import type {Id,Target,Geometry} from '../canvas-core/types.ts';
import type {SessionSnapshot,GestureState} from '../canvas-core/gestures.ts';
import {equal} from '../canvas-core/values.ts';
import type {ExternalSource} from './selectors.ts';

export type GestureSelection={readonly preview:Geometry|undefined;readonly active:boolean};
/** Derived view/target subscriptions. Values retain no document or full per-hook session. */
export function createSessionSources(source:ExternalSource<SessionSnapshot>){
 type Observer={update(snapshot:SessionSnapshot):boolean;notify():void};
 const modes=new Map<Id,Map<Id,Set<Observer>>>(),gestures=new Map<Id,Set<Observer>>();
 const targetIndexes=new WeakMap<readonly Target[],ReadonlyMap<Id,number>>();
 let stop:(()=>void)|undefined,previous:SessionSnapshot|undefined;
 function targets(gesture:GestureState){let index=targetIndexes.get(gesture.targets);if(!index){index=new Map(gesture.targets.map(target=>[target.id,target.incarnation]));targetIndexes.set(gesture.targets,index);}return index;}
 function changed(){
  if(!previous)return;const before=previous,next=source.getSnapshot();previous=next;
  const candidates=new Set<Observer>(),add=(items:Set<Observer>|undefined)=>{for(const item of items??[])candidates.add(item);};
  if(before.views!==next.views)for(const[viewId,byId]of modes){
   const old=before.views.get(viewId)?.modes,current=next.views.get(viewId)?.modes;if(old===current)continue;
   for(const id of new Set([...(old?.keys()??[]),...(current?.keys()??[])]))if(old?.get(id)!==current?.get(id))add(byId.get(id));
  }
  const old=before.gesture,current=next.gesture;
  if(old!==current){
   const ids=new Set<Id>();
   if(!old||!current||old.token!==current.token||old.viewId!==current.viewId||old.targets!==current.targets){for(const gesture of [old,current])for(const target of gesture?.targets??[])ids.add(target.id);}
   else if(old.preview!==current.preview){for(const[id,value]of old.preview)if(current.preview.get(id)!==value)ids.add(id);for(const[id,value]of current.preview)if(old.preview.get(id)!==value)ids.add(id);}
   for(const id of ids)add(gestures.get(id));
  }
  const notifications:Observer[]=[];for(const observer of candidates)if(observer.update(next))notifications.push(observer);
  let failed=false,error:unknown;for(const observer of notifications)try{observer.notify();}catch(caught){if(!failed){failed=true;error=caught;}}if(failed)throw error;
 }
 function channel<T>(read:(snapshot:SessionSnapshot)=>T,register:(observer:Observer)=>()=>void):ExternalSource<T>{
  let initialized=false,value:T;
  return Object.freeze({
   getSnapshot(){const next=read(source.getSnapshot());if(!initialized||!equal(value,next))value=next;initialized=true;return value;},
   subscribe(notify:()=>void){
    let observed=read(source.getSnapshot()),failed=false;
    const observer:Observer={notify,update(snapshot){try{const next=read(snapshot);if(!failed&&equal(observed,next))return false;observed=next;failed=false;}catch{failed=true;}return true;}};
    const remove=register(observer);
    if(!stop){previous=source.getSnapshot();try{stop=source.subscribe(changed);}catch(error){remove();previous=undefined;throw error;}}
    let active=true;return()=>{if(!active)return;active=false;remove();if(!modes.size&&!gestures.size){stop?.();stop=undefined;previous=undefined;}};
   },
  });
 }
 return Object.freeze({
  mode(viewId:Id,id:Id):ExternalSource<string|null>{return channel(snapshot=>snapshot.views.get(viewId)?.modes.get(id)??null,observer=>{
   let byId=modes.get(viewId);if(!byId){byId=new Map();modes.set(viewId,byId);}let bucket=byId.get(id);if(!bucket){bucket=new Set();byId.set(id,bucket);}bucket.add(observer);
   return()=>{bucket!.delete(observer);if(!bucket!.size)byId!.delete(id);if(!byId!.size)modes.delete(viewId);};
  });},
  gesture(viewId:Id,target:Target):ExternalSource<GestureSelection>{const{id,incarnation}=target;return channel(snapshot=>{
   const gesture=snapshot.gesture,matched=!!gesture&&targets(gesture).get(id)===incarnation;
   return{preview:matched?gesture!.preview.get(id):undefined,active:matched&&gesture!.viewId===viewId};
  },observer=>{let bucket=gestures.get(id);if(!bucket){bucket=new Set();gestures.set(id,bucket);}bucket.add(observer);return()=>{bucket!.delete(observer);if(!bucket!.size)gestures.delete(id);};});},
 });
}