Unicode

Unicode is a standard for text encoding.
It defines a mapping of integers to characters in various languages (code-points).
Various text-encodings alter how the integer is divided across byte(s),
but regardless of it's composition, the assigned number/character is constant.

For example, UTF-8 uses the first 1-5 bits of a byte to indicate the type of byte, and if the number spans multiple bytes.
The remaining bits are assembled into one large integer, that refers to a code-point/character.

UTF-1,7,8,16,32 all map to the same character set defined by unicode.

Documentation

wikipedia: unicode	https://en.wikipedia.org/wiki/Unicode
wikipedia: utf-8	https://en.wikipedia.org/wiki/UTF-8
Unicode in C++ (video)	https://www.youtube.com/watch?v=tOHnXt3Ycfo

Tutorials

tl;dr unicode

https://www.joelonsoftware.com/2003/10/08/the-absolute-minimum-every-software-developer-absolutely-positively-must-know-about-unicode-and-character-sets-no-excuses/

Endianness

Since unicode encodings use multiple bytes per character,
we need to know the order in which to read the bytes.

The first 2-bytes of a file or text-stream contain the byte order mark
which indicate if this is big endian, or little endian.

For more details, see endianness.

254 255  # (0xFEFF) big endian
255 254  # (0xFFFE) little endian

Encodings

UTF-1

UTF-7

A proposed alternative to UTF-8. It is infrequently used, and denylisted from HTML5.
See wikipedia.

UTF-8

Predominantly used by the internet.

• the smallest unit is 8-bits
• reverse compatible with ASCII
• multiple bytes may be used for characters (1-4 bytes may be used)
• UTF-8 is extendable, we can add a 5th byte later if we need it.
• byte-order NOT determined by system's endianness

01111111  # ascii only uses 7-bits, the leading bit is always 0
1.......  # utf-8 puts a '1' in the leading bit to indicate a multi-byte character

# the number of leading '1's in the first byte of a multi-byte character
# indicates how many following bytes will be used to represent the character
# (this one will have 3x bytes after it)
1110....  

# each following byte will have the prefix '10', indicating it is a 'continuation byte'
10......

# a byte beginning with '0' indicates this is ascii, and no following bytes are required
0.......

When looking up a multibyte character, all bytes are combined into one very large binary integer.

#  byte-1      byte-2       byte-3      byte-4
[1110]1001  [10]100100]   [10]010010  [10]000000

# combine to the binary number
1001 100100 010010 000000
1001100100010010000000

# in decimal
2507904

UTF-16

Predominantly used by windows.

• the smallest unit is 16-bits
• all characters use either 2-bytes, or 4-bytes
• not compatible with ASCII - it must be re-encoded
• byte order determined by endianness

UTF-32

UTF-32 always uses 4-bytes for every character.
Like ASCII, it is a 1:1 mapping between characters and code-points.

• the smallest unit is 32-bits
• byte order determined by endianness

# code-point for 'd' is 100
# in binary, this is 
0b1100100

# in UTF-32 this is
# [byte-1]   [byte-2]   [byte-3]   [byte-4]
 0b00000000 0b00000000 0b00000000 0b01100100