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Why not call it Rot8192 or Rot0x7777 ?

rot13 is (X + (26/2)) mod 26 ; this is (X + (2^16)/2) mod 2^16. (The BMP is the first 2^16 code points of unicode.)

Edit: silly formatting dropped my math punctuation.

I suspect they also really wanted the rot8 -> "rotate" joke.

AFAICS, it's actually using decimal 8000, not 2^16/2 = 0x8000, so I don't really understand how this is reversible at all unless they're just subtracting it back.

What we really need is rot88000h for the full U+0..U+10FFFF range. :)

It's using 0x8000, which is half of 0x10000 (the size of the basic multilingual plane). It doesn't extend out of BMP

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