UTF-8 Validation in Java
A character in UTF-8 can be from 1 to 4 bytes long, subjected to the following rules:
- For a 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 the 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.
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.
data = [235, 140, 4], which represented the octet sequence: 11101011 10001100 00000100.
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.
data = [197, 130, 1], which represents the octet sequence: 11000101 10000010 00000001.
It is a valid utf-8 encoding for a 2-bytes character followed by a 1-byte character.
Approach: As long as every byte in the array is of the right type, it is a valid UTF-8 encoding.
- Start from index 0, determine each byte’s type and check its validity.
- There are five kinds of valid byte type: 0**, 10**, 110**,1110** and 11110**
- Give them type numbers, 0, 1, 2, 3, 4 which are the index of the first 0 from left.
- So, the index of the first 0 determines the byte type.
- If a byte belongs to one of them: if it is type 0, continue if it is type 2 or 3 or 4, check whether the following 1, 2, and 3 byte(s) are of type 1 or not.
- If not, return false; else if a byte is type 1 or not of valid type, return false.
- Time Complexity: O(n)
- Space Complexity: O(1)
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