day9: problem 2 solution

9/src/main.rs

@@ -56,6 +56,15 @@ impl History {
             self.0[upper].push(new_value);
         }
     }
+
+    fn extrapolate2(&mut self) {
+        self.build();
+        self.0.last_mut().unwrap().insert(0, 0);
+        for (lower, upper) in (0..self.0.len()).rev().tuple_windows() {
+            let new_value = self.0[upper].first().unwrap() - self.0[lower].first().unwrap();
+            self.0[upper].insert(0, new_value);
+        }
+    }
 }
 
 // PROBLEM 1 solution
@@ -71,8 +80,14 @@ fn problem1<T: BufRead>(input: Lines<T>) -> i64 {
 }
 
 // PROBLEM 2 solution
-fn problem2<T: BufRead>(input: Lines<T>) -> u64 {
-    0
+fn problem2<T: BufRead>(input: Lines<T>) -> i64 {
+    let mut histories: Vec<History> = input.map(|s| History::from(s.unwrap().as_str())).collect();
+
+    for history in &mut histories {
+        history.extrapolate2();
+    }
+
+    histories.iter().map(|history| history.0.first().unwrap().first().unwrap()).sum()
 }
 
 #[cfg(test)]
@@ -93,6 +108,6 @@ mod tests {
     #[test]
     fn problem2_example() {
         let c = Cursor::new(EXAMPLE);
-        assert_eq!(problem2(c.lines()), 0);
+        assert_eq!(problem2(c.lines()), 2);
     }
 }