feat(2023-09): solve part 1 and 2

2023/09/Cargo.toml

@@ -0,0 +1,9 @@
+[package]
+name = "aoc-2023-09"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+aoc-2023-common = { path = "../common/" }

2023/09/input/example.txt

@@ -0,0 +1,3 @@
+0 3 6 9 12 15
+1 3 6 10 15 21
+10 13 16 21 30 45

2023/09/src/main.rs

@@ -0,0 +1,56 @@
+use aoc_2023_common::challenge_input;
+
+fn deltas(sequence: &[i64]) -> Vec<i64> {
+    sequence
+        .windows(2)
+        .filter_map(|window| match window {
+            [left, right] => Some(right - left),
+            _ => None,
+        })
+        .collect()
+}
+
+fn recursive_deltas(sequence: &[i64]) -> Vec<Vec<i64>> {
+    let mut sequences = vec![sequence.to_vec().clone()];
+    while !sequences.last().unwrap().iter().all(|&n| n == 0) {
+        let next = deltas(sequences.last().unwrap());
+        sequences.push(next);
+    }
+    sequences.pop(); // we don't care about [0, 0, ..]
+    sequences
+}
+
+fn extrapolate(sequence: &[i64]) -> i64 {
+    let mut deltas = recursive_deltas(sequence);
+
+    let len = deltas.len();
+    let smallest_delta = deltas[len - 1][0];
+    deltas[len - 1].push(smallest_delta);
+
+    for i in (0..len - 1).rev() {
+        let next = deltas[i].last().unwrap() + deltas[i + 1].last().unwrap();
+        deltas[i].push(next);
+    }
+
+    *deltas[0].last().unwrap()
+}
+
+fn main() {
+    let input = challenge_input();
+    let mut sequences: Vec<Vec<i64>> = input
+        .lines()
+        .map(|line| {
+            line.split_whitespace()
+                .filter_map(|num| num.parse().ok())
+                .collect()
+        })
+        .collect();
+
+    let part_1: i64 = sequences.iter().map(|seq| extrapolate(seq)).sum();
+    println!("{}", part_1);
+
+    sequences.iter_mut().for_each(|seq| seq.reverse());
+    let part_2: i64 = sequences.iter().map(|seq| extrapolate(seq)).sum();
+
+    println!("{}", part_2);
+}

2023/Cargo.toml

@@ -12,4 +12,5 @@ members = [
   "06",
   "07",
   "08",
+  "09",
 ]