393. UTF-8 Validation

A character in UTF8 can be from1 to 4 byteslong, subjected to the following rules:

1. For 1-byte character, the first bit is a 0, followed by its unicode code.
2. For n-bytes character, the first n-bits are all one's, the n+1 bit is 0, followed by n-1 bytes with most significant 2 bits being 10.

This is how the UTF-8 encoding would work:

   Char. number range  |        UTF
-8
 octet sequence
      (hexadecimal)    |              (binary)
   --------------------+---------------------------------------------

0000 0000-0000 007F | 0xxxxxxx

0000 0080-0000 07FF | 110xxxxx 10xxxxxx

0000 0800-0000 FFFF | 1110xxxx 10xxxxxx 10xxxxxx

0001 0000-0010 FFFF | 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx

Given an array of integers representing the data, return whether it is a valid utf-8 encoding.

Note:
The input is an array of integers. Only the least significant 8 bits of each integer is used to store the data. This means each integer represents only 1 byte of data.

Example 1:

data = [197, 130, 1], which represents the octet sequence: 
11000101 10000010 00000001.

Return 
true.
It is a valid utf-8 encoding for a 2-bytes character followed by a 1-byte character.

Example 2:

data = [235, 140, 4], which represented the octet sequence: 
11101011 10001100 00000100.

Return 
false.
The first 3 bits are all one's and the 4th bit is 0 means it is a 3-bytes character.
The next byte is a continuation byte which starts with 10 and that's correct.
But the second continuation byte does not start with 10, so it is invalid.

Solution: Logic is straightforward. But it is quite easy to have bugs in the code.

    public boolean validUtf8(int[] data) {
        if (data == null || data.length == 0) {
            return false;
        }

        return helper(data, 0);
    }

    private boolean helper(int[] data, int start) {
        if (start == data.length) {
            return true;
        }
        if (((data[start]>>7) & 1) == 0) {
            // System.out.println("1-byte");
            //1-byte
            return helper(data, start + 1);
        }

        for (int nByte = 2; nByte <= 4; nByte++) {
            if (((data[start]>>(7 - nByte)) & 1) == 0 && ((data[start]>>(8 - nByte))|0)%(1<<nByte) == ((1<<nByte) - 1) ) {
                if (start + nByte - 1 >= data.length) {
                    return false;
                }
                boolean passed = true;
                for (int i = start + 1; i <= start + nByte - 1; i++) {
                    passed &= (((data[i]>>6) & 1) == 0 && ((data[i]>>7)|0)%2 == 1);
                }
                if (passed) {
                    return helper(data, start + nByte);
                } else {
                    return false;
                }
            }
        }

        return false;
    }