UTF-8 hex to unicode code point (only math)

Question

Let's take this table with characters and HEX encodings in Unicode and UTF-8.
Does anyone know how it is possible to convert UTF-8 hex to Unicode code point using only math operations?
E.g. let's take the first row. Given 227, 129 130 how to get 12354?
Is there any simple way to do it by using only math operations?

Unicode code point	UTF-8	Char
30 42 (12354)	e3 (227) 81 (129) 82 (130)	あ
30 44 (12356)	e3 (227) 81 (129) 84 (132)	い
30 46 (12358)	e3 (227) 81 (129) 86 (134)	う

* Source: https://www.utf8-chartable.de/unicode-utf8-table.pl?start=12288&unicodeinhtml=hex

Answer 1

This video is the perfect source (watch from 6:15), but here is its summary and code sample in golang. With letters I mark bits taken from UTF-8 bytes, hopefully it makes sense. When you understand the logic it's easy to apply bitwise operators):

Bytes	Char	UTF-8 bytes	Unicode code point	Explanation
1-byte (ASCII)	E	1. 0xxx xxxx 0100 0101 or 0x45	1. 0xxx xxxx 0100 0101 or U+0045	no conversion needed, the same value in UTF-8 and unicode code point
2-byte	Ê	1. 110x xxxx 2. 10yy yyyy 1100 0011 1000 1010 or 0xC38A	0xxx xxyy yyyy 0000 1100 1010 or U+00CA	1. First 5 bits of the 1st byte 2. First 6 bits of the 2nd byte
3-byte	あ	1. 1110 xxxx 2. 10yy yyyy 3. 10zz zzzz 1110 0011 1000 0001 1000 0010 or 0xE38182	xxxx yyyy yyzz zzzz 0011 0000 0100 0010 or U+3042	1. First 4 bits of the 1st byte 2. First 6 bits of the 2nd byte 3. First 6 bits of the 3rd byte
4-byte	𐄟	1. 1111 0xxx 2. 10yy yyyy 3. 10zz zzzz 4. 10ww wwww 1111 0000 1001 0000 1000 0100 1001 1111 or 0xF090_849F	000x xxyy yyyy zzzz zzww wwww 0000 0001 0000 0001 0001 1111 or U+1011F	1. First 3 bits of the 1st byte 2. First 6 bits of the 2nd byte 3. First 6 bits of the 3rd byte 4. First 6 bits of the 4th byte

2-byte UTF-8

func get(byte1 byte, byte2 byte) {
    int1 := uint16(byte1 & 0b_0001_1111) << 6
    int2 := uint16(byte2 & 0b_0011_111)
    return rune(int1 + int2)
}

3-byte UTF-8

func get(byte1 byte, byte2 byte, byte3 byte) {
    int1 := uint16(byte1 & 0b_0000_1111) << 12
    int2 := uint16(byte2 & 0b_0011_111) << 6
    int3 := uint16(byte3 & 0b_0011_111)
    return rune(int1 + int2 + int3)
}

4-byte UTF-8

func get(byte1 byte, byte2 byte, byte3 byt3, byte4 byte) {
    int1 := uint(byte1 & 0b_0000_1111) << 18
    int2 := uint(byte2 & 0b_0011_111) << 12
    int3 := uint(byte3 & 0b_0011_111) << 6
    int4 := uint(byte4 & 0b_0011_111)
    return rune(int1 + int2 + int3 + int4)
}