String+Substring.swift

import Foundation

public extension String {

    /// just a wrapper for int type
    typealias IndexInt = Int

    /// just a wrapper for int type
    typealias DistanceInt = Int

    /**
     Gets the subtring from start[inclusive] and end index[inclusive]
     - Parameter start: int index where you want to start substring
     - Parameter end: int index where you want it stop
     - Returns: Another string acording limits
     */
    func substring(start: IndexInt, end: IndexInt) -> String {
        let startIndex = self.index(self.startIndex, offsetBy: start)

        var endI: String.Index?
        
        if end < 0 {
            endI = self.index(self.startIndex, offsetBy: self.count + end)
        } else {
            endI = self.index(self.startIndex, offsetBy: end + 1)
        }
        
        guard let endIndex = endI else {
            return ""
        }
        
        return "\(self[startIndex..<endIndex])"
    }

    /**
     Gets the subtring from start[inclusive] and distance
     - Parameter start: int index where you want to start substring
     - Parameter distance: int indicating how spaces it should move from start
     - Returns: Another string acording limits
     */
    func substring(start: IndexInt, distance: DistanceInt) -> String {
        return substring(start: start, end: start + distance - 1)
    }

    /**
     Gets the subtring from start[inclusive] to the end of the string
     - Parameter start: int index where you want to start substring
     - Returns: Another string acording limits
     */
    func substring(from: IndexInt) -> String {
        return substring(start: from, end: self.count - 1)
    }

    /**
     Gets the character at specific position, could be negative
     - Parameter index: int index where you want the character
     - Returns: Another string with the character
     */
    func character(at index: IndexInt) -> String {
        if index < 0 {
            if self.count + index < 0 {
                return ""
            }
            
            return substring(start: self.count + index, distance: 1)
        }
        
        return substring(start: index, distance: 1)
    }
}