day(06): add solution

Signed-off-by: Matej Focko <mfocko@redhat.com>

src/Utils.kt

@@ -2,12 +2,18 @@ import java.io.File
 import java.math.BigInteger
 import java.security.MessageDigest
 
+private fun openFile(day: Int, name: String) = File("src/day%02d".format(day), "$name.txt")
+
 /**
  * Reads lines from the given input txt file.
  */
-fun readInput(day: Int, name: String) = File("src/day%02d".format(day), "$name.txt").readLines()
+fun readInput(day: Int, name: String) = openFile(day, name).readLines()
 
 fun readInputAsInts(day: Int, name: String) = readInput(day, name).map { it.toInt() }
+fun readInputAsCommaSeparatedInts(day: Int, name: String) = openFile(day, name)
+    .readText()
+    .split(",")
+    .map { it.toInt() }
 
 /**
  * Converts string to md5 hash.

src/day06/Day06.kt

@@ -0,0 +1,29 @@
+package day06
+
+import readInputAsCommaSeparatedInts
+
+fun howManyAfter(input: List<Int>, days: Int): Long {
+    var counts = List(9) { i -> input.count { it == i }.toLong() }
+
+    for (day in 1..days) {
+        counts = List(9) { i ->
+            when (i) {
+                6 -> counts[0] + counts[7]
+                else -> counts[(i + 1) % 9]
+            }
+        }
+    }
+
+    return counts.sum()
+}
+
+fun main() {
+    val testInput = readInputAsCommaSeparatedInts(6, "test_input")
+    val input = readInputAsCommaSeparatedInts(6, "input")
+
+    check(howManyAfter(testInput, 80) == 5934.toLong())
+    println(howManyAfter(input, 80))
+    
+    check(howManyAfter(testInput, 256) == 26984457539)
+    println(howManyAfter(input, 256))
+}