Hacky way of obtaining Unicode input as UTF-8 in Lua(JIT)

lua-unicode-input.lua

local ffi = require("ffi")

ffi.cdef[[
    struct _iobuf {
        char *_ptr;
        int   _cnt;
        char *_base;
        int   _flag;
        int   _file;
        int   _charbuf;
        int   _bufsiz;
        char *_tmpfname;
    };
    typedef struct _iobuf FILE;

    FILE *__cdecl __acrt_iob_func(unsigned int _Ix);
    int __cdecl _setmode(int _FileHandle, int _Mode);
    wchar_t *__cdecl fgetws(wchar_t *_Buffer, int _BufferCount, FILE *_Stream);
    size_t __cdecl wcslen(const wchar_t *_String);
    int __stdcall WideCharToMultiByte(unsigned int CodePage, unsigned long dwFlags, const wchar_t *lpWideCharStr, int cchWideChar, char *lpMultiByteStr, int cbMultiByte, const char *lpDefaultChar, int *lpUsedDefaultChar);
    size_t __cdecl strlen(const char *_Str);
]]

local cstdin = ffi.C.__acrt_iob_func(0)
ffi.C._setmode(0, 0x20000)

local function readLine(max)
    max = type(max) == "number" and max or 2000
    max = math.floor(max)
    max = math.abs(max)
    if max > (0x7fffffff - 1) or max < 1 then error("maxmimum chars must be <= 2147483646 and >= 1") end

    local utf16buf = ffi.new("wchar_t[?]", max + 1)
    ffi.C.fgetws(utf16buf, max + 1, cstdin)

    local charsRead = tonumber(ffi.C.wcslen(utf16buf))
    local utfBufLen = (charsRead * 4) + 1;
    local utfBuf = ffi.new("char[?]", utfBufLen)

    ffi.C.WideCharToMultiByte(65001, 0, utf16buf, charsRead + 1, utfBuf, utfBufLen, nil, nil)
    return ffi.string(utfBuf, ffi.C.strlen(utfBuf) - 1)
end