import { useEffect, useRef, useState } from 'react';
import type { Patient } from 'fhir/r4';
import { usePatientServiceContext } from '../../../services/PatientService';
import { useOptionalActiveEncounterContext } from './ActiveEncounterContext';
import { openVisitPatientId } from './openVisitPatientId';

/**
 * The open visit's patient, read from the app-level active-encounter slot so leaving the overview
 * does not lose it. Undefined whenever no visit is open.
 */
export const useOpenVisitPatient = (): Patient | undefined => {
    const active = useOptionalActiveEncounterContext()?.active;
    const patientId = openVisitPatientId(active?.encounter);
    const { fetchPatient, patientState } = usePatientServiceContext();
    const [patient, setPatient] = useState<Patient | undefined>();
    const token = useRef(0);
    const known = patientState?.id === patientId ? patientState : undefined;

    useEffect(() => {
        const mine = ++token.current;
        if (!patientId || known) { setPatient(undefined); return; }
        void fetchPatient(patientId)
            .then((record) => { if (mine === token.current) setPatient(record); })
            .catch(() => { if (mine === token.current) setPatient(undefined); });
    }, [patientId, known, fetchPatient]);

    // Identity gate at render: a record for any other id is never returned.
    if (!patientId) return undefined;
    if (known) return known;
    return patient?.id === patientId ? patient : undefined;
};