Solved day09 part 2

Author: Gentleman1983

build.gradle

@@ -4,7 +4,7 @@ plugins {
 
 // project meta data
 group 'de.havox_design.aoc2023'
-version '0.8.4'
+version '0.8.5'
 
 // Switch to gradle "all" distribution.
 wrapper {

day09/src/main/kotlin/de/havox_design/aoc2023/day09/Day09.kt

@@ -4,19 +4,27 @@ class Day09(private var filename: String) {
     fun solvePart1(): Long =
         convertInput()
             .sumOf { history ->
-            processOasisReport(history)
-        }
+                processOasisReport(history, false)
+            }
 
     fun solvePart2(): Long =
-        0L
+        convertInput()
+            .sumOf { history ->
+                processOasisReport(history, true)
+            }
 
-    private fun processOasisReport(oasisHistory: List<Long>): Long {
+    private fun processOasisReport(oasisHistory: List<Long>, calculateLeftHistory: Boolean): Long {
         var value = 0L
         var factor = 1
         var current = oasisHistory
 
         while (current.any { it != 0L }) {
-            value += current.last()
+            if (calculateLeftHistory) {
+                value += current.first() * factor
+                factor *= -1
+            } else {
+                value += current.last()
+            }
 
             current = current.windowed(2).map { (a, b) -> b - a }
         }

day09/src/test/kotlin/de/havox_design/aoc2023/day09/Day09Test.kt

@@ -36,10 +36,10 @@ class Day09Test {
         @JvmStatic
         private fun getDataForTestSolvePart2(): Stream<Arguments> =
             Stream.of(
-                Arguments.of("sample1.txt", 0L),
+                Arguments.of("sample1.txt", -3L),
                 Arguments.of("sample2.txt", 0L),
-                Arguments.of("sample3.txt", 0L),
-                Arguments.of("sample4.txt", 0L)
+                Arguments.of("sample3.txt", 5L),
+                Arguments.of("sample4.txt", 2L)
             )
     }
 }