import type { Observation } from 'fhir/r4';

const INTERPRETATION_SYSTEM = 'http://terminology.hl7.org/CodeSystem/v3-ObservationInterpretation';

export const INTERPRETATIONS: { code: string; display: string }[] = [
    { code: 'N', display: 'Normal' },
    { code: 'L', display: 'Low' },
    { code: 'H', display: 'High' },
    { code: 'LL', display: 'Critically low' },
    { code: 'HH', display: 'Critically high' },
    { code: 'A', display: 'Abnormal' },
];

export const interpretationLabel = (observation: Observation): string => {
    const first = observation.interpretation?.[0];
    return first?.text ?? first?.coding?.[0]?.display ?? first?.coding?.[0]?.code ?? '';
};

export const valueChanges = (observation: Observation, typed: string): boolean => {
    const value = String(typed ?? '').trim();
    if (!value) return false;
    if (observation.valueQuantity) {
        const number = Number(value);
        return Number.isFinite(number) && number !== observation.valueQuantity.value;
    }
    return value !== (observation.valueString ?? '');
};

// A correction that changes a report's number has to change the Observation that holds it, and the old
// reading of that number ("in your usual range") must be restated, never carried onto the new value.
// `interpretation` is a code, '' for none, or undefined when it has not been restated yet.
export const correctedMeasurement = (observation: Observation, typed: string, interpretation: string | undefined): Observation | undefined => {
    if (!valueChanges(observation, typed) || interpretation === undefined) return undefined;
    const value = String(typed).trim();
    const { interpretation: _stale, ...rest } = observation;
    const reading = INTERPRETATIONS.find((i) => i.code === interpretation);
    return {
        ...rest,
        status: 'corrected',
        ...(observation.valueQuantity
            ? { valueQuantity: { ...observation.valueQuantity, value: Number(value) } }
            : { valueString: value }),
        ...(reading && { interpretation: [{ coding: [{ system: INTERPRETATION_SYSTEM, code: reading.code, display: reading.display }] }] }),
    };
};