AoC/2015/Day18.swift

let input = """
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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")