/**
 * Availability form: keep the lesson-length choices honest.
 *
 * A window is stored as consecutive lesson-length slots, so one shorter than the
 * chosen lesson length holds no slots at all and saves nothing. Picking 5:30–6:00
 * PM while the length select sat on its default of 60 minutes used to do exactly
 * that, silently. The server now rejects it with a message; this narrows the
 * choices first so the mistake is hard to make.
 *
 * This is a convenience only — AvailabilityController and the REST endpoint both
 * validate the same window server-side regardless of what happens here.
 */
(function () {
    'use strict';

    const form = document.getElementById('usc-add-availability');
    if (!form) return;

    const startEl    = document.getElementById('start_dt');
    const endEl      = document.getElementById('end_dt');
    const durationEl = document.getElementById('duration_minutes');
    const warningEl  = document.getElementById('usc-duration-warning');
    const submitEl   = form.querySelector('input[type="submit"], button[type="submit"]');

    if (!startEl || !endEl || !durationEl) return;

    /**
     * Minutes between the two datetime-local inputs, or 0 when the pair is not a
     * usable window yet — empty, unparseable, backwards, or spanning two days
     * (which the server rejects on its own terms, with its own message).
     */
    function windowMinutes() {
        const start = new Date(startEl.value);
        const end   = new Date(endEl.value);

        if (!startEl.value || !endEl.value || isNaN(start) || isNaN(end)) return 0;
        if (end <= start) return 0;
        if (startEl.value.slice(0, 10) !== endEl.value.slice(0, 10)) return 0;

        return Math.round((end - start) / 60000);
    }

    function refresh() {
        const minutes = windowMinutes();
        const options = Array.from(durationEl.options);

        // No usable window yet: leave every choice alone rather than fighting
        // someone part-way through typing a date.
        if (minutes === 0) {
            options.forEach((option) => {
                option.hidden   = false;
                option.disabled = false;
            });
            setBlocked(false);
            return;
        }

        let fits = [];

        options.forEach((option) => {
            const tooLong = Number(option.value) > minutes;

            option.hidden   = tooLong;
            option.disabled = tooLong;

            if (!tooLong) fits.push(option);
        });

        if (fits.length === 0) {
            // Nothing bookable fits, so the form cannot produce a single slot.
            setBlocked(true);
            return;
        }

        setBlocked(false);

        // The selection may have just been hidden — fall back to the longest
        // length that still fits, which is what the instructor most likely wants.
        if (durationEl.selectedOptions[0] && durationEl.selectedOptions[0].disabled) {
            durationEl.value = fits.reduce(
                (longest, option) => (Number(option.value) > Number(longest.value) ? option : longest),
                fits[0]
            ).value;
        }
    }

    function setBlocked(blocked) {
        if (warningEl) warningEl.hidden = !blocked;
        if (submitEl) submitEl.disabled = blocked;
    }

    startEl.addEventListener('change', refresh);
    startEl.addEventListener('input', refresh);
    endEl.addEventListener('change', refresh);
    endEl.addEventListener('input', refresh);

    refresh();
}());