UCS2 encode a string for sending via AT modem using `%04x` hex format.

ucs2_encode.lua

local function ucs2_encode(str)
    local cc  = {}
    local bv  = {1, 2, 4, 8, 16, 32, 64, 128}
    local fmt = "%04x"
    
    -- bit_set taken from http://ricilake.blogspot.no/2007/10/iterating-bits-in-lua.html
    local function bit_set(x, p)
      return x % (p + p) >= p
    end
    
    -- get bit_val
    -- x the number
    -- n the max bit bit are enumerated 1..8 instead of 0..7 lua world
    -- i the start bit pos
    -- o offset shift up/down
    local function bit_val(x, n, i, o)
        local v = 0
        if not i then
            i = 1
        end        
        if not o then
            o = 0
        end
        
        while i <= n do
            if bit_set(x, bv[i]) == true then
                v = v + bv[i + o]
            end
            i = i + 1
        end
        return v
    end
    
    -- encode string
    local i = 1
    local n = str:len()
    local c, c2, o, v
    while i <= n do
    	-- read 1st byte, if uses one byte we got the char code 
    	-- otherwise read 2nd byte and compute new char code
        c = str:byte(i) 
        o = 1
        
        -- @see http://en.wikipedia.org/wiki/UTF-8 for UTF8-2
        if c > 127 then 
            -- read 2nd lower byte
            -- compute new char code value
            c2 = str:byte(i + 1) 
            c  = bit_val(c, 5, 3, -2) + 
                 bit_val(c, 2, 1, 6)  + 
                 bit_val(c2, 6) 
                 
            o = 2
        end
        
        table.insert(cc, string.format(fmt, c))
        i = i + o
    end

    return string.upper(table.concat(cc, ""))
end