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adding ISBN 13 validator

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api-first-development-service/src/main/java/com/csaba79coder/apifirstdevelopment/util
- ISBN13Validator.java

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	`1`	`+package com.csaba79coder.apifirstdevelopment.util;`
	`2`	`+`
	`3`	`+public class ISBN13Validator {`
	`4`	`+`
	`5`	`+ public static boolean validISBN(Long isbnLong) {`
	`6`	`+ String isbn = String.valueOf(isbnLong);`
	`7`	`+ if (isbn == null) {`
	`8`	`+ return false;`
	`9`	`+ }`
	`10`	`+ if (isbn.startsWith("-")) {`
	`11`	`+ return false;`
	`12`	`+ }`
	`13`	`+ isbn = isbn.replaceAll( "-", "" );`
	`14`	`+ if ( isbn.length() != 13 ) {`
	`15`	`+ return false;`
	`16`	`+ }`
	`17`	`+ try {`
	`18`	`+ int tot = 0;`
	`19`	`+ for (int i = 0; i < 12; i++) {`
	`20`	`+ int digit = Integer.parseInt( isbn.substring( i, i + 1 ) );`
	`21`	`+ tot += (i % 2 == 0) ? digit : digit * 3;`
	`22`	`+ }`
	`23`	`+ int checksum = 10 - (tot % 10);`
	`24`	`+ if ( checksum == 10 ) {`
	`25`	`+ checksum = 0;`
	`26`	`+ }`
	`27`	`+ return checksum == Integer.parseInt( isbn.substring( 12 ) );`
	`28`	`+ }`
	`29`	`+ catch (NumberFormatException nfe) {`
	`30`	`+ return false;`
	`31`	`+ }`
	`32`	`+ }`
	`33`	`+`
	`34`	`+ private ISBN13Validator() {`
	`35`	`+`
	`36`	`+ }`
	`37`	`+}`

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