JS函數charCodeAt的Lua實現

charCodeAt by Lua

@(Lua JavaScript charCodeAt)

I wanted to have a function charCodeAt in Lua ,and it should works exactly like javascript
but with Lua5.1 ,UTF8 and Unicode are not supported,

1: how charCodeAt works in javascript

to show Console press F12 in Chrome( MAC:CMD+alt+J)

[
'你'.charCodeAt(0),
'?'.charCodeAt(0),
'n'.charCodeAt(0)
]

it will output [20320, 241, 110] ，it means the numeric value of Unicode , '你'=20320 , '?'=241, 'n'=110.

The charCodeAt() method returns the numeric Unicode value of the character at the given index (except for unicode codepoints > 0x10000).

according to alexander-yakushev we can know how many bytes one UTF8 word takes using function utf8.charbytes
[https://github.com/alexander-yakushev/awesompd/blob/master/utf8.lua]

function utf8.charbytes (s, i)
   -- argument defaults
   i = i or 1
   local c = string.byte(s, i) 
   -- determine bytes needed for character, based on RFC 3629
   if c > 0 and c <= 127 then
      -- UTF8-1 byte
      return 1
   elseif c >= 194 and c <= 223 then
      -- UTF8-2 byte
      return 2
   elseif c >= 224 and c <= 239 then
      -- UTF8-3 byte
      return 3
   elseif c >= 240 and c <= 244 then
      -- UTF8-4 byte
      return 4
   end
end

Unicode & UTF8 convert method

but we should pay attention to 4 bytes UTF8[emoji], it works not that simple

special Method

javascript engine using UTF16，characters in Basic Multilingual Plane were the same with unicode, but if the characters were in Supplementary Plane it should use the formula below，usually we encounter Supplementary Plane emoji like?? (4 byte UTF8 character)

-- formula 1
H = Math.floor((c-0x10000) / 0x400)+0xD800 
L = (c - 0x10000) % 0x400 + 0xDC00

code is here

https://github.com/lilien1010/lua-bit

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