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""".utf8 let on = UInt8(ascii: "#") let off = UInt8(ascii: ".") func step(input: [UInt8]) -> [UInt8] { assert(input.count == 100 * 100) var result: [UInt8] = [] result.reserveCapacity(input.count) func isOn(_ x: Int, _ y: Int) -> Bool { return input[x + y * 100] == on } for y in 0..<100 { for x in 0..<100 { var onCount = 0 for i in -1...1 where (0..<100).contains(i+y) { for j in -1...1 where (0..<100).contains(x+j) && !(i == 0 && j == 0) { if isOn(x + j, y + i) { onCount += 1 } } } if (x, y) == (0, 0) || (x, y) == (0, 99) || (x, y) == (99, 0) || (x, y) == (99, 99) { result.append(on) } else if isOn(x, y) { result.append((onCount == 2 || onCount == 3) ? on : off) } else { result.append(onCount == 3 ? on : off) } } } return result } var current = Array(input) current[0] = on current[99] = on current[99 * 100 + 0] = on current[99 * 100 + 99] = on for _ in 0..<100 { current = step(input: current) } let count = current.lazy.filter { $0 == on }.count print("\(count) lights on")