string_extensions.swift

extension String
{
    func character(at index: Int) -> Character?
    {
        guard index <= self.count && index >= -self.count && index != 0 else {
            return nil
        }
        return Array(self)[(index > 0 ? index - 1 : self.count + index)]
    }
    
    func occurrences(of character: Character) -> Int
    {
        return (self.filter { $0 == character } as String).count
    }
}