393. UTF-8 Validation

393. UTF-8 Validation

A character in UTF8 can be from 1 to 4 bytes long, subjected to the following rules:

  For 1-byte character, the first bit is a 0, followed by its unicode code.
  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.

public boolean validUtf8(int[] data) {
    for(int j = 0; j < data.length; j++) {
        int first = data[j];
        int trailing = data.length;
        if(first >> 7 == 0) {
            trailing = 0;
        } else if(first >> 3 == 30) {
            trailing = 3;
        } else if(first >> 4 == 14) {
            trailing = 2;
        } else if(first >> 5 == 6) {
            trailing = 1;
        }
        if(j + trailing >= data.length) return false;
        for(int i = j+1; i <= j + trailing; i++) {
            if(data[i] >> 6 != 2) return false;
        }
        j += trailing;
    }
    return true;
}