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0 pointstheoh7y ago0 comments

Wait a minute.

"A surrogate pair can represent 20 bits, or 2^20 values. A single UTF-16 byte is of course 16 bits, or 2^16 values. 2^20 + 2^16 == 0x110000, which is precisely as many unicode code points as there are (0–0x10FFFF)."

This explanation can't be quite right, because the possible surrogate code unit values are carved out of the set of 2^16 code units. So you are double counting those surrogate code units.

From Wikipedia: "Since the ranges for the high surrogates (0xD800–0xDBFF), low surrogates (0xDC00–0xDFFF), and valid BMP characters (0x0000–0xD7FF, 0xE000–0xFFFF) are disjoint, it is not possible for a surrogate to match a BMP character, or for two adjacent code units to look like a legal surrogate pair."

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theohOP7y ago

OK, I've found the gross reason why the 2^16 + 2^20 arithmetic works:

"The Unicode standard permanently reserves these code point values for UTF-16 encoding of the high and low surrogates, and they will never be assigned a character, so there should be no reason to encode them. The official Unicode standard says that no UTF forms, including UTF-16, can encode these code points."

Ugh.

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