/**
 * WHEN DOES A WEEKLY BLOCK ACTUALLY START AND END ON A GIVEN DAY?
 *
 * Extracted from the schedules calendar so the answer can be ASSERTED rather than trusted. It was three
 * lines inline, and the one that mattered was wrong in a way no test could reach.
 *
 * WHAT WENT WRONG (, measured on staging)
 * Toggling Wednesday on defaulted it to 09:00 open and 09:00 close. The calendar treated `end <= start`
 * as a block running past midnight, so it drew **Wednesday 9:00am-12:00am plus a Thursday 12:00am-9:00am
 * that nobody had turned on** - while the dialog that saved it still summarised "Wednesday only".
 * Three surfaces, three answers, and the one the person was looking at was the one that hid it.
 */

import { addCalendarDays, zonedWallClockToInstant } from '../../../utils/practiceTimezone';

const addDays = (value: Date, days: number): Date => {
    const date = new Date(value);
    date.setDate(date.getDate() + days);
    return date;
};

const setTimeOnDate = (date: Date, time?: string): Date => {
    const next = new Date(date);
    const [hours = 0, minutes = 0, seconds = 0] = (time || '00:00:00').split(':').map(Number);
    next.setHours(hours, minutes, seconds, 0);
    return next;
};

export interface DayWindow {
    start: Date;
    end: Date;
    /** True when the block genuinely runs past midnight into the following day. */
    crossesMidnight: boolean;
}

/**
 * The concrete window a weekly `availableTime` entry occupies on one calendar day.
 *
 * AN END BEFORE THE START IS AN OVERNIGHT BLOCK, deliberately - 22:00 to 06:00 is a real schedule
 * shape and the booking side handles it on purpose (urgent care, sleep studies). That is why the wrap
 * exists at all, and it stays.
 *
 * AN END EQUAL TO THE START IS NOT OVERNIGHT. It is a day with no hours in it, and reading it as a
 * 24-hour block is what manufactured a phantom day. It returns a zero-length window, which draws as
 * nothing. **Drawing nothing for a day nobody gave hours to is the honest answer; drawing a full day is
 * the harmful one.**
 *
 * Do not "improve" this by defaulting a zero-length pair to the hours used elsewhere in the period.
 * A sensible-looking default would hide the degenerate entry rather than surface it, and the entry is
 * refused at save time now precisely so it never reaches here.
 */
export const resolveDayWindow = (day: Date, startTime?: string, endTime?: string): DayWindow => {
    const start = setTimeOnDate(day, startTime);
    const end = setTimeOnDate(day, endTime);

    const crossesMidnight = end < start;

    return {
        start,
        end: crossesMidnight ? addDays(end, 1) : end,
        crossesMidnight,
    };
};

/**
 * The same window on the SITE's clock: `dateStr` is a calendar date at the site and both times are
 * wall clock there. The rules are the ones above - an end before the start runs into the next day, an
 * end equal to the start is a day with no hours - applied to instants built in the site's zone.
 */
export const resolveZonedDayWindow = (
    dateStr: string,
    startTime: string,
    endTime: string,
    timeZone: string,
): DayWindow => {
    const start = zonedWallClockToInstant(dateStr, startTime.slice(0, 5), timeZone);
    const sameDayEnd = zonedWallClockToInstant(dateStr, endTime.slice(0, 5), timeZone);
    const crossesMidnight = sameDayEnd < start;

    return {
        start,
        end: crossesMidnight
            ? zonedWallClockToInstant(addCalendarDays(dateStr, 1), endTime.slice(0, 5), timeZone)
            : sameDayEnd,
        crossesMidnight,
    };
};