solution: day 9

data/examples/09.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/09.rs

@@ -0,0 +1,87 @@
+use std::collections::VecDeque;
+
+fn find_history(v: VecDeque<i32>) -> VecDeque<VecDeque<i32>> {
+    let mut s = VecDeque::new();
+    s.push_back(v);
+
+    loop {
+        let mut all_zeros = true;
+
+        let last = s.back().unwrap();
+        let mut new = VecDeque::new();
+        for i in 0..(last.len() - 1) {
+            let diff = last[i + 1] - last[i];
+            if diff != 0 {
+                all_zeros = false;
+            }
+
+            new.push_back(diff);
+        }
+        s.push_back(new);
+
+        if all_zeros {
+            break;
+        }
+    }
+
+    s
+}
+
+fn extrapolate_forward(v: VecDeque<i32>) -> i32 {
+    let mut s = find_history(v);
+
+    for i in (1..s.len()).rev() {
+        let adder = *s[i].back().unwrap();
+        let last = *s[i - 1].back().unwrap();
+        s[i - 1].push_back(last + adder);
+    }
+
+    *s[0].back().unwrap()
+}
+
+fn extrapolate_back(v: VecDeque<i32>) -> i32 {
+    let mut s = find_history(v);
+
+    for i in (1..s.len()).rev() {
+        let adder = *s[i].front().unwrap();
+        let first = *s[i - 1].front().unwrap();
+        s[i - 1].push_front(first - adder);
+    }
+
+    *s[0].front().unwrap()
+}
+
+pub fn part_one(input: &str) -> Option<i32> {
+    Some(
+        input
+            .lines()
+            .map(|x| x.split(' ').filter_map(|y| y.parse().ok()).collect())
+            .map(extrapolate_forward)
+            .sum(),
+    )
+}
+
+pub fn part_two(input: &str) -> Option<i32> {
+    Some(
+        input
+            .lines()
+            .map(|x| x.split(' ').filter_map(|y| y.parse().ok()).collect())
+            .map(extrapolate_back)
+            .sum(),
+    )
+}
+
+aoc::solution!(9);
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    #[test]
+    fn test_part_one() {
+        assert_eq!(part_one(&aoc::template::read_file("examples", 9)), None);
+    }
+    #[test]
+    fn test_part_two() {
+        assert_eq!(part_two(&aoc::template::read_file("examples", 9)), None);
+    }
+}