class PythonBackend {
  constructor(onStatus){
    this.pending=new Map();this.sequence=0;
    this.ready=new Promise((resolve,reject)=>{this.resolveReady=resolve;this.rejectReady=reject;});
    try{
      this.worker=new Worker('./pyodide-worker.js?v=multiclass-1', {type:'module'});
      this.worker.onmessage=({data})=>{
        if(data.type==='progress'){onStatus(data.message);return;}
        if(data.type==='ready'){this.resolveReady(data.classifiers);return;}
        if(data.type==='fatal'){this.fail(data.message);return;}
        const pending=this.pending.get(data.id);
        if(pending){this.pending.delete(data.id);data.error?pending.reject(new Error(data.error)):pending.resolve(data.model);}
      };
      this.worker.onerror=event=>this.fail(event.message||'Python worker could not start.');
    }catch(error){this.fail(error.message);}
  }
  fail(message){
    const error=new Error(message);this.failure=error;this.rejectReady(error);
    for(const p of this.pending.values()) p.reject(error);
    this.pending.clear();this.worker?.terminate();
  }
  async add(request){
    await this.ready;
    if(this.failure)throw this.failure;
    return new Promise((resolve,reject)=>{
      const id=++this.sequence;this.pending.set(id,{resolve,reject});
      this.worker.postMessage({id,request});
    });
  }
}