import {useEffect,useRef} from 'react';

// Only the top modal remains interactive, including when it is portalled to body.
const modalStack=[];
const previousInert=new Map();
let isolationObserver=null;
function isolateTopModal() {
 for(const [element,inert] of previousInert)element.inert=inert;
 previousInert.clear();
 const top=modalStack.at(-1);
 if(!top?.isConnected)return;
 for(let branch=top;branch.parentElement;branch=branch.parentElement){
  const parent=branch.parentElement;
  for(const sibling of parent.children){
   if(sibling===branch)continue;
   previousInert.set(sibling,sibling.inert);
   sibling.inert=true;
  }
  if(parent===document.body)break;
 }
}

function focusableElements(root) {
 return [...root.querySelectorAll('button, a[href], input, select, textarea, summary, [tabindex]')].filter(el=>{
  if(el.tabIndex<0 || el.matches(':disabled') || !el.getClientRects().length || el.closest('[inert]'))return false;
  if(['hidden','collapse'].includes(getComputedStyle(el).visibility))return false;
  // Closed details descendants can have rectangles but cannot receive focus.
  for(let parent=el.parentElement;parent && parent!==root;parent=parent.parentElement){
   if(parent.tagName==='DETAILS' && !parent.open && !parent.querySelector(':scope > summary')?.contains(el))return false;
  }
  return true;
 });
}

export function useModalFocus(open,onClose) {
 const ref=useRef(null),close=useRef(onClose);
 close.current=onClose;
 useEffect(()=>{
  if(!open || !ref.current)return;
  const opener=document.activeElement,root=ref.current;
  modalStack.push(root);
  isolateTopModal();
  if(!isolationObserver){
   isolationObserver=new MutationObserver(isolateTopModal);
   isolationObserver.observe(document.body,{childList:true,subtree:true});
  }
  (focusableElements(root)[0] || root).focus();
  return ()=>{
   const wasTop=modalStack.at(-1)===root;
   const index=modalStack.indexOf(root);
   if(index>=0)modalStack.splice(index,1);
   isolateTopModal();
   if(!modalStack.length){isolationObserver?.disconnect();isolationObserver=null;}
   if(wasTop && opener?.isConnected && !opener.closest('[inert]'))opener.focus();
  };
 },[open]);
 const onKeyDown=e=>{
  const root=ref.current;
  if(e.defaultPrevented || e.target.closest('[role="dialog"]')!==root)return;
  if(e.key==='Escape'){e.preventDefault();e.stopPropagation();close.current();return;}
  if(e.key!=='Tab')return;
  const elements=focusableElements(root),first=elements[0],last=elements.at(-1);
  if(!first){e.preventDefault();root.focus();}
  else if(e.shiftKey && (document.activeElement===first || document.activeElement===root)){e.preventDefault();last.focus();}
  else if(!e.shiftKey && document.activeElement===last){e.preventDefault();first.focus();}
 };
 return {ref,onKeyDown};
}