From f9f5540725a589862d5e00a431c4fd66e3452f33 Mon Sep 17 00:00:00 2001
From: Simmo Saan <simmo.saan@gmail.com>
Date: Sat, 9 Dec 2023 09:46:40 +0200
Subject: [PATCH] Solve 2023 day 9 part 2

---
 .../scala/eu/sim642/adventofcode2023/Day9.scala    | 14 ++++++++++++++
 .../eu/sim642/adventofcode2023/Day9Test.scala      | 12 ++++++++++++
 2 files changed, 26 insertions(+)

diff --git a/src/main/scala/eu/sim642/adventofcode2023/Day9.scala b/src/main/scala/eu/sim642/adventofcode2023/Day9.scala
index 41049147..9df3e07a 100644
--- a/src/main/scala/eu/sim642/adventofcode2023/Day9.scala
+++ b/src/main/scala/eu/sim642/adventofcode2023/Day9.scala
@@ -16,6 +16,19 @@ object Day9 {
 
   def sumNextValues(histories: Seq[History]): Int = histories.map(nextValue).sum
 
+  def prevValue(history: History): Int = {
+    // TODO: deduplicate
+    val differences = LazyList.iterate(history)(history =>
+      (history lazyZip history.tail).map(-_ + _)
+    )
+    val nonZeroDifferences = differences.takeWhile(!_.forall(_ == 0))
+    nonZeroDifferences
+      .map(_.head)
+      .foldRight(0)((a, b) => a - b)
+  }
+
+  def sumPrevValues(histories: Seq[History]): Int = histories.map(prevValue).sum
+
 
   def parseHistories(input: String): Seq[History] =
     input.linesIterator.map(_.split(' ').map(_.toInt).toSeq).toSeq
@@ -24,5 +37,6 @@ object Day9 {
 
   def main(args: Array[String]): Unit = {
     println(sumNextValues(parseHistories(input)))
+    println(sumPrevValues(parseHistories(input)))
   }
 }
diff --git a/src/test/scala/eu/sim642/adventofcode2023/Day9Test.scala b/src/test/scala/eu/sim642/adventofcode2023/Day9Test.scala
index b663f76b..d9d8ee65 100644
--- a/src/test/scala/eu/sim642/adventofcode2023/Day9Test.scala
+++ b/src/test/scala/eu/sim642/adventofcode2023/Day9Test.scala
@@ -23,4 +23,16 @@ class Day9Test extends AnyFunSuite {
   test("Part 1 input answer") {
     assert(sumNextValues(parseHistories(input)) == 1708206096)
   }
+
+  test("Part 2 examples") {
+    val histories = parseHistories(exampleInput)
+
+    assert(prevValue(histories(0)) == -3)
+    assert(prevValue(histories(1)) == 0)
+    assert(prevValue(histories(2)) == 5)
+  }
+
+  test("Part 2 input answer") {
+    assert(sumPrevValues(parseHistories(input)) == 1050)
+  }
 }