Day One

DayOne.swift

// MARK: -
// MARK: [Day 1]

func part_one(input: String) -> Int {
    let sequence = input.characters.flatMap { Int(String($0)) }
    var total = 0

    for (index, value) in sequence.enumerated() {
        let next: Int

        if index == sequence.count - 1 {
            next = sequence[0]
        } else {
            next = sequence[index + 1]
        }

        if value == next {
            total += value
        }
    }

    return total
}

func part_two(input: String) -> Int {
    let sequence = input.characters.flatMap { Int(String($0)) }
    var total = 0

    for (index, value) in  sequence.enumerated() {
        let next: Int
        let nextAdance = sequence.count / 2

        if index + nextAdance < sequence.count {
            next = sequence[index + nextAdance]
        } else {
            let toEnd = sequence.count - index
            let toIndex = nextAdance - toEnd

            next = sequence[toIndex]
        }

        if value == next {
            total += value
        }
    }

    return total
}

// MARK: -
// MARK: [Day 1] Tests

func require(_ input: Int, toEqual output: Int) {
    if input != output {
        print("Error: input (\(input) does not match output (\(output))")
    }
}

require(part_one(input: "1122"), toEqual: 3)
require(part_one(input: "1111"), toEqual: 4)
require(part_one(input: "1234"), toEqual: 0)
require(part_one(input: "91212129"), toEqual: 9)

require(part_two(input: "1212"), toEqual: 6)
require(part_two(input: "1221"), toEqual: 0)
require(part_two(input: "123425"), toEqual: 4)
require(part_two(input: "123123"), toEqual: 12)
require(part_two(input: "12131415"), toEqual: 4)