UTF8Validation.cpp

// Source : https://leetcode.com/problems/utf-8-validation/
// Author : Hao Chen
// Date   : 2016-09-08

/*************************************************************************************** 
 *
 * 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
 *    --------------------+---------------------------------------------
 *    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.
 ***************************************************************************************/


class Solution {
public:
    bool validUtf8(vector<int>& data) {
        int i = 0;
        while ( i < data.size() ) {
            if ( (data[i] & 0x80) == 0 ){
                i++;
                continue;
            }
            
            int len = 0;
            if ( (data[i] & 0xE0) == 0xC0 ) { // checking 110xxxxx
                len = 2;
            }else if ( (data[i] & 0xF0) == 0xE0) { // checking 1110xxxx
                len = 3;
            }else if ( (data[i] & 0xF8) == 0xF0) { // checking 11110xxx
                len = 4;
            }else {
                return false;
            }
            
            // invalid utf-8 as it doesn't have enough 10xxxxxx
            if (i + len > data.size()) {
                return false;
            }
            
            for (int j=i+1; j < i+len; j++) { //checking 10xxxxxx
                if ( (data[j] & 0xC0) != 0x80 ) {
                    return false;
                }
            }
            
            i += len ;
        }
        return true;
    }
};