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2020/day8/main.swift

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+import Foundation
+
+let input = loadData(day: 8)
+
+let scanner = Scanner(string: input)
+
+enum Instruction: String {
+    case nop
+    case acc
+    case jmp
+}
+
+struct Line {
+    var instruction: Instruction
+    var argument: Int
+}
+
+extension Scanner {
+    func instruction() -> Instruction? {
+        guard let string = scanUpToCharacters(from: .whitespaces),
+              let instruction = Instruction(rawValue: string)
+        else { return nil }
+        return instruction
+    }
+
+    func line() -> Line? {
+        guard let ins = instruction(),
+              let argument = scanInt()
+        else { return nil }
+        return Line(instruction: ins, argument: argument)
+    }
+
+    func program() -> [Line]? {
+        var program = [Line]()
+        while !isAtEnd {
+            guard let line = self.line() else { return nil }
+            program.append(line)
+        }
+        return program
+    }
+}
+
+class Computer {
+    var program: [Line]
+    var ip: Int = 0
+    var acc: Int = 0
+
+    init(program: [Line]) {
+        self.program = program
+    }
+
+    var visited: Set<Int> = []
+
+    func runLine() {
+        let line = program[ip]
+        switch line.instruction {
+        case .nop:
+            ip += 1
+
+        case .acc:
+            acc += line.argument
+            ip += 1
+
+        case .jmp:
+            ip += line.argument
+        }
+    }
+
+    func run() {
+        ip = 0
+        acc = 0
+
+        while ip < program.count {
+            let (inserted, _) = visited.insert(ip)
+            if !inserted {
+                print("Repeated instruction", acc)
+                return
+            }
+
+            runLine()
+        }
+        print("Finished", acc)
+    }
+
+    func find(ip: Int, acc: Int, changed: Bool, visited: Set<Int>) -> Int?
+    {
+        if ip >= program.count {
+            return acc
+        }
+
+        if (visited.contains(ip)) {
+            return nil
+        }
+
+        let newVisited = visited.union([ip])
+
+        let line = program[ip]
+        switch line.instruction {
+        case .acc:
+            return find(ip: ip + 1, acc: acc + line.argument, changed: changed, visited: newVisited)
+
+        case .nop:
+            if let result = find(ip: ip + 1, acc: acc, changed: changed, visited: newVisited) {
+                return result
+            }
+
+            if !changed, let result = find(ip: ip + line.argument, acc: acc, changed: true, visited: newVisited) {
+                print("Found result by changing nop to jmp at", ip)
+                return result
+            }
+
+        case .jmp:
+            if let result = find(ip: ip + line.argument, acc: acc, changed: changed, visited: newVisited) {
+                return result
+            }
+
+            if !changed, let result = find(ip: ip + 1, acc: acc, changed: true, visited: newVisited) {
+                print("Found result by changing jmp to nop at", ip)
+                return result
+            }
+        }
+
+        return nil
+    }
+}
+
+guard let program = scanner.program() else { fatalError() }
+let computer = Computer(program: program)
+computer.run()
+if let found = computer.find(ip: 0, acc: 0, changed: false, visited: []) {
+    print("found solution", found)
+}
+
+