Move all the code to a lib folder

lib/parser/isbn.go

@@ -0,0 +1,81 @@
+package parser
+
+import (
+	"strings"
+	"unicode"
+)
+
+func ISBN(orig string) string {
+	isbn := getISBN(orig)
+
+	if len(isbn) != 13 && len(isbn) != 10 {
+		return ""
+	}
+	if !validChecksum(isbn) {
+		return ""
+	}
+
+	return toISBN13(isbn)
+}
+
+func getISBN(src string) string {
+	isbn := strings.ToUpper(src)
+	isNotNumber := func(r rune) bool {
+		return !unicode.IsNumber(r)
+	}
+	isNotNumberOrX := func(r rune) bool {
+		return !unicode.IsNumber(r) && r != 'X'
+	}
+
+	isbn = strings.TrimLeftFunc(isbn, isNotNumber)
+	isbn = strings.TrimRightFunc(isbn, isNotNumberOrX)
+	isbn = strings.Replace(isbn, "-", "", -1)
+	isbn = strings.Replace(isbn, " ", "", -1)
+
+	if len(isbn) > 13 {
+		isbn = isbn[:13]
+	}
+	return isbn
+}
+
+func validChecksum(isbn string) bool {
+	if len(isbn) == 10 {
+		return rune(isbn[9]) == checkDigit10(isbn)
+	}
+	return rune(isbn[12]) == checkDigit13(isbn)
+}
+
+func toISBN13(isbn string) string {
+	if len(isbn) == 13 {
+		return isbn
+	}
+
+	isbn = "978" + isbn
+	return isbn[:12] + string(checkDigit13(isbn))
+}
+
+func checkDigit10(isbn string) rune {
+	acc := 0
+	for i, r := range isbn[:9] {
+		acc += (10 - i) * int(r-'0')
+	}
+	check := (11 - (acc % 11)) % 11
+
+	if check == 10 {
+		return 'X'
+	}
+	return rune(check + '0')
+}
+
+func checkDigit13(isbn string) rune {
+	acc := 0
+	for i, r := range isbn[:12] {
+		n := int(r - '0')
+		if i%2 == 1 {
+			n = 3 * n
+		}
+		acc += n
+	}
+	check := (10 - (acc % 10)) % 10
+	return rune(check + '0')
+}