using ErsatzTV.Core;

namespace ErsatzTV.Application.Scheduling;

/// <summary>
///     Validates the three recurrence sets shared by <c>ProgramScheduleAlternate</c> and
///     <c>PlayoutTemplate</c> (ersatztv#880). One validator called from BOTH replace handlers, mirroring
///     <c>FFmpegProfileBounds</c> — the exemplar for `api.ffmpeg-profile-numeric-bounds`, whose shape this
///     follows deliberately.
/// </summary>
/// <remarks>
///     <para>
///         An EMPTY set is rejected because the three are read CONJUNCTIVELY by
///         <c>AlternateScheduleSelector.GetScheduleForDate</c> — a miss on any one continues to the next
///         item — so an empty one matches NO date and stores an item that can never apply. Rejecting
///         rather than substituting is the point: accept-then-rewrite would make an explicit `[]`
///         indistinguishable from an omitted field, which is the very collapse this issue removed.
///     </para>
///     <para>
///         An UNCHANGED empty set that the row ALREADY holds is let through. Both PUT paths are
///         whole-list replaces, so a hard rejection would make every OTHER item in the playout
///         uneditable over a row the operator never touched — the same reason
///         `api.ffmpeg-profile-numeric-bounds` rejects only a NEWLY submitted out-of-range value. A row
///         whose stored set is NULL is NOT exempt: null means unrestricted, so submitting `[]` for it is
///         a new emptying, not an unchanged legacy value.
///     </para>
///     <para>
///         This runs on the COMMAND, after the request records have normalized an ABSENT array to the
///         All*() sets, so an empty set reaching here is one a caller sent EXPLICITLY. That also means a
///         direct (non-HTTP) caller is held to the same rule rather than being able to write a dead row.
///     </para>
/// </remarks>
public static class RecurrenceSetBounds
{
    public static Option<BaseError> Validate(
        ICollection<DayOfWeek> daysOfWeek,
        ICollection<int> daysOfMonth,
        ICollection<int> monthsOfYear,
        ICollection<DayOfWeek> storedDaysOfWeek,
        ICollection<int> storedDaysOfMonth,
        ICollection<int> storedMonthsOfYear)
    {
        if (IsNewlyEmpty(daysOfWeek, storedDaysOfWeek))
        {
            return Some(BaseError.New(Message("DaysOfWeek", "no day of the week")));
        }

        if (IsNewlyEmpty(daysOfMonth, storedDaysOfMonth))
        {
            return Some(BaseError.New(Message("DaysOfMonth", "no day of the month")));
        }

        if (IsNewlyEmpty(monthsOfYear, storedMonthsOfYear))
        {
            return Some(BaseError.New(Message("MonthsOfYear", "no month")));
        }

        return Option<BaseError>.None;
    }

    // "send null" rather than "omit the property": all three are listed in the schema's `required` array
    // in v1.json (they are nullable, not optional), so a client generated from the published contract
    // cannot omit them. Omitting also works at runtime -- Newtonsoft maps a missing property and an
    // explicit null to the same thing -- but naming only that would tell a conforming client to send
    // something its own schema forbids.
    private static string Message(string field, string consequence) =>
        $"[{field}] must not be empty; an empty set matches {consequence}, so the item would never apply. " +
        "Send null to leave it unrestricted";

    // A new item (no stored row) has `stored` null, so an empty set is newly empty and is rejected.
    // Only a stored set that is ITSELF already empty exempts an empty submission.
    private static bool IsNewlyEmpty<T>(ICollection<T> submitted, ICollection<T> stored) =>
        submitted is { Count: 0 } && stored is not { Count: 0 };
}