day 9 part 1

src/bin/day9.rs

@@ -0,0 +1,66 @@
+#![allow(clippy::needless_range_loop)]
+#![forbid(elided_lifetimes_in_paths, unsafe_code)]
+
+use aoc23::read;
+use chumsky::{prelude::*, text::int};
+
+struct History(Vec<i64>);
+
+impl History {
+	fn parser() -> impl Parser<char, Self, Error = Simple<char>> {
+		one_of(['-', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9'])
+			.repeated()
+			.at_least(1)
+			.separated_by(just(" "))
+			.map(|nums: Vec<Vec<char>>| {
+				Self(
+					nums.into_iter()
+						.map(|num| num.into_iter().collect::<String>().parse().unwrap())
+						.collect()
+				)
+			})
+	}
+}
+
+fn parser() -> impl Parser<char, Vec<History>, Error = Simple<char>> {
+	History::parser()
+		.then_ignore(just("\n"))
+		.repeated()
+		.then_ignore(end())
+}
+
+impl History {
+	fn diff(&self) -> Self {
+		// intentionally reserve one space extra for .extend()
+		let mut diffs = Vec::with_capacity(self.0.len());
+		if self.0.len() > 1 {
+			for i in 1 .. self.0.len() {
+				diffs.push(self.0[i] - self.0[i - 1]);
+			}
+		}
+		Self(diffs)
+	}
+
+	fn extend(&mut self) {
+		if self.0.iter().any(|num| *num != 0) {
+			let mut diff = self.diff();
+			diff.extend();
+			self.0.push(self.0.last().unwrap() + diff.0.last().unwrap());
+		} else {
+			self.0.push(0);
+		}
+	}
+}
+
+fn main() -> anyhow::Result<()> {
+	let histories = read("inputs/day9.txt", parser())?;
+
+	let mut sum = 0;
+	for mut h in histories {
+		h.extend();
+		sum += h.0.last().unwrap();
+	}
+	println!("{sum}");
+
+	Ok(())
+}