Advent of Code 2018, day 1

aoc2018d1.swift

// prepare the puzzle input
let input = loadFile(named: "day1") // convenience function in resources
    .split(separator: "\n")
    .compactMap(Int.init) // extension on Int to init? with String.Subsequence

// part 1
let part1 = input.reduce(0, +)

// part 2

// Make an infinite sequence from the input
let frequencies = sequence(state: input.startIndex) { (index) -> Int? in
    defer {
        index = index.advanced(by: 1)
    }
    if index == input.endIndex {
        index = input.startIndex
    }
    return input[index]
}

var history = Set<Int>([0])
var current = 0

for frequency in frequencies {
    current += frequency
    if !history.contains(current) {
        history.insert(current)
        continue
    }
    break
}

let part2 = current