393. UTF-8 Validation

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Problem

Given an integer array data representing the data, return whether it is a valid UTF-8 encoding (i.e. it translates to a sequence of valid UTF-8 encoded characters).

A character in UTF8 can be from 1 to 4 bytes long, 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.

Code

class Solution {
    public boolean validUtf8(int[] data) {
        int group = 0;

        // 1000 0000
        int mask1 = 1 << 7;
        // 0100 0000
        int mask2 = 1 << 6;

        for (int i = 0; i < data.length; i++) {
            int curr = data[i];
            if (group == 0) {
                // 开头有几个1
                // 110x xxxx - 1 group
                // 1110 xxxx - 2 group
                // 1111 0xxx - 3 group
                // 1000 0000
                int mask = 1 << 7;
                while ((mask & curr) != 0) {
                    group += 1;
                    mask = mask >> 1;
                }

                // 0xxxxxxx 1-byte - valid
                if (group == 0) {
                    continue;
                }

                if (group > 4 || group == 1) {
                    return false;
                }
            } else {
                // 后续表示要以10开头
                if (!((curr & mask1) != 0 && (mask2 & curr) == 0)) {
                    return false;
                }
            }
            group -= 1;
        }

        return group == 0;
    }
}