#![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 ways2win(t: i64, d: i64) -> i64 {
	// 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 {
		return 0;
	}
	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);

	max - min + 1
}

fn main() -> anyhow::Result<()> {
	let games = read("inputs/day6.txt", parser())?;

	println!(
		"{}",
		games
			.iter()
			.map(|game| ways2win(game.time, game.distance))
			.product::<i64>()
	);

	let mut t = 0;
	let mut d = 0;
	for game in &games {
		t = t * 10i64.pow((game.time as f64).log10().floor() as u32 + 1) + game.time;
		d = d * 10i64.pow((game.distance as f64).log10().floor() as u32 + 1)
			+ game.distance;
	}
	println!("{}", ways2win(t, d));

	Ok(())
}