Remarque :

• Le concept de cache ici pose problème puisque des méthodes qui transforment (à base de verbes) retournent en réalité quelque chose.
• Les méthodes avec des boolean ont des verbes, c'est une exception à la Yegorification du code.

// Usage :
object Main {
    fun regularUseCase() {
        // Given
        val christmas = LocalDate(2019, 12, 25)
        val calendar:HolidaysCalendar = HolidaysCalendarForYear(Year(2019))

        // When
        val daysOff:DateSet = calendar.daysOff()

        // Then
        println(daysOffs.contains(christmas))       // print true
    }

    fun cachedUseCase() {
        // Given
        val christmas = LocalDate(2019, 12, 25)
        val cache:Cache = HolidaysCalendarCache()

        // When
        val daysOff2:HolidaysCalendar = cache.value(Year(2019))
        val daysOff3:HolidaysCalendar = cache.value(Year(2019))

        // Then
        println(daysOff2.contains(christmas))       // print true
        println(daysOff2 === daysOff3)              // print true (same instance)
    }

    @JvmStatic
    fun main() {
        val main = Main()
        main.regularUseCase()
        main.cachedUseCase()
    }
}

// Interfaces
interface DateSet {
    /**
     * Determine whether or not the specified date is in this set.
     *
     * @param date
     *          The date to research.
     *
     * @return true if the date exists in this set, false otherwise.
     */
    fun contains(date:LocalDate):Boolean

    /**
     * Determine whether or not the specified date is in this set.
     *
     * @param date
     *          The date to research.
     *
     * @return true if the date exists in this set, false otherwise.
     */
    fun contains(date:Calendar):Boolean

    /**
     * Return the current set of date as an iterable collection.
     *
     * @return A collection having all the date stored in this set.
     */
    fun asCollection():Collection<LocalDate>
}

interface HolidaysCalendar {
    /**
     * Return a set of holiday dates.
     *
     * @return All holidays for a period (see implementation for more detail).
     */
    fun daysOff():DateSet

    /**
     * The period covered by this calendar.
     *
     * @return Something in the CalenadrPeriod enumeration.
     */
    fun range():CalenadrPeriod
}

class Cache<K, V> {
    /**
     * Determine whether or not the specified value exists in this cache.
     *
     * @param value
     *          The value to search.
     *
     * @return true if the value has been found, false otherwise.
     */
    fun contains(value:V):Boolean

    /**
     * Determine whether or not a key exists in this cache.
     *
     * @param key
     *          The key to search.
     *
     * @return true if the key exists, false otherwise (reminder: a key cannot exists if linked to nothing).
     */
    fun containsKey(key:K):Boolean

    /**
     * Retrieve the value related to the specified key.
     *
     * @param key
     *          The key related to the researched value.
     *
     * @return The value related to the given key.
     *
     * @throw UnexistingEntryException
     *          When the subsystem cached by this object is not able to restitute a value using the specified key.
     */
    fun value(key:K):V

    /**
     * The list of keys used by this cache.
     *
     * @return The list of keys used by this cache.
     */
    fun keys():List<K>

    /**
     * Remove the specified key in order to force an update.
     *
     * @param key
     *          The key of the cache entry to remove.
     */
    fun remove(key:K)

    /**
     * Clear all entry is the current cache.
     */
    fun reset()
}

// Implementation of HolidaysCalendar
class HolidaysCalendarForYear(private val year:Year):HolidaysCalendar
class HolidaysCalendarForMonth(private val year:Year, private val month:Month):HolidaysCalendar

// Implementation of Cache
class HolidaysCalendarCache:Cache<Year, HolidaysCalendar>