const ValidationUtils = {
    isWeekend(date) {
        if (!date) return false;
        const day = date.getDay();
        return day === 0 || day === 6;
    },

    isSat(date) {
        return date && date.getDay() === 6;
    },

    isSun(date) {
        return date && date.getDay() === 0;
    },

    // Check for conflicting shift combinations
    hasConflict(shiftCodes) {
        if (!shiftCodes || !Array.isArray(shiftCodes)) return false;

        // Rule: Morning ('m') and Afternoon/Hotline ('a') cannot overlap
        if (shiftCodes.includes('m') && shiftCodes.includes('a')) {
            return "Cannot work Morning and Afternoon simultaneously.";
        }

        // Rule: Vacation ('v') cannot be combined with working shifts
        if (shiftCodes.includes('v') && shiftCodes.length > 1) {
            return "Vacation must be the only entry.";
        }

        return false; // No conflict
    },

    getConflictMessage(shiftCodes) {
        const result = this.hasConflict(shiftCodes);
        return typeof result === 'string' ? result : '';
    }
};