[rs] Solve 2022_15 part 2

inputs/2022/2022_15.solution

@@ -1,2 +1,2 @@
 Part 1: 4811413
-Part 2: ???
+Part 2: 13171855019123

rs/src/y2022/d15.rs

@@ -11,10 +11,44 @@ fn parse_coord(coord: &str) -> i32 {
     coord.parse().unwrap()
 }
 
+fn line_ranges(sensors: &[Sensor], y: i32) -> Vec<(i32, i32)> {
+    let mut raw_ranges = vec![];
+    for sensor in sensors {
+        let dy = (y - sensor.pos.1).abs();
+        if dy > sensor.dist {
+            continue;
+        }
+        let dx = sensor.dist - dy;
+        raw_ranges.push((sensor.pos.0 - dx, sensor.pos.0 + dx));
+    }
+    raw_ranges.sort();
+
+    let mut ranges = vec![];
+    let mut raw_ranges = raw_ranges.into_iter();
+    if let Some(mut cur) = raw_ranges.next() {
+        for range in raw_ranges {
+            if cur.0 <= range.0 && range.0 <= cur.1 {
+                cur.1 = cur.1.max(range.1);
+            } else {
+                ranges.push(cur);
+                cur = range;
+            }
+        }
+        ranges.push(cur);
+    }
+    ranges
+}
+
 pub fn solve(input: String) {
     let sensors = input
         .lines()
         .map(|l| {
+            // A beacon exactly dist away has 1 field, i. e. x±0
+            // A beacon exactly dist-1 away has 3 fields, i. e. x±1
+            // ...
+            // A beacon exactly 0 away has 2dist+1 fields, i. e. x±dist
+            //
+            // Formula: Go from x-(dist-dy) to x+(dist-dy)
             let parts = l.split_whitespace().collect::<Vec<_>>();
             let pos = (parse_coord(parts[2]), parse_coord(parts[3]));
             let beac = (parse_coord(parts[8]), parse_coord(parts[9]));
@@ -23,30 +57,34 @@ pub fn solve(input: String) {
         })
         .collect::<Vec<_>>();
 
-    // A beacon exactly dist away has 1 field, i. e. x±0
-    // A beacon exactly dist-1 away has 3 fields, i. e. x±1
-    // ...
-    // A beacon exactly 0 away has 2dist+1 fields, i. e. x±dist
-    //
-    // Formula: Go from x-(dist-dy) to x+(dist-dy)
     let line_at_y = 2000000;
-    let mut part1 = HashSet::new();
-    for (i, sensor) in sensors.iter().enumerate() {
-        eprintln!("Sensor {}", i + 1);
-        let dy = (line_at_y - sensor.pos.1).abs();
-        if dy > sensor.dist {
-            continue;
+    let part1_ranges = line_ranges(&sensors, line_at_y);
+    let part1_beacons = sensors
+        .iter()
+        .filter(|s| s.beac.1 == line_at_y)
+        .map(|s| s.beac.0)
+        .collect::<HashSet<_>>();
+    let mut part1 = 0;
+    for range in part1_ranges {
+        part1 += range.1 - range.0 + 1;
+        for beacon in &part1_beacons {
+            if range.0 <= *beacon && *beacon <= range.1 {
+                part1 -= 1;
+            }
         }
+    }
+    println!("Part 1: {part1}");
 
-        let dx = sensor.dist - dy;
-        for x in (sensor.pos.0 - dx)..=(sensor.pos.0 + dx) {
-            part1.insert(x);
+    for y in 0..=4000000 {
+        let mut ranges = line_ranges(&sensors, y);
+        ranges.retain(|(s, e)| *s <= 4000000 && 0 <= *e);
+        if ranges.len() != 1 {
+            // Found our beacon!
+            assert_eq!(ranges.len(), 2);
+            let x = ranges[0].1 + 1;
+            let tuning_frequency = x as i64 * 4000000 + y as i64;
+            println!("Part 2: {tuning_frequency}");
+            break;
         }
     }
-    for sensor in &sensors {
-        if sensor.beac.1 == line_at_y {
-            part1.remove(&sensor.beac.0);
-        }
-    }
-    println!("Part 1: {}", part1.len());
 }