Darts approach (#582)

* Create config.json

* Create introduction.md

* Create snippet.txt

* Create content.md

Author: bobahop

exercises/practice/darts/.approaches/config.json

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+{
+  "introduction": {
+    "authors": ["bobahop"],
+    "contributors": []
+  },
+  "approaches": [
+    {
+      "uuid": "40396c67-efdd-4031-bbfd-12e92e7448c8",
+      "slug": "hypot-for-radius",
+      "title": "hypot for radius",
+      "blurb": "Use the hypot function to get the radius of the throw.",
+      "authors": ["bobahop"]
+    }
+  ]
+}

exercises/practice/darts/.approaches/hypot-for-radius/content.md

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+# `hypot` for radius
+
+```kotlin
+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][object] is used to define `Darts` as essentially a [singleton][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][visibility] [`const`][const] [val][variables]s for the rings.
+The `const` values are given meaningful names instead of using the float literals as [magic numbers][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`][hypot] function is used to calculate the [radius][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`.
+
+[object]: https://kotlinlang.org/docs/object-declarations.html#object-declarations-overview
+[singleton]: https://en.wikipedia.org/wiki/Singleton_pattern
+[visibility]: https://kotlinlang.org/docs/visibility-modifiers.html
+[const]: https://www.geeksforgeeks.org/whats-the-difference-between-const-and-val-in-kotlin/
+[variables]: https://kotlinlang.org/docs/basic-syntax.html#variables
+[magic-numbers]: https://en.wikipedia.org/wiki/Magic_number_(programming)
+[hypot]: https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.math/hypot.html
+[radius]: https://www.mathopenref.com/coordbasiccircle.html
+

exercises/practice/darts/.approaches/hypot-for-radius/snippet.txt

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+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

exercises/practice/darts/.approaches/introduction.md

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+# Introduction
+
+There are several ways to solve Darts.
+One approach is to use the [`hypot`][hypot] function to get the [radius][radius] of the throw.
+
+## Approach: `hypot` for radius
+
+```kotlin
+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
+    }
+}
+```
+
+For more information, check the [`hypot` for radius approach][approach-hypot-for-radius].
+
+[approach-hypot-for-radius]: https://exercism.org/tracks/kotlin/exercises/darts/approaches/hypot-for-radius
+[hypot]: https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.math/hypot.html
+[radius]: https://www.mathopenref.com/coordbasiccircle.html