Kotlin provides built-in support for ranges of primitive types. In the previous recipes, we worked with the IntRange and CharRange types, which are included in the Kotlin standard library. However, it is possible to implement a custom progression for any type by implementing the Comparable interface. In this recipe, we will learn how to create a progression of the LocalDate type and discover how to traverse the dates the easy way.

In order to accomplish the task, we need to start by getting familiar with the ClosedRange and Iterator interfaces. We need to use them to declare a custom progression for the LocalDate class:

public interface ClosedRange<T: Comparable<T>> {
    public val start: T
    public val endInclusive: T
    public operator fun contains(value: T): Boolean {
        return value >= start && value <= endInclusive
    }                                      
    public fun isEmpty(): Boolean = start > endInclusive
}

The Iterator interface provides information about the subsequent values and their availability:

public interface Iterator<out T> {
    public operator fun next(): T
    public operator fun hasNext(): Boolean
}

The ClosedRange interface provides the minimum and maximum values of the range. It also provides the contains(value: T): Boolean and isEmpty(): Boolean functions, which check whether a given value belongs to the range and whether the range is empty respectively. Those two functions have default implementations provided in the ClosedRange interface. As the result, we don't need to override them in our custom implementation of the ClosedRange interface.

1. Let's start with implementing the Iterator interface for the LocalDate type. We are going to create a custom LocalDateIterator class, which implements the Iterator<LocalDate> interface:

class DateIterator(startDate: LocalDate,
                   val endDateInclusive: LocalDate,
                   val stepDays: Long) : Iterator<LocalDate> {
    private var currentDate = startDate
    override fun hasNext() = currentDate <= endDateInclusive
    override fun next(): LocalDate {
        val next = currentDate
        currentDate = currentDate.plusDays(stepDays)
        return next
    }
}

2. Now, we can implement the progression for the LocalDate type. Let's create a new class called DateProgression, which is going to implement the Iterable<LocalDate> and ClosedRange<LocalDate> interfaces:

class DateProgression(override val start: LocalDate,
                      override val endInclusive: LocalDate,
                      val stepDays: Long = 1) : 
                                                Iterable<LocalDate>, 
                                                ClosedRange<LocalDate> {
    override fun iterator(): Iterator<LocalDate> {
        return DateIterator(start, endInclusive, stepDays)
    } 

    infix fun step(days: Long) = DateProgression(start, endInclusive, days)
}

3. Finally, declare a custom rangeTo operator for the LocalDate class:

operator fun LocalDate.rangeTo(other: LocalDate) = DateProgression(this, other)

val startDate = LocalDate.of(2020, 1, 1)
val endDate = LocalDate.of(2020, 12, 31)
for (date in startDate..endDate step 7) {
    println("${date.dayOfWeek} $date ")
}

WEDNESDAY 2020-01-01
WEDNESDAY 2020-01-08
WEDNESDAY 2020-01-15
WEDNESDAY 2020-01-22
WEDNESDAY 2020-01-29
WEDNESDAY 2020-02-05
...
WEDNESDAY 2020-12-16
WEDNESDAY 2020-12-23
WEDNESDAY 2020-12-30

The DateIterator class holds three properties—currentDate: LocalDate, endDateInclusive: LocalDate, and stepDays: Long. In the beginning, the currentDate property is initialized with the startDate value passed in the constructor. Inside the next() function, we are returning the currentDate value and updating it to the next date value using a given stepDays property interval.

The DateProgression class combines the functionalities of the Iterable<LocalDate> and ClosedRange<LocalDate> interfaces. It provides the Iterator object required by the Iterable interface by returning the DateIterator instance. It also overrides the start and endInclusive properties of the ClosedRange interface. There is also the stepDays property with a default value equal to 1. Note that every time the step function is called, a new instance of the DateProgression class is being created.

You can follow the same pattern to implement custom progressions for any class that implements the Comparable interface.