393. UTF-8 Validation

• Difficulty: Medium

• Topics: Bit Manipulation

• Similar Questions:

Problem:

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. </pre> </p>

Solutions:

class Solution {
public:
    bool validUtf8(vector<int>& data) {
        int pos = 0;
        while (pos < data.size()) {
            bool ret = getToken(data, pos);
            //cout << pos << endl;
            if (!ret)   return false;
        }
        return true;
    }

private:
    bool getToken(const vector<int>& data, int& pos) {
        if (pos >= data.size()) return false;
        int firstByte = data[pos++];
        if (firstByte < 128)    return true;
        if (firstByte < 192)    return false;
        if (firstByte < 224) {
            if (pos >= data.size()) return false;
            int secondByte = data[pos++];
            return secondByte >= 128 && secondByte < 192;
        }   
        if (firstByte < 240) {
            if (pos + 1 >= data.size()) return false;
            int bytes[2];
            bytes[0] = data[pos++];
            bytes[1] = data[pos++];
            for (int i = 0 ; i < 2; ++i) {
                if (bytes[i] < 128 || bytes[i] >= 192)    return false;
            }

            return true;
        }
        if (firstByte < 248) {
            if (pos + 2 >= data.size()) return false;
            int bytes[3];
            bytes[0] = data[pos++];
            bytes[1] = data[pos++];
            bytes[2] = data[pos++];
            for (int i = 0 ; i < 3; ++i) {
                if (bytes[i] < 128 || bytes[i] >= 192)    return false;
            }
            return true;
        }

        return false;
    }

};