hypot for radius

import kotlin.math.hypot

object Darts {
    private const val innerRing = 1.0
    private const val middleRing = 5.0
    private const val outerRing = 10.0

    fun score(x: Number, y: Number): Int {
        val toss = hypot(x.toDouble(), y.toDouble())
        fun throwWithin(ring: Double) = toss <= ring

        if (throwWithin(innerRing)) return 10
        if (throwWithin(middleRing)) return 5
        if (throwWithin(outerRing)) return 1
        return 0
    }
}

An object declaration is used to define Darts as essentially a singleton object instantiation of the class. This is sufficient, since there is no object state that needs to change with each call of the score method.

The object defines private const vals for the rings. The const values are given meaningful names instead of using the float literals as magic numbers. A val is immutable, as is a const. A const val means that the value of the val is known at compile time.

The hypot function is used to calculate the radius of the dart throw from the x and y coordinates.

The throwWithin function returns if the radius is within the ring passed in.

Due to the naming of the function and varables, the if statements read much like natural language.

The throwWithin function is passed the ring. If it returns true, then the function returns with the score for throwing within that ring.

If the throw is not within a defined ring, then the function returns 0.