Codechange: use setting name instead of index for HandleOldDiffCustom() (#9311)

src/table/settings/gameopt_settings.ini

@@ -15,6 +15,7 @@
 
 [pre-amble]
 static const uint GAME_DIFFICULTY_NUM = 18;
+static const std::array<std::string, GAME_DIFFICULTY_NUM> _old_diff_settings{"max_no_competitors", "competitor_start_time", "number_towns", "industry_density", "max_loan", "initial_interest", "vehicle_costs", "competitor_speed", "competitor_intelligence", "vehicle_breakdowns", "subsidy_multiplier", "construction_cost", "terrain_type", "quantity_sea_lakes", "economy", "line_reverse_mode", "disasters", "town_council_tolerance"};
 static uint16 _old_diff_custom[GAME_DIFFICULTY_NUM];
 uint8 _old_diff_level;                                 ///< Old difficulty level from old savegames
 uint8 _old_units;                                      ///< Old units from old savegames

src/table/settings/settings.ini

@@ -94,7 +94,7 @@ startup  = false
 
 
 ; Saved settings variables.
-; Do not ADD or REMOVE something in this "difficulty.XXX" table or before it. It breaks savegame compatibility.
+; The next 18 entries are important for savegame compatibility. Do NOT remove those. See HandleOldDiffCustom() for more details.
 [SDT_VAR]
 var      = difficulty.max_no_competitors
 type     = SLE_UINT8
@@ -106,10 +106,14 @@ interval = 1
 post_cb  = MaxNoAIsChange
 cat      = SC_BASIC
 
-[SDT_NULL]
-length   = 1
+[SDT_VAR]
+var      = difficulty.competitor_start_time
+type     = SLE_UINT8
 from     = SLV_97
 to       = SLV_110
+def      = 2
+min      = 0
+max      = 3
 
 [SDT_VAR]
 var      = difficulty.number_towns
@@ -192,10 +196,14 @@ strhelp  = STR_CONFIG_SETTING_CONSTRUCTION_SPEED_HELPTEXT
 strval   = STR_AI_SPEED_VERY_SLOW
 cat      = SC_BASIC
 
-[SDT_NULL]
-length   = 1
+[SDT_VAR]
+var      = difficulty.competitor_intelligence
+type     = SLE_UINT8
 from     = SLV_97
 to       = SLV_110
+def      = 0
+min      = 0
+max      = 2
 
 [SDT_VAR]
 var      = difficulty.vehicle_breakdowns