ace-teknologi/abra

package abra

import (
    "fmt"
    "math"
    "strconv"
    "strings"
)

// ABN represents an actual Australian Business Number
type ABN struct {
    IdentifierValue         string  `xml:"identifierValue,omitempty"`
    IdentifierStatus        string  `xml:"identifierStatus,omitempty"`
    IsCurrentIndicator      string  `xml:"isCurrentIndicator,omitempty"`
    ReplacedIdentifierValue string  `xml:"replacedIdentifierValue,omitempty"`
    ReplacedFrom            abnDate `xml:"replacedFrom,omitempty"`
}

// IsValid checks whether your ABN has a valid identifier
// (https://www.abr.business.gov.au/HelpAbnFormat.aspx)
func (a *ABN) IsValid() (bool, error) {
    return ValidateABN(a.IdentifierValue)
}

// String returns the ABN as a formatted string
func (a *ABN) String() string {
    return a.IdentifierValue
}

// ValidateABN tests a string to see if it is a valid ABN
func ValidateABN(abn string) (bool, error) {
    // Strip whitespace
    raw := strings.Replace(abn, " ", "", -1)
    if len(raw) != 11 {
        return false, fmt.Errorf("Invalid ABN: found a %d character string", len(raw))
    }

    var checksum int
    // compute the checksum
    for i, char := range raw {
        digit, err := strconv.Atoi(string(char))
        if err != nil {
            return false, fmt.Errorf("Invalid ABN: found character %s", string(char))
        }
        if i == 0 {
            // For the first digit, subtract 1 then multiply by ten
            checksum += (digit - 1) * 10
        } else {
            // For all other digits, multiply by (2i - 1)
            checksum += (digit * (2*i - 1))
        }
    }

    if math.Mod(float64(checksum), float64(89)) > 0 {
        return false, fmt.Errorf("Invalid checksum: %d", checksum)
    }

    return true, nil
}