Add solution to day 9

src/Day09.kt

@@ -0,0 +1,64 @@
+fun main() {
+    fun part1(input: List<String>): Int {
+        return input.toHistories()
+            .sequences()
+            .extrapolate()
+            .sumOf { it.last().last() }
+            .also { it.println() }
+    }
+
+    fun part2(input: List<String>): Int {
+        return input.toHistories()
+            .sequences()
+            .extrapolatePrevious()
+            .sumOf { it.last().first() }
+            .also { it.println() }
+    }
+
+    // test if implementation meets criteria from the description, like:
+    val testInput = readInput("Day09_test")
+    check(part1(testInput) == 114)
+
+    val input = readInput("Day09")
+    part1(input).println()
+    check(part2(testInput) == 2)
+    part2(input).println()
+}
+
+fun List<String>.toHistories(): List<List<Int>> = map { line -> line.split(' ').map { it.toInt() } }
+
+fun List<List<Int>>.sequences() = map { history ->
+    val differences = mutableListOf(history.toMutableList())
+    var nextSequence = history.toMutableList()
+    while (nextSequence.any { it != 0 }) {
+        nextSequence = nextSequence.zipWithNext { a, b -> b - a }.toMutableList()
+        differences.add(nextSequence)
+    }
+    differences
+}
+
+fun List<MutableList<MutableList<Int>>>.extrapolate() = map { history ->
+    val bottomUpHistory = history.asReversed()
+    bottomUpHistory.mapIndexed { index, sequence ->
+        if (index == 0) {
+            sequence.add(0)
+        } else {
+            val other = bottomUpHistory[index - 1].last()
+            sequence.add(sequence.last() + other)
+        }
+        sequence
+    }
+}
+
+fun List<MutableList<MutableList<Int>>>.extrapolatePrevious() = map { history ->
+    val bottomUpHistory = history.asReversed()
+    bottomUpHistory.mapIndexed { index, sequence ->
+        if (index == 0) {
+            sequence.add(0, 0)
+        } else {
+            val other = bottomUpHistory[index - 1].first()
+            sequence.add(0, sequence.first() - other)
+        }
+        sequence
+    }
+}