From 57bd3fd46448fc461a2e977c4f08f1341005ee6e Mon Sep 17 00:00:00 2001
From: Lucas Resch <lucas.resch@gmx.de>
Date: Sat, 9 Dec 2023 14:45:23 +0100
Subject: [PATCH] 2023-09: Solve second puzzle

---
 2023-rust/README.md     | 18 +++++++++---------
 2023-rust/src/bin/09.rs | 39 ++++++++++++++++++++++++++++++---------
 2 files changed, 39 insertions(+), 18 deletions(-)

diff --git a/2023-rust/README.md b/2023-rust/README.md
index af50556..a457c19 100644
--- a/2023-rust/README.md
+++ b/2023-rust/README.md
@@ -9,17 +9,17 @@ Solutions for [Advent of Code](https://adventofcode.com/) in [Rust](https://www.
 
 | Day | Part 1 | Part 2 |
 | :---: | :---: | :---:  |
-| [Day 1](./src/bin/01.rs) | `64.6µs` | `651.1µs` |
-| [Day 2](./src/bin/02.rs) | `42.9µs` | `50.4µs` |
-| [Day 3](./src/bin/03.rs) | `445.6µs` | `428.1µs` |
-| [Day 4](./src/bin/04.rs) | `103.4µs` | `19.7ms` |
+| [Day 1](./src/bin/01.rs) | `65.9µs` | `668.0µs` |
+| [Day 2](./src/bin/02.rs) | `42.1µs` | `48.7µs` |
+| [Day 3](./src/bin/03.rs) | `429.6µs` | `418.1µs` |
+| [Day 4](./src/bin/04.rs) | `96.3µs` | `19.4ms` |
 | [Day 5](./src/bin/05.rs) | `30.3µs` | `26.5s` |
-| [Day 6](./src/bin/06.rs) | `642.0ns` | `37.2ms` |
-| [Day 7](./src/bin/07.rs) | `205.1µs` | `-` |
-| [Day 8](./src/bin/08.rs) | `257.5µs` | `1.7ms` |
-| [Day 9](./src/bin/09.rs) | `101.1µs` | `-` |
+| [Day 6](./src/bin/06.rs) | `573.0ns` | `40.4ms` |
+| [Day 7](./src/bin/07.rs) | `200.0µs` | `-` |
+| [Day 8](./src/bin/08.rs) | `256.2µs` | `1.7ms` |
+| [Day 9](./src/bin/09.rs) | `106.4µs` | `110.1µs` |
 
-**Total: 26560.95ms**
+**Total: 26563.94ms**
 <!--- benchmarking table --->
 
 ## Usage
diff --git a/2023-rust/src/bin/09.rs b/2023-rust/src/bin/09.rs
index 6f9c2b4..c517396 100644
--- a/2023-rust/src/bin/09.rs
+++ b/2023-rust/src/bin/09.rs
@@ -8,16 +8,41 @@ pub fn part_one(input: &str) -> Option<i32> {
     Some(
         sensor_readings
             .iter()
-            .map(|readings| extrapolate(readings))
+            .map(|readings| extrapolate_forwards(readings))
             .sum(),
     )
 }
 
-pub fn part_two(input: &str) -> Option<u32> {
-    None
+pub fn part_two(input: &str) -> Option<i32> {
+    let sensor_readings = parse_input(input);
+
+    Some(
+        sensor_readings
+            .iter()
+            .map(|readings| extrapolate_backwards(readings))
+            .sum(),
+    )
 }
 
-fn extrapolate(readings: &[i32]) -> i32 {
+fn extrapolate_forwards(readings: &[i32]) -> i32 {
+    // Add the last row of differences together to extrapolate the next number
+    calc_differences(readings)
+        .iter()
+        .map(|differences| differences.last().unwrap())
+        .sum()
+}
+
+fn extrapolate_backwards(readings: &[i32]) -> i32 {
+    let differences = calc_differences(readings);
+    differences
+        .iter()
+        .map(|differences| differences.first().copied().unwrap())
+        .rev()
+        .reduce(|acc, number| number - acc)
+        .unwrap()
+}
+
+fn calc_differences(readings: &[i32]) -> Vec<Vec<i32>> {
     let mut differences: Vec<Vec<i32>> = vec![readings.to_vec()];
 
     let mut i = 0;
@@ -35,11 +60,7 @@ fn extrapolate(readings: &[i32]) -> i32 {
         i += 1;
     }
 
-    // Add the last row of differences together to extrapolate the next number
     differences
-        .iter()
-        .map(|differences| differences.last().unwrap())
-        .sum()
 }
 
 fn parse_input(input: &str) -> Vec<Vec<i32>> {
@@ -66,6 +87,6 @@ mod tests {
     #[test]
     fn test_part_two() {
         let result = part_two(&advent_of_code::template::read_file("examples", DAY));
-        assert_eq!(result, None);
+        assert_eq!(result, Some(2));
     }
 }