03b

Sources/aoc2021/03.swift

@@ -16,15 +16,15 @@ struct Day03 {
         let commands = input
             .components(separatedBy: .newlines)
             .filter { !$0.isEmpty }
-            //.compactMap { MoveCommand(line: $0) }
         runA(commands)
-        //runB(commands)
+        runB(commands)
     }
     
     func runA(_ commands: [String]) {
         let neededForMajority = commands.count / 2
         let lastBitIndex = commands.first!.count
         var countsOnes = [Int](repeating: 0, count: lastBitIndex)
+        // There's probably some clever bitwise operation to solve this
         for command in commands {
             let m = command.map { Bool.init(digit: $0)! }.map { $0 ? 1 : 0 }
             countsOnes.enumerated().forEach { (idx, elem) in
@@ -42,11 +42,81 @@ struct Day03 {
             }
             return (gamma, episolon)
         })
-        print(dec.episolon * dec.gamma)<
+        print(dec.episolon * dec.gamma)
     }
     
     func runB(_ commands: [String]) {
+        // Incredibly naive, iterates through the list many, many times
+        // Still runs plenty fast (~70ms on my machine)
+        // Too tired to optimize this
         
+        let lastBitIndex = commands.first!.count
+        var countsOnes = [Int](repeating: 0, count: lastBitIndex)
+        for command in commands {
+            let m = command.map { Bool.init(digit: $0)! }.map { $0 ? 1 : 0 }
+            countsOnes.enumerated().forEach { (idx, elem) in
+                countsOnes[idx] += m[idx]
+            }
+        }
+        
+        var oxygenCommands = commands
+        var scrubberCommands = commands
+        for pos in 0..<lastBitIndex {
+            var countsOnesOxygen = [Int](repeating: 0, count: lastBitIndex)
+            for command in oxygenCommands {
+                let m = command.map { Bool.init(digit: $0)! }.map { $0 ? 1 : 0 }
+                countsOnesOxygen.enumerated().forEach { (idx, elem) in
+                    countsOnesOxygen[idx] += m[idx]
+                }
+            }
+            
+            var countsOnesScrubber = [Int](repeating: 0, count: lastBitIndex)
+            for command in scrubberCommands {
+                let m = command.map { Bool.init(digit: $0)! }.map { $0 ? 1 : 0 }
+                countsOnesScrubber.enumerated().forEach { (idx, elem) in
+                    countsOnesScrubber[idx] += m[idx]
+                }
+            }
+            let neededForMajorityOxygen = Int(ceil(Double(oxygenCommands.count) / 2.0))
+            for command in oxygenCommands {
+                let idx = command.index(command.startIndex, offsetBy: pos)
+                let isMostCommon = countsOnesOxygen[pos] >= neededForMajorityOxygen
+                let digit = Bool(digit: command[idx])
+                let keepOxygen = digit == isMostCommon
+                if !keepOxygen, oxygenCommands.count > 1, let coi = oxygenCommands.firstIndex(of: command){
+                    oxygenCommands.remove(at: coi)
+                }
+            }
+            
+            let neededForMajorityScrubber = Int(ceil(Double(scrubberCommands.count) / 2.0))
+            for command in scrubberCommands {
+                let idx = command.index(command.startIndex, offsetBy: pos)
+                let isLeastCommon = countsOnesScrubber[pos] < neededForMajorityScrubber
+                let digit = Bool(digit: command[idx])
+                let keepScrubber = digit == isLeastCommon
+                if !keepScrubber, scrubberCommands.count > 1, let ci = scrubberCommands.firstIndex(of: command) {
+                    scrubberCommands.remove(at: ci)
+                }
+            }
+            
+        }
+        let finalOxygenNumber = oxygenCommands.first!
+        let finalScruberNumber = scrubberCommands.first!
+        
+        print(ToDecimal(num: finalOxygenNumber) * ToDecimal(num: finalScruberNumber))
+    }
+    
+    private func ToDecimal(num: String) -> Int {
+        var pot = -1
+        var val = 0
+        for c in num.reversed() {
+            pot += 1
+            if c == "0" {
+                continue
+            }
+            val += Int(truncating: NSDecimalNumber(decimal: pow(2, pot)))
+        }
+        return val
     }
 }