Kotlin Example of Custom Iterable

/**
* Example of making custom class iterable
*/
data class MyDate(val year: Int, val month: Int, val dayOfMonth: Int) : Comparable<MyDate> {
    override fun compareTo(other: MyDate): Int {
        if (year != other.year) return year - other.year
        if (month != other.month) return month - other.month
        return dayOfMonth - other.dayOfMonth
    }
}

operator fun MyDate.rangeTo(other: MyDate) = DateRange(this, other)

import java.util.Calendar

/*
 * Returns the following date after the given one.
 * For example, for Dec 31, 2019 the date Jan 1, 2020 is returned.
 */
fun MyDate.followingDate(): MyDate {
    val c = Calendar.getInstance()
    c.set(year, month, dayOfMonth)
    val millisecondsInADay = 24 * 60 * 60 * 1000L
    val timeInMillis = c.timeInMillis + millisecondsInADay
    val result = Calendar.getInstance()
    result.timeInMillis = timeInMillis
    return MyDate(result.get(Calendar.YEAR), result.get(Calendar.MONTH), result.get(Calendar.DATE))
}

import java.util.*
import java.util.function.Consumer

class DateRange(val start: MyDate, val end: MyDate): Iterable<MyDate> {
    override fun forEach(action: Consumer<in MyDate>?) {
        super.forEach(action)
    }

    override fun iterator(): Iterator<MyDate> {
        return object : Iterator<MyDate> {
            var current: MyDate = start

            override fun next(): MyDate {
                if (!hasNext()) throw NoSuchElementException()
                val result = current
                current = current.followingDate()
                return result
            }

            override fun hasNext(): Boolean = current <= end
        }
    }

    override fun spliterator(): Spliterator<MyDate> {
        return super.spliterator()
    }
}

fun iterateOverDateRange(firstDate: MyDate, secondDate: MyDate, handler: (MyDate) -> Unit) {
    for (date in firstDate..secondDate) {
        handler(date)
    }
}

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