import type { Encounter, Reference } from 'fhir/r4';

/**
 * Add a diagnosis to a visit, or say there is nothing to add.
 *
 * WHY THIS IS A FUNCTION AND NOT FOUR LINES INSIDE THE SERVICE. The service call around it is a
 * read-modify-write against a live Encounter, so exercising it needs the whole provider stack and a
 * mocked transport. The decision it makes - what the encounter should look like afterwards - is pure,
 * and it is the half that can be wrong in a way nobody sees.
 *
 * IT PRESERVES EVERY OTHER ENTRY, and that is the point rather than a nicety. `Encounter.diagnosis` is
 * an array of everything found in the visit; a second diagnosis that replaced the array instead of
 * appending would delete the first, and the screen would look right because the newest one is the one
 * being read back.
 *
 * NULL MEANS "ALREADY THERE", NOT "FAILED". Returning the encounter unchanged would send a PUT that
 * writes nothing, and a write that does nothing still makes a new version of the resource and still
 * costs the history a row that says a change happened when none did.
 */
export const encounterWithDiagnosis = (
    encounter: Encounter,
    conditionRef: Reference,
): Encounter | null => {
    const target = conditionRef.reference;
    if (!target) return null;

    const existing = encounter.diagnosis ?? [];
    if (existing.some((entry) => entry.condition?.reference === target)) return null;

    return { ...encounter, diagnosis: [...existing, { condition: conditionRef }] };
};