Utf8Validation.java

// 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 class Utf8Validation {
    public boolean validUtf8(int[] data) {
        int count = 0;

        for(int i : data) {
            if(count == 0) {
                if((i >> 5) == 0b110) {
                    count = 1;
                } else if((i >> 4) == 0b1110) {
                    count = 2;
                } else if((i >> 3) == 0b11110) {
                    count = 3;
                } else if((i >> 7) == 0b1) {
                    return false;
                }
            } else {
                if((i >> 6) != 0b10) {
                    return false;
                }

                count--;
            }
        }
        
        return count == 0;
    }
}