local pattern = '[%z\1-\127\194-\244][\128-\191]*'

-- helper function
local posrelat =
	function (pos, len)
		if pos < 0 then
			pos = len + pos + 1
		end

		return pos
	end

utf8 = {}

-- maps f over s's utf8 characters f can accept args: (visual_index, utf8_character, byte_index)
utf8.map =
	function (s, f, no_subs)
		local i = 0

		if no_subs then
			for b, e in s:gmatch('()' .. pattern .. '()') do
				i = i + 1
				local c = e - b
				f(i, c, b)
			end
		else
			for b, c in s:gmatch('()(' .. pattern .. ')') do
				i = i + 1
				f(i, c, b)
			end
		end
	end

-- generator for the above -- to iterate over all utf8 chars
utf8.chars =
	function (s, no_subs)
		return coroutine.wrap(function () return utf8.map(s, coroutine.yield, no_subs) end)
	end

-- returns the number of characters in a UTF-8 string
utf8.len =
	function (s)
		-- count the number of non-continuing bytes
		return select(2, s:gsub('[^\128-\193]', ''))
	end

utf8.GetMaxLenString =
    function (s, maxlen, symbol)
        -- s:待裁剪字符串，maxlen:最大长度，symbol:裁剪后补全符号
        symbol = symbol or ""
        local len = utf8.len(s)
        local dstString = s
        -- 超长，裁剪，加symbol
        if len > maxlen then
            dstString = utf8.sub(s, 1, maxlen)
            dstString = dstString..symbol
        end
        return dstString
    end

-- replace all utf8 chars with mapping
utf8.replace =
	function (s, map)
		return s:gsub(pattern, map)
	end

-- reverse a utf8 string
utf8.reverse =
	function (s)
		-- reverse the individual greater-than-single-byte characters
		s = s:gsub(pattern, function (c) return #c > 1 and c:reverse() end)

		return s:reverse()
	end

-- strip non-ascii characters from a utf8 string
utf8.strip =
	function (s)
		return s:gsub(pattern, function (c) return #c > 1 and '' end)
	end

-- like string.sub() but i, j are utf8 strings
-- a utf8-safe string.sub()
utf8.sub =
	function (s, i, j)
		local l = utf8.len(s)

		i =       posrelat(i, l)
		j = j and posrelat(j, l) or l

		if i < 1 then i = 1 end
		if j > l then j = l end

		if i > j then return '' end

		local diff = j - i
		local iter = utf8.chars(s, true)

		-- advance up to i
		for _ = 1, i - 1 do iter() end

		local c, b = select(2, iter())

		-- i and j are the same, single-charaacter sub
		if diff == 0 then
			return string.sub(s, b, b + c - 1)
		end

		i = b

		-- advance up to j
		for _ = 1, diff - 1 do iter() end

		c, b = select(2, iter())

		return string.sub(s, i, b + c - 1)
	end