From 5b2760e9f7f396e981734fab5c84a253359cbab5 Mon Sep 17 00:00:00 2001
From: Andrew Coleman <penguincoder@gmail.com>
Date: Sun, 10 Dec 2023 14:57:00 -0500
Subject: [PATCH] day9 solution

---
 2023/src/day09.rs | 75 +++++++++++++++++++++++++++++++++++++++++++++++
 2023/src/lib.rs   |  1 +
 2 files changed, 76 insertions(+)
 create mode 100644 2023/src/day09.rs

diff --git a/2023/src/day09.rs b/2023/src/day09.rs
new file mode 100644
index 0000000..4164459
--- /dev/null
+++ b/2023/src/day09.rs
@@ -0,0 +1,75 @@
+use aoc_runner_derive::{aoc, aoc_generator};
+
+type Seq = Vec<i64>;
+
+fn all_zero(s: &Seq) -> bool {
+    s.iter().find(|t| **t != 0).is_none()
+}
+
+#[aoc_generator(day9)]
+fn parse(input: &str) -> Vec<Seq> {
+    input
+        .lines()
+        .filter_map(|line| {
+            if !line.is_empty() {
+                let nums = line.split(" ").map(|n| n.parse::<i64>().unwrap()).collect();
+                Some(nums)
+            } else {
+                None
+            }
+        })
+        .collect()
+}
+
+fn get_diffs(s: &Seq) -> Seq {
+    let mut result: Seq = vec![];
+    for index in 0..(s.len() - 1) {
+        result.push(s[index + 1] - s[index]);
+    }
+    result
+}
+
+fn next_number(s: &Seq) -> i64 {
+    if all_zero(s) {
+        0
+    } else {
+        let d = get_diffs(s);
+        s.last().unwrap() + next_number(&d)
+    }
+}
+
+#[aoc(day9, part1)]
+fn part1(input: &[Seq]) -> i64 {
+    input.iter().map(|s| next_number(&s)).sum()
+}
+
+fn previous_number(s: &Seq) -> i64 {
+    if all_zero(s) {
+        0
+    } else {
+        let d = get_diffs(s);
+        s[0] - previous_number(&d)
+    }
+}
+
+#[aoc(day9, part2)]
+fn part2(input: &[Seq]) -> i64 {
+    input.iter().map(|s| previous_number(&s)).sum()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const SAMPLE_DATA: &'static str = r#"0 3 6 9 12 15
+1 3 6 10 15 21
+10 13 16 21 30 45
+"#;
+
+    #[test]
+    fn sample_data() {
+        let input = parse(&SAMPLE_DATA);
+        assert_eq!(part1(&input), 114);
+        assert_eq!(part2(&input), 2);
+    }
+}
diff --git a/2023/src/lib.rs b/2023/src/lib.rs
index 4d0316e..63c5ba7 100644
--- a/2023/src/lib.rs
+++ b/2023/src/lib.rs
@@ -8,5 +8,6 @@ mod day05;
 mod day06;
 mod day07;
 mod day08;
+mod day09;
 
 aoc_lib! { year = 2023 }