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.

# @lc code=start
using LeetCode

# Automation
function valid_utf8(data::Vector{Int})::Bool
    # 1 represents 0xxx.., 2 represents 10xx.., 3,4,5 similarly, 6 represents error
    switch(num::Int) = num >= 248 ? 6 : findfirst('0', bitstring(Int16(num))[(end - 7):end])
    # 1 represents start, 2 represents 2-byte(need one more byte), 3, 4 similarly, 5 represents fail
    state = 1
    state_shift = Dict{Int,Vector{Int}}(1 => [1, 5, 2, 3, 4, 5])
    for i in 2:4
        state_shift[i] = [5, i - 1, 5, 5, 5, 5]
    end
    for num in data
        state = state_shift[state][switch(num)] ## new state
        state == 5 && return false
    end
    return state == 1
end
# @lc code=end

valid_utf8 (generic function with 1 method)

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