utf8_validation.py

'''
UTF-8 Validation
Leetcode #393

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.

'''
def validUtf8(data):
    """
    :type data: List[int]
    :rtype: bool
    """
    n_bytes = 0

    mask1 = 1 << 7
    mask2 = 1 << 6
    for num in data:
        # bin_rep = format(num, '#010b')[-8:]
        
        mask = 1 << 7
        if n_bytes == 0:
            while mask & num:
                n_bytes += 1
                mask = mask >> 1
            if n_bytes == 0:
                continue
            if n_bytes == 1 or n_bytes > 4:
                return False
        else:
            if not (num & mask1 and not (num & mask2)):
                return False
        
        n_bytes -= 1

    return n_bytes == 0

# Test cases
print validUtf8([197, 130, 1])