AmirSoleimani/IRValidator

package validate

import (
    "errors"
    "math/big"
    "strconv"
    "strings"
)

//IBAN Check IBAN Number
func IBAN(value string) error {

    value = strings.ToUpper(value)

    if len(value) >= 4 {
        //get the first four
        firstFour := value[:4]

        //replace the first four
        value = strings.Replace(value, firstFour, "", -1)

        //set char number to the first four
        for _, r := range firstFour[:2] {
            s := string(r)
            firstFour = strings.Replace(firstFour, s, strconv.Itoa(int([]rune(s)[0])-55), -1)
        }

        //add end of string
        value = value + firstFour

        //convert string to bigInt
        i, err := big.NewInt(0).SetString(value, 10)
        if !err {
            return errors.New("IBAN > convert problem")
        }

        //set divider value
        j := big.NewInt(97)

        //div->mod
        k := big.NewInt(0).Mod(i, j)

        //result
        if k.Cmp(big.NewInt(1)) != 0 { //Correct
            return errors.New("IBAN > problem")
        }

    }

    return nil
}