day9: problem 1 solution

9/src/main.rs

@@ -0,0 +1,98 @@
+use std::fs::File;
+use std::io::{BufRead, BufReader, Lines};
+use itertools::Itertools;
+
+// BOILERPLATE
+type InputIter = Lines<BufReader<File>>;
+
+fn get_input() -> InputIter {
+    let f = File::open("input").unwrap();
+    let br = BufReader::new(f);
+    br.lines()
+}
+
+fn main() {
+    println!("Problem 1 solution: {}", problem1(get_input()));
+    println!("Problem 2 solution: {}", problem2(get_input()));
+}
+
+// PARSE
+
+#[derive(Debug)]
+struct History(Vec<Vec<i64>>);
+
+impl From<&str> for History {
+    fn from(s: &str) -> Self {
+        let hist: Vec<i64> = s
+            .split_whitespace()
+            .map(|num| num.parse().unwrap())
+            .collect();
+        Self(vec![hist])
+    }
+}
+
+impl History {
+    fn build_row(&mut self) {
+        let last = self.0.last().unwrap();
+        self.0.push(
+            last.iter()
+                .tuple_windows()
+                .map(|(a, b)| b - a)
+                .collect(),
+        )
+    }
+
+    fn build(&mut self) {
+        while !self.0.last().unwrap().iter().all(|x| *x == 0) {
+            self.build_row();
+        }
+    }
+
+    fn extrapolate(&mut self) {
+        self.build();
+        self.0.last_mut().unwrap().push(0);
+        for (lower, upper) in (0..self.0.len()).rev().tuple_windows() {
+            let new_value = self.0[upper].last().unwrap() + self.0[lower].last().unwrap();
+            self.0[upper].push(new_value);
+        }
+    }
+}
+
+// PROBLEM 1 solution
+
+fn problem1<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.extrapolate();
+    }
+
+    histories.iter().map(|history| history.0.first().unwrap().last().unwrap()).sum()
+}
+
+// PROBLEM 2 solution
+fn problem2<T: BufRead>(input: Lines<T>) -> u64 {
+    0
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::*;
+    use std::io::Cursor;
+
+    const EXAMPLE: &str = &"0 3 6 9 12 15
+1 3 6 10 15 21
+10 13 16 21 30 45";
+
+    #[test]
+    fn problem1_example() {
+        let c = Cursor::new(EXAMPLE);
+        assert_eq!(problem1(c.lines()), 114);
+    }
+
+    #[test]
+    fn problem2_example() {
+        let c = Cursor::new(EXAMPLE);
+        assert_eq!(problem2(c.lines()), 0);
+    }
+}