Optimize string parsing using a rope

json.lua

@@ -231,7 +231,7 @@ end
 
 
 local function parse_string(str, i)
-  local res = ""
+  local res = {}
   local j = i + 1
   local k = j
 
@@ -242,32 +242,33 @@ local function parse_string(str, i)
       decode_error(str, j, "control character in string")
 
     elseif x == 92 then -- `\`: Escape
-      res = res .. str:sub(k, j - 1)
+      res[#res + 1] = str:sub(k, j - 1)
       j = j + 1
       local c = str:sub(j, j)
       if c == "u" then
         local hex = str:match("^[dD][89aAbB]%x%x\\u%x%x%x%x", j + 1)
                  or str:match("^%x%x%x%x", j + 1)
                  or decode_error(str, j - 1, "invalid unicode escape in string")
-        res = res .. parse_unicode_escape(hex)
+        res[#res + 1] = parse_unicode_escape(hex)
         j = j + #hex
       else
         if not escape_chars[c] then
           decode_error(str, j - 1, "invalid escape char '" .. c .. "' in string")
         end
-        res = res .. escape_char_map_inv[c]
+        res[#res + 1] = escape_char_map_inv[c]
       end
       k = j + 1
 
     elseif x == 34 then -- `"`: End of string
-      res = res .. str:sub(k, j - 1)
-      return res, j + 1
+      res[#res + 1] = str:sub(k, j - 1)
+      return table.concat(res), j + 1
     end
 
     j = j + 1
   end
 
   decode_error(str, i, "expected closing quote for string")
+  return table.concat(res)
 end