Make utf8.char variadic, (re)move checks

utf8.lua

@@ -1,15 +1,15 @@
-local string_char = string.char
+local string_char, table_concat = string.char, table.concat
 
 local utf8 = {}
 
-function utf8.char(codepoint)
-	if codepoint <= 0x007F then
-		-- Single byte
-		return string_char(codepoint)
+function utf8.is_valid_codepoint(codepoint)
+	-- Must be in bounds & must not be a surrogate
+	return codepoint <= 0x10FFFF and (codepoint < 0xD800 or codepoint > 0xDFFF)
 end
-	if codepoint < 0x00A0 or codepoint > 0x10FFFF then
-		-- Out of range
-		return -- TODO (?) error instead
+
+local function utf8_char(codepoint)
+	if codepoint <= 0x007F then -- single byte
+		return string_char(codepoint) -- UTF-8 encoded string
 	end
 	local result = ""
 	local i = 0
@@ -19,7 +19,18 @@ function utf8.char(codepoint)
 		codepoint = (codepoint - remainder) / 64
 		i = i + 1
 	until codepoint <= 2 ^ (8 - i - 2)
-	return string_char(0x100 - 2 ^ (8 - i - 1) + codepoint) .. result
+
+	return string_char(0x100 - 2 ^ (8 - i - 1) + codepoint) .. result -- UTF-8 encoded string
+end
+
+function utf8.char(...)
+	local n_args = select("#", ...)
+	if n_args == 1 then return utf8_char(...) end
+	local chars = {}
+	for i = 1, n_args do
+		chars[i] = utf8_char(select(i, ...))
+	end
+	return table_concat(chars)
 end
 
 return utf8