2023 - Day09 - part 2 - can probably be calculated directly (think so…

…mething along the lines of Pascal's Triangle) instead of looping through everything (twice for part 2)

src/main/kotlin/no/rodland/advent_2023/Day09.kt

@@ -5,24 +5,37 @@ import no.rodland.advent.Day
 // template generated: 09/12/2023
 // Fredrik Rødland 2023
 
-class Day09(val input: List<String>) : Day<Int, Long, List<List<Int>>> {
+
+// can probably be calculated directly (think something along the lines of Pascal's Triangle)
+// instead of looping through everything (twice for part 2)
+class Day09(val input: List<String>) : Day<Int, Int, List<List<Int>>> {
 
     private val parsed = input.parse()
 
     override fun partOne(): Int {
-        return parsed.sumOf { diffList(it) }
+        return parsed.sumOf { list -> allDiffs(list).sumOf { it.last() } }
     }
 
-    override fun partTwo(): Long {
-        return 2
+    override fun partTwo(): Int {
+        return parsed.sumOf { list ->
+            val append2 = mutableListOf<List<Int>>()
+            allDiffs(list).reversed().map { next ->
+                append2.add(diffFirst(append2.lastOrNull(), next))
+            }
+            append2.last().first()
+        }
     }
 
-    private fun diffList(list: List<Int>): Int {
+    private fun allDiffs(list: List<Int>): List<List<Int>> {
         val append = mutableListOf(list)
         while (append.last().any { it != 0 }) {
             append.add(append.last().diffs())
         }
-        return append.sumOf { it.last() }
+        return append
+    }
+
+    private fun diffFirst(last: List<Int>?, next: List<Int>): List<Int> {
+        return listOf(next.first() - (last?.first() ?: 0)) + next
     }
 
     private fun List<Int>.diffs(): List<Int> = windowed(2) { it.last() - it.first() }

src/test/kotlin/no/rodland/advent_2023/Day09Test.kt

@@ -16,9 +16,9 @@ internal class Day09Test {
     )
 
     private val resultTestOne = 114
-    private val resultTestTwo = 2L
+    private val resultTestTwo = 2
     private val resultOne = 1972648895
-    private val resultTwo = 2L
+    private val resultTwo = 919
 
     val test = defaultTestSuiteParseOnInit(
         Day09(data09),