indents.swift

enum StringPaddingStyle {
    case Left, Right
}

func padString(source: String, 
        with character: Character = " ",
        fill target: Int, 
        style: StringPaddingStyle = .Left
    ) -> String {
    
        let count  = max(target - source.characters.count, 0)
        let insert = String(count: count, repeatedValue: character)

        return  style == .Left
                ? insert + source
                : source + insert
}

indents2.swift

enum StringPaddingStyle {
    case Left, Right, Center
}


extension String {
    
    func padded(with character: Character = " ",
            to target: Int,
            style: StringPaddingStyle = .Left
        ) -> String {
            
            let total = max(target - characters.count, 0)
            let left: Int
            
            switch style {
                case .Left:   left = 0
                case .Right:  left = total
                case .Center: left = total / 2
            }
            
            let 🔨 = { String(count: $0, repeatedValue: character) }
            
            return 🔨(left) + self + 🔨(total - left)
    }
}

"string".padded(with: "_", to: 10, style: .Center)
"another".padded(to: 15)

indents3.swift

enum StringPaddingStyle {
    case Left, Right, Center
}


func *(style: StringPaddingStyle, length: Int) -> Int {
  
    switch style {
        case .Left:   return 0
        case .Right:  return length
        case .Center: return length / 2
    }
    
}


extension String {
    
    func padded(with character: Character = " ",
            to target: Int,
            style: StringPaddingStyle = .Left
        ) -> String {
            
            let 🔨 = { String(count: $0, repeatedValue: character) }
            
            let total = max(target - characters.count, 0)
            let left  = style * total
            let right = total - left
            
            return 🔨(left) + self + 🔨(right)
    }
    
}

"string".padded(with: "_", to: 10, style: .Center)
"another".padded(to: 15)