Solve day 9

input/09-test

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

src/bin/09.rs

@@ -0,0 +1,54 @@
+use aoc2023::*;
+
+const INPUT: &str = include_str!("../../input/09");
+const TEST_INPUT: &str = include_str!("../../input/09-test");
+
+fn main() {
+    let (test1, test2) = solve(TEST_INPUT);
+    let (part1, part2) = solve(INPUT);
+
+    assert_eq!(test1, 114, "Part 1");
+    println!("Part 1: {}", part1);
+    assert_eq!(test2, 2, "Part 2");
+    println!("Part 2: {}", part2);
+}
+
+fn solve(input: &str) -> (i64, i64) {
+    let (forward, backward): (Vec<i64>, Vec<i64>) = input
+        .lines()
+        .map(parse_ws_separated)
+        .map(extrapolate)
+        .unzip();
+    (forward.into_iter().sum(), backward.into_iter().sum())
+}
+
+/// Extrapolate (forward, backward).
+fn extrapolate(mut input: impl Iterator<Item = i64>) -> (i64, i64) {
+    let start = input.next().unwrap();
+
+    // We do not need to store all values,
+    // only the first and last difference values are important.
+    let mut forward = vec![start];
+    let mut backward = vec![start];
+
+    // Build the list of differences
+    for next in input {
+        let mut propagate_diff = next;
+        for diff in &mut forward {
+            let previous = *diff;
+            *diff = propagate_diff;
+            propagate_diff -= previous;
+        }
+        let double_0 = *forward.last().unwrap() == 0 && propagate_diff == 0;
+        if !double_0 {
+            forward.push(propagate_diff);
+            backward.push(propagate_diff);
+        }
+    }
+
+    // Extrapolate
+    let forward = forward.into_iter().sum();
+    let backward = backward.into_iter().rev().fold(0, |a, b| b - a);
+
+    (forward, backward)
+}