Up to Day 21 (excluding day 16)

src/bin/day09.rs

@@ -0,0 +1,48 @@
+use anyhow::{Context, Result};
+
+fn extrapolate(sequence: &mut Vec<i64>) -> Option<(i64, i64)> {
+    let diff = |seq: &mut Vec<i64>| {
+        (0..seq.len() - 1).for_each(|i| seq[i] = seq[i + 1] - seq[i]);
+        seq.pop();
+    };
+
+    let mut fwd = 0;
+    let mut bwd = 0;
+    let mut f = 1;
+
+    while sequence.len() > 1 {
+        fwd += sequence[sequence.len() - 1];
+        bwd += f * sequence[0];
+        diff(sequence);
+        f *= -1;
+
+        if sequence.iter().all(|&num| num == 0) {
+            return Some((bwd, fwd));
+        }
+    }
+
+    None
+}
+
+fn main() -> Result<()> {
+    let filename = std::env::args()
+        .nth(1)
+        .context("./day09 <path to puzzle input>")?;
+    let input = std::fs::read_to_string(filename)?;
+
+    let mut part1 = 0;
+    let mut part2 = 0;
+
+    for line in input.lines() {
+        let parse_i64 = |str: &str| str.parse::<i64>().unwrap();
+        let mut initial: Vec<i64> = line.split_ascii_whitespace().map(parse_i64).collect();
+        let (p2, p1) = extrapolate(&mut initial).context("malformed input")?;
+        part1 += p1;
+        part2 += p2;
+    }
+
+    println!("1) {}", part1);
+    println!("2) {}", part2);
+
+    Ok(())
+}