Simple, non-standard compliant UTF8 iterator for vanilla Lua

iutf8.lua

local function utf8_iter(s, i)
	if i >= #s then return end
	local b, nbytes = s:byte(i+1,i+1), 1

	-- determine width of the codepoint by counting the number if set bits
	-- from top to bottom. not 100% to standard, but it works well enough
	if     b/4  >= 63 then nbytes = 6
	elseif b/8  >= 31 then nbytes = 5
	elseif b/16 >= 15 then nbytes = 4
	elseif b/32 >= 7  then nbytes = 3
	elseif b/64 >= 3  then nbytes = 2
	end
	return i+nbytes, s:sub(i+1,i+nbytes), nbytes
end

local function iutf8(s)
	return utf8_iter, s, 0
end

str = 'abcdeäÄ«ÖÓÚÜöüßå'
print(str) --> abcdeäÄ«ÖÓÚÜöüßå

-- i is the position of the *last* byte of the character
-- c contains the unicode character 
-- nbytes is the width (in bytes) of the character
for i, c, nbytes in iutf8(str) do
	print(i, c, nbytes)
end
-- output of the loop:
-- 1       a       1
-- 2       b       1
-- 3       c       1
-- 4       d       1
-- 5       e       1
-- 7       ä       2
-- 9       Ä       2
-- 11      «       2
-- 13      Ö       2
-- 15      Ó       2
-- 17      Ú       2
-- 19      Ü       2
-- 21      ö       2
-- 23      ü       2
-- 25      ß       2
-- 27      å       2