解决string.len 处理 utf-8 中文字符不正确的问题

解决string.len 处理 utf-8 中文字符不正确的问题。因为这个返回的结果是字节数（不是字符数），比如 UTF-8 里一个中文得到 3，在 GBK 里一个中文得到2。

据说是有个 string.utf8len 还是 string.lenutf8 可以用，但是试了一下发现貌似没有这个函数

幸亏GitHub上有大神贴出了代码，效果非常好

https://github.com/airtonix/ouf_tags/blob/master/utf8.lua

贴出主要部分

local function utf8charbytes (s, i)
	-- argument defaults
	i = i or 1

	-- argument checking
	if type(s) ~= "string" then
		error("bad argument #1 to "utf8charbytes" (string expected, got ".. type(s).. ")")
	end
	if type(i) ~= "number" then
		error("bad argument #2 to "utf8charbytes" (number expected, got ".. type(i).. ")")
	end

	local c = s:byte(i)

	-- determine bytes needed for character, based on RFC 3629
	-- validate byte 1
	if c > 0 and c <= 127 then
		-- UTF8-1
		return 1

	elseif c >= 194 and c <= 223 then
		-- UTF8-2
		local c2 = s:byte(i + 1)

		if not c2 then
			error("UTF-8 string terminated early")
		end

		-- validate byte 2
		if c2 < 128 or c2 > 191 then
			error("Invalid UTF-8 character")
		end

		return 2

	elseif c >= 224 and c <= 239 then
		-- UTF8-3
		local c2 = s:byte(i + 1)
		local c3 = s:byte(i + 2)

		if not c2 or not c3 then
			error("UTF-8 string terminated early")
		end

		-- validate byte 2
		if c == 224 and (c2 < 160 or c2 > 191) then
			error("Invalid UTF-8 character")
		elseif c == 237 and (c2 < 128 or c2 > 159) then
			error("Invalid UTF-8 character")
		elseif c2 < 128 or c2 > 191 then
			error("Invalid UTF-8 character")
		end

		-- validate byte 3
		if c3 < 128 or c3 > 191 then
			error("Invalid UTF-8 character")
		end

		return 3

	elseif c >= 240 and c <= 244 then
		-- UTF8-4
		local c2 = s:byte(i + 1)
		local c3 = s:byte(i + 2)
		local c4 = s:byte(i + 3)

		if not c2 or not c3 or not c4 then
			error("UTF-8 string terminated early")
		end

		-- validate byte 2
		if c == 240 and (c2 < 144 or c2 > 191) then
			error("Invalid UTF-8 character")
		elseif c == 244 and (c2 < 128 or c2 > 143) then
			error("Invalid UTF-8 character")
		elseif c2 < 128 or c2 > 191 then
			error("Invalid UTF-8 character")
		end
		
		-- validate byte 3
		if c3 < 128 or c3 > 191 then
			error("Invalid UTF-8 character")
		end

		-- validate byte 4
		if c4 < 128 or c4 > 191 then
			error("Invalid UTF-8 character")
		end

		return 4

	else
		error("Invalid UTF-8 character")
	end
end

local function utf8len (s)
	-- argument checking
	if type(s) ~= "string" then
		error("bad argument #1 to "utf8len" (string expected, got ".. type(s).. ")")
	end

	local pos = 1
	local bytes = s:len()
	local len = 0

	while pos <= bytes and len ~= chars do
		local c = s:byte(pos)
		len = len + 1

		pos = pos + utf8charbytes(s, pos)
	end

	if chars ~= nil then
		return pos - 1
	end

	return len
end

print(utf8len("中国人"))