2022 Day 15

2022/day15.swift

@@ -0,0 +1,117 @@
+import Foundation
+
+
+let input = try String(contentsOf: URL(fileURLWithPath: "day15.input"))
+let scanner = Scanner(string: input)
+
+struct Point {
+    var x: Int
+    var y: Int
+}
+
+struct Sensor {
+    var position: Point
+    var beacon: Point
+    
+    var distance: Int {
+        abs(position.x - beacon.x) + abs(position.y - beacon.y)
+    }
+    
+    func coveredXRange(at y: Int) -> ClosedRange<Int>? {
+        let xDistance = distance - abs(position.y - y)
+        guard xDistance > 0 else { return nil }
+        return position.x - xDistance ... position.x + xDistance
+    }
+}
+
+extension Scanner {
+    func point() -> Point? {
+        guard scanString("x=") != nil, let x = scanInt(), scanString(", y=") != nil, let y = scanInt() else {
+            return nil
+        }
+        return Point(x: x, y: y)
+    }
+    
+    func sensor() -> Sensor? {
+        guard scanString("Sensor at") != nil, let sensor = point(), scanString(": closest beacon is at") != nil, let beacon = point() else {
+            return nil
+        }
+        
+        return Sensor(position: sensor, beacon: beacon)
+    }
+}
+
+let testY = 2000000
+
+
+var sensors: [Sensor] = []
+
+while !scanner.isAtEnd {
+    guard let sensor = scanner.sensor() else {
+        fatalError()
+    }
+    sensors.append(sensor)
+}
+
+
+
+func coveredRanges(at testY: Int) -> [ClosedRange<Int>] {
+    var events: [(Int, Bool)] = []
+    for sensor in sensors {
+        guard let range = sensor.coveredXRange(at: testY) else {
+            continue
+        }
+        events.append((range.lowerBound, true))
+        events.append((range.upperBound, false))
+    }
+
+    events.sort { $0.0 < $1.0 }
+
+    var mergedRanges: [ClosedRange<Int>] = []
+    
+    var count = 0
+    var currentStart: Int? = nil
+    var currentEnd: Int? = nil
+
+    for (value, isStart) in events {
+        count += isStart ? 1 : -1
+        if isStart && currentStart == nil {
+            currentStart = value
+        } else if isStart, currentEnd == value {
+            currentEnd = nil
+        } else if isStart, let start = currentStart, let end = currentEnd {
+            currentStart = value
+            currentEnd = nil
+            mergedRanges.append(start ... end)
+        } else if !isStart && count == 0 {
+            currentEnd = value
+        }
+    }
+
+    if let start = currentStart, let end = currentEnd {
+        mergedRanges.append(start ... end)
+    }
+
+    return mergedRanges    
+}
+
+
+var impossiblePositions = 0
+for range in coveredRanges(at: testY) {
+    impossiblePositions += range.count
+}    
+print("Part 1:", impossiblePositions)
+
+
+let possibleRange = 0...4000000
+
+for y in 0...4000000 {
+    let ranges = coveredRanges(at: y)
+    
+    let end = ranges.lazy.map(\.upperBound).filter { possibleRange.contains($0) }.first
+    if let end {
+        let x = end + 1
+        print("Part 2:", x * 4000000 + y)
+        break
+    }
+}