CHECKSUM_ALGORITHMS.md

Check Digit Algorithms

This page defines the basic check digit algorithms used by couriers to verify that a tracking number is valid.

This reference page is useful for anyone wishing to use this repository to create a new tracking number library in your language. The examples below are in ruby, and are taken directly from tracking_number. Keep in mind that the implementation in your language may already exist!

  def validates_mod7?(sequence, check_digit, extras = {})
    # standard mod 7 check
    return true if sequence.to_i % 7 == check_digit.to_i
  end

  def validates_mod10?(sequence, check_digit, extras = {})
    total = 0
    sequence.chars.each_with_index do |c, i|
      x = if c[/[0-9]/] # numeric
        c.to_i
      else
        (c[0].ord - 3) % 10
      end

      if extras[:odds_multiplier] && i.odd?
        x *= extras[:odds_multiplier].to_i
      elsif extras[:evens_multiplier] && i.even?
        x *= extras[:evens_multiplier].to_i
      end

      total += x
    end

    check = (total % 10)
    check = (10 - check) unless (check.zero?)

    return (check.to_i == check_digit.to_i)
  end

  def validates_s10?(sequence, check_digit, extras = {})
    weighting = [8,6,4,2,3,5,9,7]

    total = 0
    sequence.chars.to_a.zip(weighting).each do |(a,b)|
      total += a.to_i * b.to_i
    end

    remainder = total % 11
    check = case remainder
    when 1
      0
    when 0
      5
    else
      11 - remainder
    end

    return check.to_i == check_digit.to_i
  end

  def validates_sum_product_with_weightings_and_modulo?(sequence, check_digit, extras = {})
    weighting = extras[:weightings] || []

    total = 0
    sequence.chars.to_a.zip(weighting).each do |(a,b)|
      total += a.to_i * b
    end
    return (total % extras[:modulo1] % extras[:modulo2]) == check_digit.to_i
  end

  def validates_mod_37_36?(sequence, check_digit, extras = {})
    # From https://esolutions.dpd.com/dokumente/DPD_Parcel_Label_Specification_2.4.1_EN.pdf

    mod = 36
    weights = {A: 10, B: 11, C: 12, D: 13, E: 14, F: 15, G: 16, H: 17, I: 18, J: 19, K: 20, L: 21, M: 22, N: 23, O: 24, P: 25, Q: 26, R: 27, S: 28, T: 29, U: 30, V: 31, W: 32, X: 33, Y: 34, Z: 35}
    cd = mod
    sequence.chars.to_a.each do |char, i|
      val = (char =~ /[A-Za-z]/  ? weights[char.to_sym] : char.to_i)

      cd = val + cd
      cd = cd - mod if cd > mod
      cd = cd * 2
      cd = cd - (mod + 1) if cd > mod
    end

    cd = (mod + 1) - cd
    cd = 0 if cd == mod
    computed = if cd >= 10
                 weights.find { |a, val| val == cd }[0].to_s
               else
                 cd.to_s
               end

    computed == check_digit
  end