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2020/day14/main.swift

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+import Foundation
+let input = loadData(day: 14)
+
+class Problem {
+    let scanner: Scanner
+
+    /// Each bit where the mask is "1" is set here
+    var maskBits: UInt64 = 0
+
+    /// Each bit where the mask is "X" is set here
+    var maskUsed: UInt64 = 0
+    var mem: [UInt64:UInt64] = [:]
+
+    var writeMem: (Problem, UInt64, UInt64) -> Void
+
+    init(_ input: String, writeMem: @escaping (Problem, UInt64, UInt64) -> Void) {
+        scanner = Scanner(string: input)
+        self.writeMem = writeMem
+    }
+
+    func run() {
+        while !scanner.isAtEnd {
+            if scanner.string("mask = "), let mask = scanner.scanCharacters(from: Self.maskSet) {
+                readMask(mask)
+            } else if scanner.string("mem["), let addr = scanner.scanUInt64(), scanner.string("] = "), let value = scanner.scanUInt64() {
+                writeMem(self, addr, value)
+            } else {
+                assertionFailure("Invalid input")
+            }
+        }
+
+
+        print("sum", mem.values.reduce(0, +))
+    }
+
+    private func readMask(_ mask: String) {
+        assert(mask.count == 36)
+
+        maskBits = 0
+        maskUsed = 0
+
+        for char in mask {
+            maskBits <<= 1
+            maskUsed <<= 1
+
+            switch char {
+            case "1": maskBits |= 1
+            case "0": break
+            case "X": maskUsed |= 1
+            default: assertionFailure("Invalid character in mask")
+            }
+        }
+    }
+
+    static private let maskSet = CharacterSet(charactersIn: "01X")
+}
+
+
+
+let part1 = Problem(input) { problem, addr, value in
+    problem.mem[addr] = (value & problem.maskUsed) | problem.maskBits
+}
+part1.run()
+
+
+
+func possibleValues(_ mask: UInt64) -> [UInt64] {
+    var values: [UInt64] = []
+    var mask = mask
+    for i in 0..<36 {
+        if mask & 1 != 0 {
+            let bit: UInt64 = 1 << i
+            if values.isEmpty {
+                values.append(0)
+                values.append(bit)
+            } else {
+                values.append(contentsOf: values.map { $0 | bit })
+            }
+        }
+        mask >>= 1
+    }
+
+    return values
+}
+
+let part2 = Problem(input) { problem, addr, value in
+    for i in possibleValues(problem.maskUsed) {
+        let effective = i | ((addr | problem.maskBits) & ~problem.maskUsed)
+        problem.mem[effective] = value
+    }
+}
+
+part2.run()