Improve expression parsing

Now uses the game's builtin expression evaluator
(`game.evaluate_expression`) instead of using Lua's `load` function.

Also improves behaviour when invalid expressions are confirmed in the
GUI, falling back to the previous valid value instead.

Fixes #18

src/changelog.txt

@@ -1,4 +1,10 @@
 ---------------------------------------------------------------------------------------------------
+Version: 0.4.4
+Date: 2023-04-16
+  Changes:
+    - Switch to using built-in expression evaluator in Factorio.
+    - Now falls back to previous valid value if invalid signal count is entered.
+---------------------------------------------------------------------------------------------------
 Version: 0.4.3
 Date: 2023-04-15
   Bugfixes:

src/scripts/expression.lua

@@ -6,23 +6,21 @@ local NONMATH_PATTERN = "[^0-9%.+%-*/%%^()]"
 local expr = {}
 
 --- @param input string?
+--- @param fallback number?
 --- @return number
-function expr.parse(input)
+function expr.parse(input, fallback)
+  fallback = fallback or 0
   log:debug("parsing expression: ", input)
-  if not input then return 0 end
-  local e = gsub(input, NONMATH_PATTERN, "")
-  local f, err = load("return " .. e, nil, "t", {})
-  if not f then
-    log:warn("The given input '", input, "' could not be parsed as a math expression: ", err)
-    return 0
-  end
-  local success, result = pcall(f)
+  if not input then return fallback end
+
+  local success, result = pcall(game.evaluate_expression, input)
+
   if not success then
     log:warn("The given input '", input, "' could not be evaluated as a math expression: ", result)
-    return 0
+    return fallback
   end
-  log:debug("expression '", input, "' parsed as: ", result)
-  return tonumber(result) or 0
+
+  return result
 end
 
 return expr

src/scripts/gui.lua

@@ -101,8 +101,9 @@ end
 
 --- @param element LuaGuiElement
 --- @param player PlayerIdentification?
+--- @param fallback number?
 --- @return number?
-local function resolve_textfield_number(element, player)
+local function resolve_textfield_number(element, player, fallback)
   if not element or not element.text then return nil end
   local min = tonumber(element.tags.min) or constants.INT32_MIN
   local max = tonumber(element.tags.max) or constants.INT32_MAX
@@ -115,9 +116,9 @@ local function resolve_textfield_number(element, player)
   local value
 
   if enable_expressions then
-    value = expression.parse(element.text)
+    value = expression.parse(element.text, fallback)
   else
-    value = tonumber(element.text)
+    value = tonumber(element.text) or fallback
   end
 
   if not value then return nil end
@@ -245,7 +246,7 @@ local function set_cs_signal_value(element, player, update_element)
   if not state then return end
   local signal_name = element.tags.signal_name --[[@as string]]
   local current = state.combinator:get_cs_value(signal_name)
-  local value = resolve_textfield_number(element, player)
+  local value = resolve_textfield_number(element, player, current)
   if value then
     log:debug("cs_signal_value_changed: ", signal_name, " changed to ", value)
   else
@@ -437,7 +438,7 @@ local function handle_signal_value_confirmed(event)
   local state = get_player_state(event.player_index)
   if not state or not state.selected_slot then return end
   local current = state.combinator:get_item_slot(state.selected_slot)
-  local value = resolve_textfield_number(state.signal_value_items, event.player_index)
+  local value = resolve_textfield_number(state.signal_value_items, event.player_index, current.count or 0)
   if not value then
     if current and current.count then
       state.signal_value_items.text = tostring(current.count)
@@ -456,7 +457,8 @@ end
 local function handle_signal_value_confirm(event)
   local state = get_player_state(event.player_index)
   if not state or not state.selected_slot then return end
-  local value = resolve_textfield_number(state.signal_value_items, event.player_index)
+  local current = state.combinator:get_item_slot(state.selected_slot)
+  local value = resolve_textfield_number(state.signal_value_items, event.player_index, current.count or 0)
   if not value then
     state.signal_value_confirm.enabled = false
     return