const TABBABLE_NODES = /input|select|textarea|button|object/

export const FOCUS_SELECTOR =
  'a, input, select, textarea, button, object, [tabindex]'

function hidden(element: HTMLElement) {
  if (process.env.NODE_ENV === 'test') {
    return false
  }

  return element.style.display === 'none'
}

function visible(element: HTMLElement) {
  let parentElement: HTMLElement = element

  while (parentElement) {
    if (parentElement === document.body) {
      break
    }

    if (hidden(parentElement)) {
      return false
    }

    parentElement = parentElement.parentNode as HTMLElement
  }

  return true
}

function getElementTabIndex(element: HTMLElement) {
  let tabIndex = element.getAttribute('tabindex')

  if (tabIndex === null) {
    tabIndex = undefined
  }

  return parseInt(tabIndex, 10)
}

export function focusable(element: HTMLElement) {
  const nodeName = element.nodeName.toLowerCase()
  const isTabIndexNotNaN = !Number.isNaN(getElementTabIndex(element))

  const results =
    (TABBABLE_NODES.test(nodeName) && !(element as any).disabled) ||
    (element instanceof HTMLAnchorElement
      ? element.href || isTabIndexNotNaN
      : isTabIndexNotNaN)

  return results && visible(element)
}

export function tabbable(element: HTMLElement) {
  const tabIndex = getElementTabIndex(element)
  const isTabIndexNaN = Number.isNaN(tabIndex)
  return (isTabIndexNaN || tabIndex >= 0) && focusable(element)
}

export function findTabbableDescendants(element: HTMLElement): HTMLElement[] {
  return Array.from(
    element.querySelectorAll<HTMLElement>(FOCUS_SELECTOR)
  ).filter(tabbable)
}