393. UTF-8 Validation

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.

Rust Solution

struct Solution;

impl Solution {
    fn valid_utf8(data: Vec<i32>) -> bool {
        let mut count = 0;
        for x in data {
            if count == 0 {
                if x >> 3 & 0b11111 == 0b11110 {
                    count += 3;
                    continue;
                }
                if x >> 4 & 0b1111 == 0b1110 {
                    count += 2;
                    continue;
                }
                if x >> 5 & 0b111 == 0b110 {
                    count += 1;
                    continue;
                }
                if x >> 7 & 0b1 == 0 {
                    continue;
                }
            } else {
                if x >> 6 & 0b11 == 0b10 {
                    count -= 1;
                    continue;
                }
            }
            return false;
        }
        count == 0
    }
}

#[test]
fn test() {
    let data = vec![197, 130, 1];
    assert_eq!(Solution::valid_utf8(data), true);
    let data = vec![235, 140, 4];
    assert_eq!(Solution::valid_utf8(data), false);
}

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