day(09): add solution

Signed-off-by: Matej Focko <mfocko@redhat.com>

samples/day09.txt

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

src/bin/day09.rs

@@ -0,0 +1,64 @@
+use aoc_2023::*;
+
+type Output1 = i32;
+type Output2 = Output1;
+
+struct Day09 {
+    values: Vec<Vec<i32>>,
+}
+impl Solution<Output1, Output2> for Day09 {
+    fn new<P: AsRef<Path>>(pathname: P) -> Self {
+        let values = file_to_lines::<Vec<_>, P>(pathname)
+            .iter()
+            .map(|vals| parse_ws_separated(vals))
+            .collect_vec();
+
+        Self { values }
+    }
+
+    fn part_1(&mut self) -> Output1 {
+        self.values
+            .iter()
+            .map(|xs| {
+                let mut ds = vec![];
+
+                let mut last_row = xs.clone();
+                while last_row.iter().any(|&x| x != 0) {
+                    let next_row = last_row.windows(2).map(|s| s[1] - s[0]).collect_vec();
+
+                    ds.push(*next_row.last().unwrap());
+                    last_row = next_row;
+                }
+
+                xs.last().unwrap() + ds.iter().rev().sum::<i32>()
+            })
+            .sum()
+    }
+
+    fn part_2(&mut self) -> Output2 {
+        self.values
+            .iter()
+            .map(|xs| {
+                let mut ds = vec![];
+
+                let mut last_row = xs.clone();
+                last_row.reverse();
+
+                while last_row.iter().any(|&x| x != 0) {
+                    let next_row = last_row.windows(2).map(|s| s[1] - s[0]).collect_vec();
+
+                    ds.push(*next_row.last().unwrap());
+                    last_row = next_row;
+                }
+
+                xs.first().unwrap() + ds.iter().rev().sum::<i32>()
+            })
+            .sum()
+    }
+}
+
+fn main() -> Result<()> {
+    Day09::main()
+}
+
+test_sample!(day_09, Day09, 114, 2);