import type { Extension, HealthcareService } from 'fhir/r4';

// The form->FHIR transform (`transformToFhirHealthcareService`) maps form FIELDS only; FHIR extensions
// are not form fields, so the transform never emits an `extension` array. Feature data lives there -
// per-service appointment duration, patient-self-bookable default, and price - each written by its own
// editor via an immutable filter-then-append helper. When a service is re-saved through an editor that
// does NOT own those features (the full HealthcareService editor, the simple modal), the transform output
// has no extensions, so without this the existing ones are silently wiped on save.
//
// Carry the source resource's extensions onto a freshly-transformed resource before the save. Merge, don't
// clobber: any extension the transform already set wins by `url` (today it sets none), and the source's
// remaining extensions are appended. Feature helpers (e.g. `applyServicePrice`) still run AFTER this and
// filter-then-append their own extension, so the value the user just edited replaces the carried-over one.
export const preserveHealthcareServiceExtensions = (
    next: HealthcareService,
    source?: HealthcareService | null,
): HealthcareService => {
    const sourceExtensions = source?.extension ?? [];
    if (sourceExtensions.length === 0) {
        return next;
    }

    const nextExtensions = next.extension ?? [];
    const nextUrls = new Set(nextExtensions.map((extension) => extension.url));
    const carried = sourceExtensions.filter((extension) => !nextUrls.has(extension.url));
    const merged: Extension[] = [...nextExtensions, ...carried];

    return merged.length > 0 ? { ...next, extension: merged } : next;
};