export function capitalizeFirstLetter(val) {
  return String(val).charAt(0).toUpperCase() + String(val).slice(1);
}

const _mapRecursiveStructure = mapping => transform => root => {
  const found = mapping.get(root);
  if (found) {
    // already mapped
    // return existing result to prevent endless recursion
    return found;
  }
  // note the indirection (wrapped in lamda), this allows the user to recursively map the children (which may refer to the root) without yet having finished mapping the root.
  let memo;
  const result = () => {
    // memoization is necessary for correctness
    return memo || (memo = transform(root, _mapRecursiveStructure(mapping)(transform)));
  };
  mapping.set(root, result);
  return result;
};

export const mapRecursiveStructure = _mapRecursiveStructure(new Map());

export const memoize = callback => {
  let called = false
  let result;
  return () => {
    if (!called) {
      result = callback();
      called = true;
    }
    return result;
  };
};