list_slicing_extension.kt

fun lsxUsage() {
    val list = listOf(2, 3, 5, 7, 11, 13, 17, 19, 23, 29)

    println(list[1..3])          // [3, 5, 7]
    println(list[1 til 3])       // [3, 5]
    println(list[4..1])          // [11, 7, 5, 3]
    println(list[4 til 1])       // [11, 7, 5]
    println(list[2..-1])         // [5, 7, 11, 13, 17, 19, 23, 29]
    println(list[2 til -1])      // [5, 7, 11, 13, 17, 19, 23]
    println(list[3..0 by 1])     // [7, 5, 3, 2]
    println(list[3 til 0 by 1])  // [7, 5, 3]
    println(list[-1..1 by 2])    // [29, 19, 13, 7, 3]
    println(list[-1 til 1 by 2]) // [29, 19, 13, 7]
}

data class OpenRange(val first: Int, val last: Int) 

// I spitted classes because `begin til end by step by step` is messy
data class Range(val first: Int, val last: Int, val step: Int = 1, val closed: Boolean) {
    init {
        if (step <= 0) throw IllegalArgumentException("Step must be positive, was: $step.")
    }
}

infix fun Int.til(last: Int) = OpenRange(this, last)

// this function is extension because I wanted highlighting
infix fun OpenRange.by(step: Int) = Range(first, last, step, closed = false)

infix fun IntRange.by(step: Int) = Range(first, last, step, closed = true)


operator fun <T> List<T>.get(range: OpenRange): List<T> = this[range by 1]

operator fun <T> List<T>.get(range: IntRange): List<T> = this[range by 1]

operator fun <T> List<T>.get(range: Range): List<T> =
        (range adaptTo size).map { get(it) }


private infix fun Int.adaptTo(size: Int) = this + if(this < 0) size else 0

// this function is not member because I wanted it to be file private
private infix fun Range.adaptTo(size: Int): IntProgression {
    val first = first adaptTo size
    var last  = last adaptTo size

    if (!closed) when {
        first == last -> return 1..0 // empty range
        first < last  -> last -= 1
        first > last  -> last += 1
    }

    return when {
        first <= last -> first..last step step
        else -> first downTo last step step
    }
}