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// Copyright 2015 Rodrigo Moraes. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
/*
Package isbn provides functions to validate ISBN strings, calculate ISBN
check digits and convert ISBN-10 to ISBN-13.
*/
package isbn
import (
"fmt"
"strconv"
)
// sum10 returns the weighted sum of the provided ISBN-10 string. It is used
// to calculate the ISBN-10 check digit or to validate an ISBN-10.
//
// The provided string must have a length of 9 or 10 and no formatting
// characters (spaces or hyphens).
func sum10(isbn string) (int, error) {
s := 0
w := 10
for k, v := range isbn {
if k == 9 && v == 88 {
// Handle "X" as the digit.
s += 10
} else {
n, err := strconv.Atoi(string(v))
if err != nil {
return -1, fmt.Errorf("Failed to convert ISBN-10 character to int: %s", string(v))
}
s += n * w
}
w--
}
return s, nil
}
// sum13 returns the weighted sum of the provided ISBN-13 string. It is used
// to calculate the ISBN-13 check digit or to validate an ISBN-13.
//
// The provided string must have a length of 12 or 13 and no formatting
// characters (spaces or hyphens).
func sum13(isbn string) (int, error) {
s := 0
w := 1
for _, v := range isbn {
n, err := strconv.Atoi(string(v))
if err != nil {
return -1, fmt.Errorf("Failed to convert ISBN-13 character to int: %s", string(v))
}
s += n * w
if w == 1 {
w = 3
} else {
w = 1
}
}
return s, nil
}
// CheckDigit10 returns the check digit for an ISBN-10.
//
// The provided string must have a length of 9 or 10 and no formatting
// characters (spaces or hyphens). For a 10-length string, the last character
// (the digit) is ignored since that is what is being (re)calculated.
func CheckDigit10(isbn10 string) (string, error) {
if len(isbn10) != 9 && len(isbn10) != 10 {
return "", fmt.Errorf("A string of length 9 or 10 is required to calculate the ISBN-10 check digit. Provided was: %s", isbn10)
}
s, err := sum10(isbn10[:9])
if err != nil {
return "", err
}
d := (11 - (s % 11)) % 11
if d == 10 {
return "X", nil
}
return strconv.Itoa(d), nil
}
// CheckDigit13 returns the check digit for an ISBN-13.
//
// The provided string must have a length of 12 or 13 and no formatting
// characters (spaces or hyphens). For a 13-length string, the last character
// (the digit) is ignored since that is what is being (re)calculated.
func CheckDigit13(isbn13 string) (string, error) {
if len(isbn13) != 12 && len(isbn13) != 13 {
return "", fmt.Errorf("A string of length 12 or 13 is required to calculate the ISBN-13 check digit. Provided was: %s", isbn13)
}
s, err := sum13(isbn13[:12])
if err != nil {
return "", err
}
d := 10 - (s % 10)
if d == 10 {
return "0", nil
}
return strconv.Itoa(d), nil
}
// Validate returns true if the provided string is a valid ISBN-10 or ISBN-13.
//
// The provided string must have a length of 10 or 13 and no formatting
// characters (spaces or hyphens).
func Validate(isbn string) bool {
switch len(isbn) {
case 10:
return Validate10(isbn)
case 13:
return Validate13(isbn)
}
return false
}
// Validate10 returns true if the provided string is a valid ISBN-10.
//
// The provided string must have a length of 10 and no formatting
// characters (spaces or hyphens).
func Validate10(isbn10 string) bool {
if len(isbn10) == 10 {
s, _ := sum10(isbn10)
return s%11 == 0
}
return false
}
// Validate13 returns true if the provided string is a valid ISBN-13.
//
// The provided string must have a length of 13 and no formatting
// characters (spaces or hyphens).
func Validate13(isbn13 string) bool {
if len(isbn13) == 13 {
s, _ := sum13(isbn13)
return s%10 == 0
}
return false
}
// To13 converts an ISBN-10 to an ISBN-13.
//
// The provided string must have a length of 9 or 10 and no formatting
// characters (spaces or hyphens).
func To13(isbn10 string) (string, error) {
if len(isbn10) != 9 && len(isbn10) != 10 {
return "", fmt.Errorf("A string of length 9 or 10 is required to convert an ISBN-10 to an ISBN-13. Provided was: %s", isbn10)
}
isbn13 := "978" + isbn10[:9]
d, err := CheckDigit13(isbn13)
if err != nil {
return "", err
}
return isbn13 + d, nil
}
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