Factor out a filterMaxBy() function on Iterable

It filters the collection by the maximum element according to
the given selector function. This enables writing maximum filters
without breaking the code's flow.
See ReindeerOlympics.kt as an example.

Author: rhaendel

src/main/kotlin/de/ronny_h/aoc/extensions/collections/Lists.kt

@@ -26,3 +26,12 @@ fun List<String>.split(delimiter: String = ""): List<List<String>> {
     }
     return result
 }
+
+/**
+ * @return The [Iterable] of elements that maximize the given [selector].
+ */
+inline fun <T, R : Comparable<R>> Iterable<T>.filterMaxBy(selector: (T) -> R): Iterable<T> {
+    if (none()) return this
+    val max = maxOf(selector)
+    return filter { selector(it) == max }
+}

src/main/kotlin/de/ronny_h/aoc/year2015/day14/ReindeerOlympics.kt

@@ -1,6 +1,7 @@
 package de.ronny_h.aoc.year2015.day14
 
 import de.ronny_h.aoc.AdventOfCode
+import de.ronny_h.aoc.extensions.collections.filterMaxBy
 import kotlin.math.min
 
 fun main() = ReindeerOlympics().run(2660, 1256)
@@ -15,12 +16,10 @@ fun List<Reindeer>.maxReindeerDistanceIn(secondsTotal: Int) = map { it.reindeerD
 fun List<Reindeer>.pointsOfWinnerIn(secondsTotal: Int): Int {
     val reindeerPoints = MutableList(size) { 0 }
     for (seconds in 1..secondsTotal) {
-        val distances = map { it.reindeerDistanceIn(seconds) }.withIndex()
-        val leaderDistance = distances.maxOf { it.value }
-        val leaders = distances.filter { it.value == leaderDistance }
-        leaders.forEach {
-            reindeerPoints[it.index]++
-        }
+        map { it.reindeerDistanceIn(seconds) }
+            .withIndex()
+            .filterMaxBy(IndexedValue<Int>::value)
+            .forEach { reindeerPoints[it.index]++ }
     }
     return reindeerPoints.max()
 }

src/test/kotlin/de/ronny_h/aoc/extensions/collections/ListsTest.kt

@@ -31,4 +31,16 @@ class ListsTest : StringSpec({
     "split a list with multiple blocks ending with the delimiter line" {
         listOf("a", "b", "", "c", "d", "").split("") shouldBe listOf(listOf("a", "b"), listOf("c", "d"))
     }
+
+    "filterMaxBy an empty list returns an empty list" {
+        emptyList<String>().filterMaxBy(String::length) shouldBe emptyList()
+    }
+
+    "filterMaxBy with a unique max returns exactly that" {
+        listOf("1", "123", "12").filterMaxBy(String::length) shouldBe listOf("123")
+    }
+
+    "filterMaxBy with more than one max element returns all max elements" {
+        listOf("1", "123", "321").filterMaxBy(String::length) shouldBe listOf("123", "321")
+    }
 })