07_more_space.swift

import Foundation

func add(_ char:Character, to input:String, pattern:[UInt]) -> String {
    guard pattern.reduce(0, +) > 0 else { return input }
    var str = input
    var index:String.Index? = str.startIndex
    var i = 0
    while index != nil {
        let offset = pattern[i%pattern.count]
        i += 1
        guard offset > 0 else {
            continue
        }
        guard let newIndex = index else {
            return str
        }
        index = str.index(newIndex,
                          offsetBy: Int(offset),
                          limitedBy: str.endIndex)
        guard let solidIndex = index else {
            return str
        }
        guard solidIndex != str.endIndex else {
            return str
        }
        str.insert(char, at: solidIndex)
        index = str.index(solidIndex,
                          offsetBy:1,
                          limitedBy: str.endIndex)
    }
    return str
}


var str = """

Helloplayground,howareyou?
Helloplayground,howareyou?
Helloplayground,howareyou?
"""
let offsets:[UInt] = [6, 11, 3, 3, 4]

let spaced = add(" ", to: str, pattern: offsets)
print(spaced)

print(add("|",to: "aaa", pattern: []))

print(add("|",to: "aaaaaaa", pattern: [0]))

print(add("|", to: "aaaaa", pattern: [0, 0, 0, 0]))

print(add("|", to: "", pattern: []))

print(add("|", to: "aaaaaaa", pattern: [1]))