1 files changed, 155 insertions, 0 deletions
diff --git a/skate/isbn/isbn.go b/skate/isbn/isbn.go
new file mode 100644
index 0000000..c50eaaa
--- /dev/null
+++ b/skate/isbn/isbn.go
@@ -0,0 +1,155 @@
+// 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
+}