day3 was too much reading

src/day3/main.rs

@@ -8,6 +8,19 @@ struct Report {
 	vals: Vec<u16>,
 }
 
+fn segment(vals: &[u16], m: u16) -> (Vec<u16>, Vec<u16>) {
+	let mut a = Vec::new();
+	let mut b = Vec::new();
+	for val in vals {
+		if (val & m) != 0 {
+			a.push(*val);
+		} else {
+			b.push(*val);
+		}
+	}
+	(a, b)
+}
+
 impl Report {
 	fn from_reader<R: Read>(r: R) -> Result<Report, Box<dyn Error>> {
 		let r = BufReader::new(r);
@@ -37,6 +50,27 @@ impl Report {
 
 		g * (!g & ((1 << self.n) - 1))
 	}
+
+	fn part2(&self) -> usize {
+		let mask = 1 << (self.n - 1);
+		let (mut a, mut b) = segment(&self.vals, mask);
+
+		let mut m = mask >> 1;
+		while a.len() > 1 {
+			let (aa, bb) = segment(&a, m);
+			a = if aa.len() >= bb.len() { aa } else { bb };
+			m >>= 1;
+		}
+
+		let mut m = mask >> 1;
+		while b.len() > 1 {
+			let (aa, bb) = segment(&b, m);
+			b = if aa.len() < bb.len() { aa } else { bb };
+			m >>= 1;
+		}
+
+		a[0] as usize * b[0] as usize
+	}
 }
 
 fn main() -> Result<(), Box<dyn Error>> {
@@ -53,6 +87,7 @@ fn main() -> Result<(), Box<dyn Error>> {
 	)?)?;
 
 	println!("Part #1: {}", report.part1());
+	println!("Part #2: {}", report.part2());
 
 	Ok(())
 }