ReScript-aware `memo`

// https://github.com/facebook/react/blob/857ee8c/packages/shared/shallowEqual.js#L18

// `is` and `hasOwnProperty` are polyfill of `Object` static methods
function shallowEqual(objA: mixed, objB: mixed): boolean {
  if (is(objA, objB)) {
    return true;
  }

  if (
    typeof objA !== 'object' ||
    objA === null ||
    typeof objB !== 'object' ||
    objB === null
  ) {
    return false;
  }

  const keysA = Object.keys(objA);
  const keysB = Object.keys(objB);

  if (keysA.length !== keysB.length) {
    return false;
  }

  // Test for A's keys different from B.
  for (let i = 0; i < keysA.length; i++) {
    const currentKey = keysA[i];
    if (
      !hasOwnProperty.call(objB, currentKey) ||
      // $FlowFixMe[incompatible-use] lost refinement of `objB`
      !is(objA[currentKey], objB[currentKey])
    ) {
      return false;
    }
  }

  return true;
}

function shallowEqualPolyvar(objA: polyvar, objB: polyvar) {
  if (objA.NAME !== objB.NAME) {
    return false;
  }
  return shallowEqual(objA, objB);
}

function shallowEqualVariant(objA: variant, objB: variant) {
  // Ok... this should be done by the compiler
  if (objA.TAG !== objB.TAG) {
    return false;
  }
  return shallowEqual(objA._0, objB._0)
}

function shallowEqual(objA: mixed, objB: mixed): boolean {
  if (is(objA, objB)) {
    return true;
  }

  if (
    typeof objA !== 'object' ||
    objA === null ||
    typeof objB !== 'object' ||
    objB === null
  ) {
    return false;
  }
  
  // We cannot skip check because there is no guarantee objs are record.
  // Or maybe we can make separate function for it.

  // isPolyvar should be provided by the compiler
  const isObjAPolyvar = isPolyvar(objA)
  const isObjBPolyvar = isPolyvar(objB)
  if (isObjAPolyvar !== isObjBPolyvar) {
    return false;
  } else if (isObjAPolyvar) {
    return shallowEqualPolyvar(objA, objB);
  }

  // isVariant should be provided by the compiler
  const isObjAVariant = isVariant(objA)
  const isObjBVariant = isVariant(objB)
  if (isObjAVariant !== isObjBVariant) {
    return false;
  } else if (isObjAVariant) {
    return shallowEqualVariant(objA, objB)
  }

  const keysA = Object.keys(objA);
  const keysB = Object.keys(objB);

  if (keysA.length !== keysB.length) {
    return false;
  }

  // Test for A's keys different from B.
  for (let i = 0; i < keysA.length; i++) {
    const currentKey = keysA[i];
    if (
      !hasOwnProperty.call(objB, currentKey) ||
      !is(objA[currentKey], objB[currentKey])
    ) {
      return false;
    }
  }

  return true;
}