BT Ma Hoa Giang

BT.java

package main;

import java.util.Arrays;

public class BT {
    final static String ALPHABET = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    final static char[] CHAR_ALPHABETS_UPPSER = ALPHABET.toCharArray();
    final static char[] CHAR_ALPHABETS_LOWER = ALPHABET.toLowerCase()
	    .toCharArray();

    /**
     * encrypt origin string with turn alphabet.
     * 
     * @param origin
     * @param k
     * @return
     */
    public static String encode(String origin, int k) {
	char[] toEncode = origin.toCharArray();
	for (int i = 0; i < toEncode.length; i++) {
	    if (Character.isLetter(toEncode[i])) {
		int index = i - k;
		if (index < 0) {
		    index = toEncode[i] + 25;
		}
		if (index > 26) {
		    index = toEncode[i] - 26;
		}
		toEncode[i] -= k;
		toEncode[i] = (char) ((toEncode[i] - 3 - (int) 'A') % 26 + (int) 'a');
	    }
	}
	origin = String.valueOf(toEncode);
	return origin;
    }

    /**
     * ham nay khong phan biet duoc chu thuong / chu hoa. tat ca chu sau khi
     * giai ma la chu thuong
     * 
     * @param encrypt
     * @param k
     * @return
     * @throws Exception
     */
    public static String decode(String encrypt, int k) throws Exception {
	// kiem tra gia tri dau vao
	if (encrypt == null) {
	    System.out.println("Can't decode null value");
	    throw new RuntimeException("Can't decode null value");
	}
	// kiem tra gia tri dau vao cho K
	if (k < 0 || k >= 25) {
	    System.out.println("Can't decode with k invalid");
	    throw new RuntimeException("Can't decode with k invalid");
	}
	if (encrypt.isEmpty()) {
	    return "";
	}
	// ------- bat dau decode (giai ma)--------
	char[] encryptChars = encrypt.toUpperCase().toCharArray();
	char[] decryptChars = new char[encrypt.length()];
	for (int i = 0; i < encryptChars.length; ++i) {
	    char afterDecodeChar = encryptChars[i];
	    // only decode with letter character.
	    if (Character.isLetter(afterDecodeChar)) {
		// find index of character in alphabet
		int foundIndex = 0;
		// co the thay the ham nay bang tim kiem trong vong for
		foundIndex = Arrays.binarySearch(CHAR_ALPHABETS_UPPSER,
			afterDecodeChar);
		// for (int index = 0; index < CHAR_ALPHABETS.length; ++index) {
		// if (afterDecodeChar == CHAR_ALPHABETS[index]) {
		// foundIndex = index;
		// break;
		// }
		// }
		// after find index of encrypt
		foundIndex -= k;
		// xoay vong nguoc kim dong ho BAZYX...
		if (foundIndex < 0) {
		    foundIndex = CHAR_ALPHABETS_UPPSER.length + foundIndex;
		}
		// xoay vong cung chieu kim dong ho
		// khong co truong hop lon hon 25. vi max length = 25 roi.
		// if (foundIndex > 25) {
		// foundIndex = CHAR_ALPHABETS.length - foundIndex;
		// }
		afterDecodeChar = CHAR_ALPHABETS_UPPSER[foundIndex];

	    }
	    // add value to result;
	    decryptChars[i] = afterDecodeChar;
	}

	return String.copyValueOf(decryptChars).toLowerCase();
    }

    /**
     * ham nay phan biet duoc chu hoa chu thuong
     * 
     * @param encrypt
     * @param k
     * @return
     * @throws Exception
     */
    public static String decodeUpLowCase(String encrypt, int k)
	    throws Exception {
	// kiem tra gia tri dau vao
	if (encrypt == null) {
	    System.out.println("Can't decode null value");
	    throw new RuntimeException("Can't decode null value");
	}
	// kiem tra gia tri dau vao cho K
	if (k < 0 || k >= 25) {
	    System.out.println("Can't decode with k invalid");
	    throw new RuntimeException("Can't decode with k invalid");
	}
	if (encrypt.isEmpty()) {
	    return "";
	}
	// ------- bat dau decode (giai ma)--------
	char[] encryptChars = encrypt.toCharArray();
	char[] decryptChars = new char[encrypt.length()];
	for (int i = 0; i < encryptChars.length; ++i) {
	    char afterDecodeChar = encryptChars[i];
	    // only decode with letter character.
	    if (Character.isLetter(afterDecodeChar)) {
		char[] searchChar = CHAR_ALPHABETS_UPPSER;
		if (Character.isLowerCase(afterDecodeChar)) {
		    searchChar = CHAR_ALPHABETS_LOWER;
		}
		int foundIndex = Arrays.binarySearch(searchChar,
			afterDecodeChar);
		// find index of character in alphabet

		// tru nguoc di k vi tri
		foundIndex -= k;
		// xoay vong nguoc kim dong ho BAZYX...
		if (foundIndex < 0) {
		    foundIndex = ALPHABET.length() + foundIndex;
		}
		afterDecodeChar = searchChar[foundIndex];
	    }
	    // add value to result;
	    decryptChars[i] = afterDecodeChar;
	}

	return String.copyValueOf(decryptChars);
    }

    public static void main(String[] args) throws Exception {
	// test 1
	String encrypt1 = "ab";
	System.out.println("After decode 1: " + BT.decode(encrypt1, 2));

	// test 2
	String encrypt2 = "123xyz";
	System.out.println("After decode 2: " + BT.decode(encrypt2, 2));

	// test 3
	String encrypt3 = "%^&***^&xyz";
	System.out.println("After decode 3: " + BT.decode(encrypt3, 2));

	// test 4 // test chu hoa chu thuong
	String encrypt4 = "Ab";
	System.out
		.println("After decode 4: " + BT.decodeUpLowCase(encrypt4, 2));
    }
}

Result:
After decode 1: yz
After decode 2: 123vwx
After decode 3: %^&***^&vwx
After decode 4: Yz