Day 16

day16/main.swift

@@ -0,0 +1,116 @@
+import Foundation
+
+let input = loadData(day: 16)
+let scanner = Scanner(string: input)
+
+extension Scanner {
+    func scanRange() -> ClosedRange<Int>? {
+        guard let low = scanInt(),
+              string("-"),
+              let high = scanInt()
+        else { return nil }
+        return low...high
+    }
+
+    func scanFieldDefinition() -> (String, ClosedRange<Int>, ClosedRange<Int>)? {
+        var location = scanLocation
+        guard let name = scanUpToString(":"),
+              string(":"),
+              let first = scanRange(),
+              string("or"),
+              let second = scanRange()
+        else {
+            scanLocation = location
+            return nil
+        }
+        return (name, first, second)
+    }
+
+    func scanFieldDefinitions() -> [(String, ClosedRange<Int>, ClosedRange<Int>)] {
+        var result = [(String, ClosedRange<Int>, ClosedRange<Int>)]()
+        while let field = scanFieldDefinition() {
+            result.append(field)
+        }
+
+        return result
+    }
+
+    func scanTicket() -> [Int]? {
+        var result = [Int]()
+        repeat {
+            guard let field = scanInt() else { return nil }
+            result.append(field)
+        } while string(",")
+        return result
+    }
+
+    func scanYourTicket() -> [Int]? {
+        guard string("your ticket:"),
+              let ticket = scanTicket()
+        else { return nil }
+        return ticket
+    }
+
+    func scanNearbyTickets() -> [[Int]]? {
+        guard string("nearby tickets:") else { return nil }
+        var result = [[Int]]()
+        while !isAtEnd {
+            guard let ticket = scanTicket() else { return nil }
+            result.append(ticket)
+        }
+        return result
+    }
+}
+
+let definitions = scanner.scanFieldDefinitions()
+guard let yourTicket = scanner.scanYourTicket(),
+      let nearbyTickets = scanner.scanNearbyTickets()
+else { fatalError() }
+
+
+func isValid(_ value: Int) -> Bool {
+    definitions.contains {
+        $0.1.contains(value) || $0.2.contains(value)
+    }
+}
+
+func isValid(_ ticket: [Int]) -> Bool {
+    ticket.allSatisfy(isValid)
+}
+
+let invalidSums = nearbyTickets
+    .flatten()
+    .filter( { !isValid($0) } )
+    .reduce(0, +)
+
+print(invalidSums)
+
+
+var possibleFields: [Set<String>] = Array(repeating: Set(definitions.map { $0.0 }), count: yourTicket.count)
+
+for ticket in nearbyTickets where isValid(ticket) {
+    for (index, value) in ticket.enumerated() {
+        for (name, first, second) in definitions {
+            if !(first.contains(value) || second.contains(value)) {
+                possibleFields[index].remove(name)
+            }
+        }
+    }
+}
+
+for index in possibleFields.indices.sorted(by: { possibleFields[$0].count < possibleFields[$1].count }) {
+    guard possibleFields[index].count == 1, let key = possibleFields[index].first else { fatalError() }
+    for i in possibleFields.indices where i != index {
+        possibleFields[i].remove(key)
+    }
+}
+
+let mapping = possibleFields.map { $0.first! }
+
+let result = mapping.enumerated()
+    .map { ($0.element, yourTicket[$0.offset]) }
+    .filter { $0.0.hasPrefix("departure") }
+    .map { $0.1 }
+    .reduce(1, *)
+
+print(result)