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static final char[] DIGITS =

{ '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f', 'g',

'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x',

'y', 'z', 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O',

'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z' };

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public String shorten(String longUrl, int urlLength) {

if (urlLength < 0 || urlLength > 6) {

throw new IllegalArgumentException("the length of url must be between 0 and 6");

}

String md5Hex = DigestUtils.md5Hex(longUrl);

// 6 digit binary can indicate 62 letter & number from 0-9a-zA-Z

int binaryLength = urlLength * 6;

long binaryLengthFixer = Long.valueOf(StringUtils.repeat("1", binaryLength), BINARY);

for (int i = 0; i < 4; i++) {

String subString = StringUtils.substring(md5Hex, i * 8, (i + 1) * 8);

subString = Long.toBinaryString(Long.valueOf(subString, 16) & binaryLengthFixer);

subString = StringUtils.leftPad(subString, binaryLength, "0");

StringBuilder sbBuilder = new StringBuilder();

for (int j = 0; j < urlLength; j++) {

String subString2 = StringUtils.substring(subString, j * 6, (j + 1) * 6);

int charIndex = Integer.valueOf(subString2, BINARY) & NUMBER_61;

sbBuilder.append(DIGITS[charIndex]);

}

String shortUrl = sbBuilder.toString();

if (lookupLong(shortUrl) != null) {

continue;

} else {

return shortUrl;

}

}

// if all 4 possibilities are already exists

return null;

}

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private AtomicLong sequence = new AtomicLong(0);

@Override

protected String shorten(String longUrl) {

long myseq = sequence.incrementAndGet();

String shortUrl = to62RadixString(myseq);

return shortUrl;

}

private String to62RadixString(long seq) {

StringBuilder sBuilder = new StringBuilder();

while (true) {

int remainder = (int) (seq % 62);

sBuilder.append(DIGITS[remainder]);

seq = seq / 62;

if (seq == 0) {

break;

}

}

return sBuilder.toString();

}