Implement 2022 day 15 part 2

2022/src/common.rs

@@ -2,6 +2,7 @@
 
 use std::cmp::Ordering;
 use std::ops::Add;
+use std::ops::Div;
 use std::ops::Index;
 use std::ops::IndexMut;
 use std::ops::Sub;
@@ -202,6 +203,14 @@ impl Sub<Self> for Vec2 {
     }
 }
 
+impl Div<i32> for Vec2 {
+    type Output = Self;
+
+    fn div(self, rhs: i32) -> Self::Output {
+        Self(self.0.map(|v| v / rhs))
+    }
+}
+
 impl Index<usize> for Vec2 {
     type Output = i32;
 

2022/src/day15.rs

@@ -18,6 +18,22 @@ use crate::common::Vec2;
 struct Beacon {
     pos: Vec2,
     closest: Vec2,
+    range: i32,
+}
+
+impl Beacon {
+    pub fn can_contain_unseen(&self, lower: Vec2, upper: Vec2) -> bool {
+        let corners = [
+            lower,
+            upper,
+            Vec2([lower[0], upper[1]]),
+            Vec2([upper[0], lower[1]]),
+        ];
+
+        corners
+            .into_iter()
+            .any(|c| (c - self.pos).l1() > self.range)
+    }
 }
 
 fn parse_pos(input: &[u8]) -> IResult<&[u8], Vec2> {
@@ -34,7 +50,11 @@ fn parse_beacons(input: &[u8]) -> IResult<&[u8], Vec<Beacon>> {
             preceded(tag("Sensor at "), parse_pos),
             preceded(tag(": closest beacon is at "), parse_pos),
         ),
-        |(pos, closest)| Beacon { pos, closest },
+        |(pos, closest)| Beacon {
+            pos,
+            closest,
+            range: (pos - closest).l1(),
+        },
     );
 
     many0(terminated(parse_beacon, newline))(input)
@@ -51,8 +71,7 @@ fn part1_generic<const Y: i32>(input: &[u8]) -> Result<String> {
     let mut on_line = AHashSet::new();
 
     for beacon in beacons {
-        let distance = (beacon.closest - beacon.pos).l1();
-        let horizontal_distance = distance - (beacon.pos[1] - Y).abs();
+        let horizontal_distance = beacon.range - (beacon.pos[1] - Y).abs();
 
         if horizontal_distance >= 0 {
             not_blocking_yet.push(Reverse((
@@ -92,8 +111,53 @@ pub fn part1(input: &[u8]) -> Result<String> {
     part1_generic::<2000000>(input)
 }
 
-pub fn part2(_input: &[u8]) -> Result<String> {
-    anyhow::bail!("not implemented")
+fn part2_generic<const MAX: i32>(input: &[u8]) -> Result<String> {
+    let beacons = parse_input(input, parse_beacons)?;
+
+    fn find_unseen<const MAX: i32>(beacons: &[Beacon]) -> Result<Vec2> {
+        let mut todo = vec![(Vec2([0, 0]), Vec2([MAX, MAX]))];
+
+        while let Some((lower, upper)) = todo.pop() {
+            if lower == upper {
+                if beacons
+                    .iter()
+                    .all(|beacon| (beacon.pos - lower).l1() > beacon.range)
+                {
+                    return Ok(lower);
+                }
+            } else {
+                let mid = (lower + upper) / 2;
+
+                let quads = [
+                    (lower, mid),
+                    (Vec2([lower[0], mid[1] + 1]), Vec2([mid[0], upper[1]])),
+                    (Vec2([mid[0] + 1, lower[1]]), Vec2([upper[0], mid[1]])),
+                    (Vec2([mid[0] + 1, mid[1] + 1]), upper),
+                ];
+
+                for (lower_new, upper_new) in quads {
+                    if lower_new[0] > upper_new[0] || lower_new[1] > upper_new[1] {
+                        // Empty quad in quad tree, skip
+                    } else if beacons
+                        .iter()
+                        .all(|beacon| beacon.can_contain_unseen(lower_new, upper_new))
+                    {
+                        todo.push((lower_new, upper_new));
+                    }
+                }
+            }
+        }
+
+        anyhow::bail!("Did not find position")
+    }
+
+    let Vec2([x, y]) = find_unseen::<MAX>(&beacons)?;
+
+    Ok((i64::from(x) * 4000000 + i64::from(y)).to_string())
+}
+
+pub fn part2(input: &[u8]) -> Result<String> {
+    part2_generic::<4000000>(input)
 }
 
 #[cfg(test)]
@@ -106,4 +170,9 @@ mod tests {
     fn sample_part1() {
         assert_eq!(part1_generic::<10>(SAMPLE).unwrap(), "26");
     }
+
+    #[test]
+    fn sample_part2() {
+        assert_eq!(part2_generic::<20>(SAMPLE).unwrap(), "56000011");
+    }
 }