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

/**
 * The line that says which visit an item is about to be filed to.
 *
 * Drawn in `mockups/practitioner/encounter.html` directly under the modal's heading, in two forms:
 *
 *     Filed to this visit - today, 4:05 PM
 *     Filed to Tuesday's visit - September 2, finished 4:12 PM
 *
 * IT IS THERE FOR THE EVENING, WHICH IS THE HARDER HALF OF THIS WHOLE FLOW. The deck: *"a practitioner
 * writing up at six o'clock is still documenting the four o'clock visit, and the line at the top says
 * which one."* Filing to the right encounter after the visit has ended is one of the named requirements
 * of this surface, and without this line nothing on screen tells the clinician which visit they are in -
 * so somebody finishing three notes in a row has no way to notice they are in the wrong one.
 *
 * IT NEVER GUESSES. A visit with no start time is described without one rather than being given a
 * plausible hour, and no encounter at all is said plainly instead of the line claiming a visit that is
 * not there. A wrong confident answer here misfiles clinical data.
 *
 * DATES ARE WRITTEN THE WAY THE REST OF THE PRODUCT WRITES THEM. The deck renders `2 September`; every
 * other surface here reads `Aug 16, 2026`, and these are US practices. A drawing's date order is not a
 * ruling about locale, so the product's own convention wins.
 */

const time = (value: Date): string =>
    value.toLocaleTimeString('en-US', { hour: 'numeric', minute: '2-digit' });

const day = (value: Date): string =>
    value.toLocaleDateString('en-US', { month: 'long', day: 'numeric' });

const weekday = (value: Date): string =>
    value.toLocaleDateString('en-US', { weekday: 'long' });

const isSameDay = (a: Date, b: Date): boolean =>
    a.getFullYear() === b.getFullYear() && a.getMonth() === b.getMonth() && a.getDate() === b.getDate();

const parse = (value?: string): Date | undefined => {
    if (!value) return undefined;
    const parsed = new Date(value);
    return Number.isNaN(parsed.getTime()) ? undefined : parsed;
};

/** True once the visit is over, by either of the two things that say so. */
const hasEnded = (encounter: Encounter): boolean =>
    encounter.status === 'finished' || Boolean(encounter.period?.end);

export const describeFilingTarget = (encounter: Encounter | undefined, now: Date): string => {
    // NO VISIT IS NOT "THIS VISIT". An item can still be filed - it simply is not attached to an
    // encounter - and saying so is the difference between a record entry and a misfiled one.
    if (!encounter) return 'Filed to the patient’s record';

    const started = parse(encounter.period?.start);
    const ended = parse(encounter.period?.end);
    const anchor = started ?? ended;

    if (!anchor) return 'Filed to this visit';

    const today = isSameDay(anchor, now);

    // "this visit" only while it IS this one. On any other day the weekday names it, and the date
    // beside it removes the ambiguity a weekday alone would carry a fortnight later.
    const which = today ? 'this visit' : `${weekday(anchor)}’s visit`;
    const when = today ? 'today' : day(anchor);

    const at = hasEnded(encounter) && ended
        ? `finished ${time(ended)}`
        : started
            ? time(started)
            : undefined;

    return at ? `Filed to ${which} · ${when}, ${at}` : `Filed to ${which} · ${when}`;
};

/** Where items added from a note's suggestion list are filed; the save row states the note itself. */
export const describeWhereAddedItemsGo = (encounter: Encounter | undefined, now: Date): string =>
    `Anything you add here is filed to ${describeFilingTarget(encounter, now).replace(/^Filed to /, '')}`;

export default describeFilingTarget;