Compress decimal escapes for string.format("%q").

Lua 5.2+ escape all control characters using decimal escape
sequences. The previous code did the same but always used three
decimals (possibly zero-padded) no matter the following character.
The new version only uses all three decimals when another decimal
character is following.

Author: siffiejoe

compat53/module.lua

@@ -671,8 +671,8 @@ if lua_version < "5.3" then
                ["\""] = "\\\""
             }
 
-            local function addquoted(c)
-               return addqt[c] or string_format("\\%03d", c:byte())
+            local function addquoted(c, d)
+               return (addqt[c] or string_format(d~="" and "\\%03d" or "\\%d", c:byte()))..d
             end
 
             function M.string.format(fmt, ...)
@@ -684,7 +684,7 @@ if lua_version < "5.3" then
                      if kind == "s" then
                         args[i] = _G.tostring(args[i])
                      elseif kind == "q" then
-                        args[i] = '"'..string_gsub(args[i], "[%z%c\\\"\n]", addquoted)..'"'
+                        args[i] = '"'..string_gsub(args[i], "([%z%c\\\"\n])(%d?)", addquoted)..'"'
                         return lead.."%"..mods.."s"
                      end
                   end

tests/test.lua

@@ -505,7 +505,7 @@ do
          return _tostring(v)
       end
    end
-   print("string.format()", string.format("%q", "\"\\\0000\0010\r0\n0\t0\""))
+   print("string.format()", string.format("%q", "\"\\\0000\0010\002\r\n0\t0\""))
    print("string.format()", string.format("%12.3fx%%sxx%.6s", 3.1, {}))
    print("string.format()", string.format("%-3f %%%s %%s", 3.1, true))
    print("string.format()", string.format("% 3.2g %%d %%%s", 3.1, nil))