From 1d285ee38c9f919669304e2b5b4f59853e65b5f4 Mon Sep 17 00:00:00 2001
From: Dominic <git@msrd0.de>
Date: Wed, 6 Dec 2023 09:04:10 +0100
Subject: [PATCH] day 6 part 1

---
 inputs/day6.txt |  2 ++
 src/bin/day6.rs | 64 +++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 66 insertions(+)
 create mode 100644 inputs/day6.txt
 create mode 100644 src/bin/day6.rs

diff --git a/inputs/day6.txt b/inputs/day6.txt
new file mode 100644
index 0000000..57f1bf3
--- /dev/null
+++ b/inputs/day6.txt
@@ -0,0 +1,2 @@
+Time:        42     68     69     85
+Distance:   284   1005   1122   1341
diff --git a/src/bin/day6.rs b/src/bin/day6.rs
new file mode 100644
index 0000000..595e431
--- /dev/null
+++ b/src/bin/day6.rs
@@ -0,0 +1,64 @@
+#![forbid(elided_lifetimes_in_paths, unsafe_code)]
+
+use aoc23::read;
+use chumsky::{prelude::*, text::int};
+
+struct Game {
+	time: i64,
+	distance: i64
+}
+
+fn parser() -> impl Parser<char, Vec<Game>, Error = Simple<char>> {
+	let int_list = just(" ")
+		.repeated()
+		.ignore_then(int(10).separated_by(just(" ").repeated().at_least(1)));
+	just("Time:")
+		.ignore_then(int_list)
+		.then_ignore(just("\nDistance:"))
+		.then(int_list)
+		.map(|(times, dists): (Vec<String>, Vec<String>)| {
+			times
+				.into_iter()
+				.zip(dists)
+				.map(|(time, distance)| Game {
+					time: time.parse().unwrap(),
+					distance: distance.parse().unwrap()
+				})
+				.collect()
+		})
+		.then_ignore(just("\n"))
+		.then_ignore(end())
+}
+
+fn main() -> anyhow::Result<()> {
+	let games = read("inputs/day6.txt", parser())?;
+
+	let mut result = 1;
+	for game in &games {
+		let t = game.time;
+		let d = game.distance + 1;
+		// we want the minimum a s.t. (t-a)*a > d,
+		// and the maximum a s.t.     (t-a)*a < d
+		// that means we have a quadratic function -a² + ta - d and we want to find
+		// its intersections with the axis
+
+		let discriminant = t * t - 4 * d;
+		if discriminant < 0 {
+			continue;
+		}
+		let sqrt = (discriminant as f64).sqrt();
+		let min = ((t as f64 - sqrt) / 2.0).ceil() as i64;
+		let max = ((t as f64 + sqrt) / 2.0).floor() as i64;
+		eprintln!(
+			"t={t}, d={d}, min={min} ({}), max={max} ({})",
+			(t - min) * min,
+			(t - max) * max
+		);
+		assert!(max >= min);
+
+		result *= max - min + 1;
+	}
+	println!("{result}");
+
+	Ok(())
+}