Validator of national identification number used in the Czech Republic.

RCValidator.java

import static java.util.Calendar.DAY_OF_MONTH;
import static java.util.Calendar.MONTH;
import static java.util.Calendar.YEAR;

import java.util.Calendar;

/**
 * 
 * Validator of national identification number used in the Czech Republic.
 * 
 * See wiki <a href="http://cs.wikipedia.org/wiki/Rodn%C3%A9_%C4%8D%C3%ADslo">http://cs.wikipedia.org/wiki/Rodn%C3%A9_%C4%8D%C3%ADslo</a>
 * 
 * @author banterCZ
 */
public class RCValidator {

	/**
	 * Validates the given national identification number. 
	 * 
	 * @param value to validate (without slash, e.g. 7310285169), may be <code>null</code> or an empty string but returns <code>false</code> in that case
	 * @return <code>true</code> when number is valid, <code>false</code> otherwise
	 */
	public boolean isValid(String value) {
		if (value == null) {
			return false;
		}
		
		if (value.length() < 9 || value.length() > 10) {
			return false;
		}
		
		int year;
		int month;
		int day;
		
		try {
			year = Integer.parseInt(value.substring(0, 2));
			month = Integer.parseInt(value.substring(2, 4));
			day = Integer.parseInt(value.substring(4, 6));
		} catch (NumberFormatException e) {
			return false;
		}
		
		if (value.length() == 9 && (year >= 54)) {
			return false;
		}
		
		if (value.length() == 10) {
			long mod = Long.parseLong(value.substring(0, 9)) % 11;
			int lastNumber = Integer.parseInt(value.substring(9));
			if (mod == 10) {
				if (lastNumber != 0) {
					return false;
				}
			} else if (lastNumber != mod) {					
				return false;
			}
		}
		
		return isDayAndMonthValid(value, year, month, day);
	}

	private boolean isDayAndMonthValid(String value, int year, int month, int day) {
		year = getYearWithHunders(year, value);
		month = getRealMonth(year, month);

		if (month == 0 || month > 12) {
			return false;
		}

		if (day == 0) {
			return false;
		}

		Calendar calendar = Calendar.getInstance();
		calendar.clear();
		calendar.set(YEAR, year);
		calendar.set(MONTH, month - 1);
		
		int daysInMonth = calendar.getActualMaximum(DAY_OF_MONTH);
		
		if (daysInMonth < day) {
			return false;
		}
		
		return true;
	}

	/**
	 * Removes possible offset (20, 50, or 70).
	 * 
	 * @param year
	 * @param month may be with offset
	 * @return month without offset
	 */
	private int getRealMonth(int year, int month) {
		if ((month > 70) && (year > 2003)) {
			month -= 70;
		} else if (month > 50) {
			month -= 50;
		} else if ((month > 20) && (year > 2003)) {
			month -= 20;
		}
		return month;
	}

	/**
	 * Possible problem. People born before 1954 usually have the national identification number of length 9 but might be also 10.
	 * Little bit risky for somebody who is hundred-year old.
	 * 
	 * @param year
	 * @param value
	 * @return
	 */
	private int getYearWithHunders(int year, String value) {
		if (value.length() == 9) {
			return year + 1900;
		}
		
		if (year > Calendar.getInstance().get(YEAR) % 100) {
			return year + 1900;
		} else {
			return year + 2000;
		}
		
	}
}

RCValidatorTest.java

@RunWith(Parameterized.class)
public class RCValidatorTest {
	
	private RCValidator validator;
	private String rcToTest;
	private boolean expectedResult;
	private String errorMessage;

	@Before
	public void setUp() {
		validator = new RCValidator();
	}
	
	public RCValidatorTest(String rc, boolean expect, String errorMessage) {
		this.rcToTest = rc;
		this.expectedResult = expect;
		this.errorMessage = errorMessage;
	}
	
	@Test
	public void tesRc() {
		boolean actual = validator.isValid(rcToTest);
		assertEquals(errorMessage, expectedResult, actual);
	}
	
	@Parameters
	public static Collection rcData() {
		return Arrays.asList(new Object[][] {
				{"", false, "empty"},
				{null, false, "null value"},
				{"x123456789", false, "wrong number format"},
				{"8405011330", true, "valid mod 11"},
				{"73602851", false, "too short"},
				{"73102851691", false, "too long"},
				{"7310285169", true, "normal man"},
				{"7360005158", false, "zero day"},
				{"7360335158", false, "invalid day"},
				{"1202300011", false, "invalid day in february"},
				{"1102290002", false, "invalid day in leap year"},
				{"1202290001", true, "valid leap year"},
				{"8356078656", false, "invalid" },
				{"5305061234", true, "old valid"},
				{"530506123", true, "old valid 2"},
				{"1202031237", true, "valid after 2K"},
				{"1200010009", false, "invalid month"}
		});
	}
}