mod10.py

def mod10(sequence):
    factor = 1
    intermediate = ""
    for i in range(len(sequence) - 1, -1, -1):
        factor = 2 if factor == 1 else 1
        intermediate += str(int(sequence[i]) * factor)
    remainder = sum([int(d) for d in intermediate]) % 10

    return 0 if remainder == 0 else 10 - remainder
    
    
assert mod10("8220000215048200974123220154098290108605940") == 1
assert mod10("261533") == 4
assert mod10("2615332") == 0

mod10.rb

def mod10(sequence)
    factor = 1
    intermediate = ""
    sequence.reverse.split("").each do |d|
        factor = factor == 1 ? 2 : 1
        intermediate << (d.to_i * factor).to_s
    end
    remainder = intermediate.split("").collect{|d| d.to_i}.inject{|sum, i| sum + i} % 10
    
    remainder == 0 ? 0 : 10 - remainder
end

mod10("8220000215048200974123220154098290108605940") == 1 or raise "Failed!" 
mod10("261533") == 4 or raise "Failed!" 
mod10("2615332") == 0 or raise "Failed!"