1 files changed, 43 insertions, 0 deletions
diff --git a/src/bin/day9.rs b/src/bin/day9.rs
new file mode 100644
index 0000000..ec008fb
--- /dev/null
+++ b/src/bin/day9.rs
@@ -0,0 +1,43 @@
+static DAY: u8 = 9;
+
+fn main() {
+    let input = advent::read_lines(DAY);
+    println!("{DAY}a: {}", sum_extrapolations(&input));
+    println!("{DAY}b: {}", 0);
+}
+
+fn extrapolate(input: &[i32]) -> i32 {
+    if input.iter().all(|&n| n == 0) {
+        /* all zeroes */
+        return 0;
+    }
+
+    let diffs = input.iter()
+                     .as_slice()
+                     .windows(2)
+                     .map(|x| x[1] - x[0])
+                     .collect::<Vec<_>>();
+    input.last().unwrap() + extrapolate(&diffs)
+}
+
+fn sum_extrapolations(input: &[String]) -> i32 {
+    input.iter()
+         .map(|x| x.split(' ').map(|n| n.parse().unwrap()).collect::<Vec<_>>())
+         .map(|numbers| extrapolate(&numbers))
+         .sum()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test() {
+        let input = [
+            "0 3 6 9 12 15",
+            "1 3 6 10 15 21",
+            "10 13 16 21 30 45",
+        ].iter().map(|&x| String::from(x)).collect::<Vec<_>>();
+        assert_eq!(sum_extrapolations(&input), 114);
+    }
+}