day 6 part 2

src/bin/day6.rs

@@ -30,35 +30,48 @@ fn parser() -> impl Parser<char, Vec<Game>, Error = Simple<char>> {
 		.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())?;
 
-	let mut result = 1;
+	println!(
+		"{}",
+		games
+			.iter()
+			.map(|game| ways2win(game.time, game.distance))
+			.product::<i64>()
+	);
+
+	let mut t = 0;
+	let mut d = 0;
 	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;
+		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!("{result}");
+	println!("{}", ways2win(t, d));
 
 	Ok(())
 }