GST(Indian) check digit validation algorithm2 min read

GST number validation is done through 2 steps

  1. Check if there is a character in position of a character and a number in position of a number.
  2. Validate the check digit that is the last character in the GST no. with  Luhn mod N algorithm (https://en.wikipedia.org/wiki/Luhn_mod_N_algorithm)

An implementation in javascript

let GST = {
  /*
      Luhn mod N algorithm    
      https://en.wikipedia.org/wiki/Luhn_mod_N_algorithm
  */
  chars: '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ',
  pattern: /[0-9]{2}[a-zA-Z]{5}[0-9]{4}[a-zA-Z]{1}[1-9A-Za-z]{1}[Zz1-9A-Ja-j]{1}[0-9a-zA-Z]{1}/,

  generate_check_charcter: function(input) {
    factor = 2;
    sum = 0;
    n = this.number_of_valid_input_chars();

    for (i = input.length - 1; i >= 0; i--) {
      code_point = this.code_point_from_char(input[i]);
      addend = factor * code_point;

      factor = (factor == 2) ? 1 : 2;

      addend = (addend / n) + (addend % n);
      sum += Math.floor(addend);
    }

    remainder = sum % n;
    check_code_point = (n - remainder) % n;

    return this.char_from_code_point(check_code_point);
  },

  validate_check_character: function(input) {
    factor = 1;
    sum = 0;
    n = this.number_of_valid_input_chars();

    for (i = input.length - 1; i >= 0; i--) {
      code_point = this.code_point_from_char(input[i]);
      addend = factor * code_point;

      factor = (factor == 2) ? 1 : 2;

      addend = parseInt(addend / n) + (addend % n);
      sum += addend;
    }

    remainder = sum % n;

    return (remainder == 0);
  },

  number_of_valid_input_chars: function() {
    return this.chars.length;
  },

  code_point_from_char: function(input) {
    let str = this.chars;
    for (let i = 0, q = str.length; i < q; i++) {
      if (input == str[i]) {
        return i;
      }
    }
  },

  char_from_code_point: function(input) {
    let str = this.chars;
    for (let i = 0, q = str.length; i < q; i++) {
      if (input == i) {
        return str[i];
      }
    }
  },

  /*
      Function called to check if a GSTno is valid.
      Full GST.no is passed to the func
  */
  is_valid_num: function(gst_no) {
    //Check for pattern
    var patt = new RegExp(this.pattern);
    if (!patt.test(gst_no)) {
      console.log('pattern wrong');
      return false;
    }

    //Validate the check digit
    return this.validate_check_character(gst_no);
  }

};

USAGE:

GST.is_valid_num(gst_no);

true  is returned if the GST.no is valid. false  if not.

Comment below if you have any questions.

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