Added day09 part 2

Author: Gentleman1983

build.gradle

@@ -4,7 +4,7 @@ plugins {
 
 // project meta data
 group 'de.havox_design.aoc2023'
-version '0.8.3'
+version '0.8.4'
 
 // Switch to gradle "all" distribution.
 wrapper {

day09/README.md

@@ -89,3 +89,28 @@ If you find the next value for each history in this example and add them togethe
 
 Analyze your OASIS report and extrapolate the next value for each history. **What is the sum of these extrapolated 
 values**?
+
+# Part Two
+Of course, it would be nice to have **even more history** included in your report. Surely it's safe to just 
+**extrapolate backwards** as well, right?
+
+For each history, repeat the process of finding differences until the sequence of differences is entirely zero. Then, 
+rather than adding a zero to the end and filling in the next values of each previous sequence, you should instead add 
+a zero to the **beginning** of your sequence of zeroes, then fill in new **first** values for each previous sequence.
+
+In particular, here is what the third example history looks like when extrapolating back in time:
+```
+_5_ 10  13  16  21  30  45
+  _5_  3   3   5   9  15
+   _-2_  0   2   4   6
+      _2_  2   2   2
+        _0_  0   0
+```
+Adding the new values on the left side of each sequence from bottom to top eventually reveals the new left-most history 
+value: **`5`**.
+
+Doing this for the remaining example data above results in previous values of **`-3`** for the first history and **`0`** 
+for the second history. Adding all three new values together produces **`2`**.
+
+Analyze your OASIS report again, this time extrapolating the previous value for each history. **What is the sum of 
+these extrapolated values**?

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

@@ -27,16 +27,19 @@ class Day09Test {
         @JvmStatic
         private fun getDataForTestSolvePart1(): Stream<Arguments> =
             Stream.of(
-                Arguments.of("part1sample1.txt", 18L),
-                Arguments.of("part1sample2.txt", 28L),
-                Arguments.of("part1sample3.txt", 68L),
-                Arguments.of("part1sample4.txt", 114L)
+                Arguments.of("sample1.txt", 18L),
+                Arguments.of("sample2.txt", 28L),
+                Arguments.of("sample3.txt", 68L),
+                Arguments.of("sample4.txt", 114L)
             )
 
         @JvmStatic
         private fun getDataForTestSolvePart2(): Stream<Arguments> =
             Stream.of(
-                Arguments.of("part2sample1.txt", 0L)
+                Arguments.of("sample1.txt", 0L),
+                Arguments.of("sample2.txt", 0L),
+                Arguments.of("sample3.txt", 0L),
+                Arguments.of("sample4.txt", 0L)
             )
     }
 }