Ticket #1604: isbn.js

// Implementation of ISBN check-digit verification
// Based on ISBN Users' Manual (http://www.isbn.org/standards/home/isbn/international/html/usm4.htm)
// and the Wikipedia treatment of ISBN (http://en.wikipedia.org/wiki/International_Standard_Book_Number)

// This does not validate multiple ISBNs in one field,
// but it will gracefully ignore all non-number detritus,
// such as hyphens, spaces, and comments.

// Takes the first 10 valid digits and tries to read an ISBN 10,
// and takes the first 13 valid digits to try to read an ISBN 13
// Returns an four-element array:
//      [0] the validity as an ISBN 10
//      [1] detected ISBN 10 that checks
//      [2] validity as an ISBN 13
//      [3] detected ISBN 13 that checks
// If there is no valid ISBN of a type detected, then elements 1 and/or 3 will
// be empty. There could conceivably be a valid ISBN-13 with an ISBN-10
// substring; this should probably be interpreted as the latter, but it is a
// client UI issue.
ISBNCheck = function(isbn) {
        // For ISBN 10, multiple by these coefficients, take the sum mod 11
        // and subtract from 11
        var isbn10 = [10, 9, 8, 7, 6, 5, 4, 3, 2];

        // For ISBN 13, multiple by these coefficients, take the sum mod 10
        // and subtract from 10
        var isbn13 = [1, 3, 1, 3, 1, 3, 1, 3, 1, 3, 1, 3];

        // We make a single pass through the provided string, interpreting the
        // first 10 valid characters as an ISBN-10, and the first 13 as an
        // ISBN-13. We then return an array of booleans and valid detected
        // ISBNs.

        var j = 0;
        var sum10 = 0;
        var num10 = "";
        var sum13 = 0;
        var num13 = "";
        var chars = [];

        for (var i=0; i < isbn.length; i++) {
                if (isbn.charAt(i) == " ") {
                        // Since the space character evaluates as a number,
                        // it is a special case.
                } else if (j < 9 && ((isbn.charAt(i) - 0) == isbn.charAt(i))) {
                        sum10 += isbn.charAt(i) * isbn10[j];
                        sum13 += isbn.charAt(i) * isbn13[j];
                        num10 += isbn.charAt(i);
                        num13 += isbn.charAt(i);
                        j++;
                } else if (j == 9 &&
                        (isbn.charAt(i) == "X" || isbn.charAt(i) == "x" ||
                                ((isbn.charAt(i) - 0) == isbn.charAt(i)))) {
                        // In ISBN-10, an X represents the check digit "10".
                        if(isbn.charAt(i) == "X" || isbn.charAt(i) == "x") {
                                var check10 = 10;
                                num10 += "X";
                        } else {
                                var check10 = isbn.charAt(i);
                                sum13 += isbn.charAt(i) * isbn13[j];
                                num10 += isbn.charAt(i);
                                num13 += isbn.charAt(i);
                                j++;
                        }
                } else if(j < 12 && ((isbn.charAt(i) - 0) == isbn.charAt(i))) {
                        sum13 += isbn.charAt(i) * isbn13[j];
                        num13 += isbn.charAt(i);
                        j++;
                } else if (j == 12 && ((isbn.charAt(i) - 0) == isbn.charAt(i))) {
                        var check13 = isbn.charAt(i);
                        num13 += isbn.charAt(i);
                }
        }
        var valid10 = (11 - (sum10 % 11) == check10);
        var valid13 = (10 - (sum13 % 10) == check13);
        if(!valid10) {num10 = ""};
        if(!valid13) {num13 = ""};
        return [valid10, num10, valid13, num13];
}