Aoc2018Day11.swift

let totalSize = 300

func powerLevel(x: Int, y: Int, serialNumber: Int) -> Int {
    let rackID = x + 10
    return ((rackID * y + serialNumber) * rackID / 100) % 10 - 5
}

func part2(serialNumber: Int) -> String {
    let grid = (1...totalSize).map { y in
        (1...totalSize).map { x in
            powerLevel(x: x, y: y, serialNumber: serialNumber)
        }
    }
    
    let partialX = grid.map { $0.scan(0, +) }
    let partial = (0..<totalSize).map { x in partialX.lazy.map { $0[x] }.scan(0, +) }
    
    var maxSum = Int.min
    var solution: (x: Int, y: Int, size: Int)!
    
    for size in 1...totalSize {
        for x in 0..<(totalSize - size) {
            for y in 0..<(totalSize - size) {
                let sum = partial[x + size][y + size]
                    - partial[x][y + size]
                    - partial[x + size][y]
                    + partial[x][y]
                
                if sum > maxSum {
                    maxSum = sum
                    solution = (x + 1, y + 1, size)
                }
            }
        }
    }
    
    return "\(solution.x),\(solution.y),\(solution.size)"
}

Scan.swift

extension Sequence {
    func scan<T>(_ initial: T, _ next: (T, Element) -> T) -> [T] {
        var result = [initial]
        var last = initial
        
        for element in self {
            last = next(last, element)
            result.append(last)
        }
        
        return result
    }
}