double转成 8 个字节

local to_double , from_double
do
     local s , e , d
     -- Can't use with to_double as we can't strip debug info :(
     d = string.dump ( loadstring ( [[return 523123.123145345]] ) )
     s , e = d:find ( "\3\54\208\25\126\204\237\31\65" )
     s = d:sub ( 1 , s )
     e = d:sub ( e+1 , -1 )

     function to_double ( n )
          -- Should be the 8 bytes following the second \3 (LUA_TSTRING == '\3')
          local str = string.dump ( loadstring ( [[return ]] .. n ) )
          local loc , en , mat = str:find ( "\3(........)" , str:find ( "\3" ) + 1 )
          return mat
     end
     function from_double ( str )
          assert ( #str == 8 )
          str = s .. str .. e
          return loadstring ( str ) ( )
     end
end