this is a chatgpt chat summarised , by chatgpt for my personal reference,
if you can understand this, ggwp “so utf-8 only has like 8 bits , so it can only represent 256 charecters?”
No. 8 bits is the size of one byte, but UTF-8 lets a character use 1, 2, 3 or 4 bytes. That yields well over a million possible characters.
“wait, wont 2 bytes be utf 16 ?”
A 2-byte sequence could be either (a) one UTF-16 code-unit or (b) a 2-byte UTF-8 character. The bit pattern tells which.
“that means i can make every charecter using a utf8 block , why use a utf-16 block.”
UTF-8 wins on file size for Western text; UTF-16 gives cheaper random-access by 16-bit units and is a bit smaller for pure CJK data. Windows, Java, .NET and browser DOMs already store text internally as UTF-16.
“so because of the block start address, utf -8 is not good for larger data, show me with an example how utf -16 solves it.”
To reach character n in UTF-8 you may have to scan the variable-length bytes before it. In UTF-16 most everyday characters are one fixed 16-bit slot, so their byte offset is simply index × 2 (unless you hit a surrogate pair).
“what if 😃 had come as the second charecter, then show me”
UTF-8 offsets shift by +3 extra bytes (😃 is 4 bytes vs. 1-byte ASCII). UTF-16 offsets shift by +2 bytes (😃 is two 16-bit units), so later characters move less.
“the byte offset for third charecter in utf-16 is 6 , it should have been 4?”
Because 😃 happens earlier and takes two 16-bit units, every following character starts 2 bytes later than the simple index × 2 rule.
“what is a BMP? what is a code-unit index? and what is a code-point index?”
BMP: the first Unicode plane (U+0000–U+FFFF). Code-unit index: position counted in 16-bit chunks (UTF-16) or 8-bit chunks (UTF-8). Code-point index: position counted in actual characters people see.
“so you are saying, 16 bits is good enough for most charecters? including kanji, mandarin , hindi , emojies everything?”
One 16-bit unit covers every BMP script (Latin, Hindi, common CJK). Emoji and rare Han characters sit above U+FFFF and need two units.
“what is a surrogate pair? and wasent 16 bits advantage it is easier to jump to nth charecter ? wont surrogate pair screw it ?”
A surrogate pair = two reserved 16-bit values (D800–DBFF + DC00–DFFF) that together store one character ≥ U+10000. They keep constant-time math for code-unit positions; libraries just keep a tiny side-table to map them back to character counts.
“what do you mean subtracts? how did we get to 20 bit numbers , did we add 4 bits randomly ?”
For any code-point ≥ U+10000 you subtract 0x10000, giving a value 0…0xFFFFF. That occupies exactly 20 bits—no bits added.
“how is upper 0x3d, wont it be , 0xf603 is 16 bits not 20 bits”
0xF603 is padded to 20 bits: 0000 1111 0110 0000 0011. Upper 10 bits = 0x3D, lower 10 bits = 0x203.
“and why will you add it to 0xD800”
Adding to 0xD800 (or 0xDC00) drops the 10-bit payload into a block reserved only for surrogates, so decoders instantly know what they’re looking at.
“so after 16bits, you use 20 bits and normalise it back to 16 bits?”
We expand 20 bits into two 16-bit units (32 bits total). Nothing shrinks back to 16.
“but 3D D8 03 DE is 16 bits. so you got it down to 16 bits while representing it”
It’s four bytes: 3D D8 (first unit) + 03 DE (second unit) = 32 bits.
“oh yes, i mean it is a 32 bit code, so you work with 20 bits and then expand it to 32 bits? and i am guessing it cannot be more than 20 bits now?”
Correct. 20-bit payload → stored in 32 bits. Unicode is permanently capped at plane 16, so 20 bits will always be enough.
“20 bits gives me 2^20 possibilities, that covers everything? !!”
Yes. Planes 1-16 have 1 048 576 slots (2²⁰). Plane 0 adds another 65 536. Total Unicode capacity = 1 114 112 code-points.
“so this is how the counting goes … i have like 2^16+2^20 possibilites?”
Exactly: 65 536 (BMP) + 1 048 576 (Supplementary) = 1 114 112.
HOW A SURROGATE PAIR IS BUILT (😃 = U+1F603) 1F603 – 10000 = 0F603 → 20-bit payload
Upper 10 bits = 0x3D, lower 10 bits = 0x203
High surrogate = D800 + 3D = D83D Low surrogate = DC00 + 203 = DE03 Stored little-endian: 3D D8 03 DE (32 bits)
TOTAL CAPACITY Plane 0 (BMP): 2¹⁶ = 65 536
Planes 1-16: 2²⁰ = 1 048 576
Overall limit: 1 114 112 code-points (Unicode stops at U+10FFFF)
Assigned so far: ~154 000 → plenty of space left.
WHEN TO USE WHICH ENCODING • UTF-8 — default for web, JSON, logs; smallest for English-heavy text; ASCII-compatible. • UTF-16 — good if your engine already uses it (Windows, Java, .NET) or if your data is mostly Chinese/Japanese/Korean. • UTF-32 — trivially simple (one 32-bit int per character) but twice the size of UTF-16 and four times ASCII.