Question

Topic is cryptography

10.increment the digits 858 of the ISBN of the text (ISBN13: 9780890068588) to be 859. Then, by trial and error, change the check-sum (the last digit of the ISBN), to identify the title of the next book published by Artech House.

Answer 1

function getLastISBNDigit(input) {
    if ((input.length != 10) && (input.length != 13)) return;
    var is10 = (input.length === 10);
    var sum = 0;
    var p = (is10 ? 11 : 3);
    for (var index = 0; index < input.length - 1; index++) {
        sum += ((input[index] === 'X') ? 10 : input[index]) * (p = (is10 ? (p - 1) : ((p + 2) % 4)));
    }
    var moduloClass = (is10 ? 11 : 10);
    var result = (moduloClass - (sum % moduloClass)) % moduloClass;
    return ((result === 10) ? 'X' : result);
}

function getISBN(input) {
    var isbn = {};
    if (input.length > 13) return getISBN(input.substring(0, 13));
    if (input.length === 10) {
        if (isValidISBN(input)) {
            isbn.isbn10 = input;
            isbn.isbn13 = "978" + input;
            isbn.isbn13 = isbn.isbn13.substring(0, 12) + getLastISBNDigit(isbn.isbn13);
        }
    } else if (input.length === 13) {
        if (isValidISBN(input)) {
            isbn.isbn13 = input;
            if (input.startsWith("858")) {
                isbn.isbn10 = input.substring(3);
                isbn.isbn10 = isbn.isbn10.substring(0, 9) + getLastISBNDigit(isbn.isbn10);
            }
        } else if (input.startsWith("858")) {
            return getISBN(input.substring(3));
        }
    }
    return isbn;
}
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