class Alphanumeric(object):
    
    ALPHABET_LENGTH = 26

    def __init__(self, nrange=None):      
        self.current_letter = 'z'
        self.current_number = 0
        self.nrange = nrange

    def forward_letter(self, letter, positions):        
        if letter.islower():
            unicode_point = ord('a')
        else:
            unicode_point = ord('A')

        start = ord(letter) - unicode_point
        offset = ((start + positions) % self.ALPHABET_LENGTH) + unicode_point
        self.current_letter = chr(offset)
        return self.current_letter

    def backward_letter(self, letter, positions):        
        if letter.islower():
            unicode_point = ord('a')
        else:
            unicode_point = ord('A')

        start = ord(letter) - unicode_point
        offset = ((start - positions) % self.ALPHABET_LENGTH) + unicode_point
        self.current_letter = chr(offset)
        return self.current_letter

    def next_letter(self):        
        return self.forward_letter(self.current_letter, 1)

    def previous_letter(self):        
        return self.backward_letter(self.current_letter, 1)

    def forward_number(self, number, positions):        
        if not self.nrange:
            self.current_number = number + positions
            return self.current_number

        index = self.nrange.index(number)
        start = index + positions
        offset = (start % len(self.nrange))
        self.current_number = self.nrange[offset]
        return self.current_number

    def backward_number(self, number, positions):        
        if not self.nrange:
            return number - positions

        index = self.nrange.index(number)
        start = index - positions
        offset = (start % len(self.nrange))
        self.current_number = self.nrange[offset]
        return self.current_number

    def next_number(self):        
        return self.forward_number(self.current_number, 1)

    def previous_number(self):       
        return self.backward_number(self.current_number, 1)

    def forward_alphanumeric(self, alpha, positions, ignore_numbers=False, ignore_letters=False):        
        result = ""
        for char in alpha:
            if char.isdigit() and not ignore_numbers:
                char = str(self.forward_number(int(char), positions))

            if char.isalpha() and not ignore_letters:
                char = self.forward_letter(char, positions)

            result += char

        return result

    def backward_alphanumeric(self, alpha, positions, ignore_numbers=False, ignore_letters=False):        
        result = ""
        for char in alpha:
            if char.isdigit() and not ignore_numbers:
                char = str(self.backward_number(int(char), positions))

            if char.isalpha() and not ignore_letters:
                char = self.backward_letter(char, positions)

            result += char

        return result