ISIN code

GetISINcode.cs

public static string GetIsincode(string country, string cusip)
{
  string s = (country + cusip).ToUpper();
  StringBuilder sb = new StringBuilder();
  foreach (char c in s)
  {
    if (c <= 'Z' && c >= 'A') sb.Append(((c - 'A') + 10).ToString());
    else sb.Append(c.ToString());
  }

  bool isTimes2 = true;
  int r = 0;
  for (int i = sb.Length - 1; i >= 0; i--)
  {
    int d = sb[i] - '0';
    if (isTimes2) r += (d * 2) / 10 + (d * 2) % 10;
    else r += d;
    isTimes2 = !isTimes2;
  }

  return s + (10 - (r % 10)).ToString();
}

GetISINcode.sql

IF EXISTS (SELECT 1 FROM sysobjects WHERE id = OBJECT_ID('[fn_GetISINcode]') AND xtype IN ('FN','IF','TF'))
   DROP FUNCTION [fn_GetISINcode];
GO

CREATE FUNCTION [fn_GetISINcode](@country VARCHAR(2), @cussip VARCHAR(9))
RETURNS VARCHAR(12)
BEGIN
  DECLARE @cc VARCHAR(11) = UPPER(@country) + UPPER(@cussip)
  DECLARE @tmp VARCHAR(22) = ''
  DECLARE @idx INT = 1;
  WHILE(@idx <= 11)
  BEGIN
    DECLARE @cur_ascii INT = ASCII(SUBSTRING(@cc, @idx, 1));
    IF @cur_ascii BETWEEN 65 AND 90                        --A ~ Z
    BEGIN
      SET @tmp = @tmp + CAST(@cur_ascii-55 AS VARCHAR(2))
    END
    ELSE                                                   --0 ~ 9
    BEGIN
      SET @tmp = @tmp + CAST(@cur_ascii-48 AS VARCHAR(1))
    END
    
    SET @idx = @idx + 1
  END
  
  DECLARE @check_sum INT = 0
  DECLARE @reverse_tmp VARCHAR(22) = REVERSE(@tmp)
  DECLARE @ridx INT = 1
  WHILE (@ridx <= LEN(@reverse_tmp))
  BEGIN
    DECLARE @rev_cur_ascii INT = ASCII(SUBSTRING(@reverse_tmp, @ridx, 1)) - 48;
    IF @ridx % 2 = 1 
    BEGIN
      SET @rev_cur_ascii = @rev_cur_ascii * 2
    END
    
    SET @check_sum = @check_sum + @rev_cur_ascii / 10 + @rev_cur_ascii % 10
    
    SET @ridx = @ridx + 1
  END
  
  DECLARE @check_digit INT = 10 - @check_sum % 10
  
  RETURN @cc + CAST(@check_digit AS CHAR(1))
END
GO