`set_item_category_spawn_rates` should take math

[Standing-Storm]

Is your feature request related to a problem? Please describe.

I thought "Oh, it would be neat to have certain loot categories decrease with time from the Cataclysm!" only to find out that set_item_category_spawn_rates only takes numbers.

Solution you would like.

set_item_category_spawn_rates should accept math.

Describe alternatives you have considered.

No response

Additional context

[GuardianDll]

try this

diff --git a/src/npctalk.cpp b/src/npctalk.cpp
index a04b7ebec9..6bb407b1c2 100644
--- a/src/npctalk.cpp
+++ b/src/npctalk.cpp
@@ -5290,23 +5290,23 @@ talk_effect_fun_t::func f_set_item_category_spawn_rates( const JsonObject &jo,
         std::string_view member, const std::string_view )
 {
     if( jo.has_array( member ) ) {
-        std::set<std::pair<item_category_id, float>> rates;
+        std::set<std::pair<item_category_id, dbl_or_var>> rates;
         for( const JsonObject joi : jo.get_array( member ) ) {
             item_category_id cat( joi.get_string( "id" ) );
-            float spawn_rate = joi.get_float( "spawn_rate" );
+            dbl_or_var spawn_rate = get_dbl_or_var( joi.get_member( "spawn_rate" ), "spawn_rate", true );
             rates.insert( std::make_pair( cat, spawn_rate ) );
         }
-        return [rates]( dialogue const &/* d */ ) {
-            for( const std::pair<item_category_id, float> &rate : rates ) {
-                rate.first.obj().set_spawn_rate( rate.second );
+        return [rates]( dialogue const & d ) {
+            for( const std::pair<item_category_id, dbl_or_var> &rate : rates ) {
+                rate.first.obj().set_spawn_rate( rate.second.evaluate( d ) );
             }
         };
     } else {
         JsonObject joi = jo.get_member( member );
         item_category_id cat( joi.get_string( "id" ) );
-        float spawn_rate = joi.get_float( "spawn_rate" );
-        return [cat, spawn_rate]( dialogue const &/* d */ ) {
-            cat.obj().set_spawn_rate( spawn_rate );
+        dbl_or_var spawn_rate = get_dbl_or_var( joi.get_member( "spawn_rate" ), "spawn_rate", true );
+        return [cat, spawn_rate]( dialogue const & d ) {
+            cat.obj().set_spawn_rate( spawn_rate.evaluate( d ) );
         };
     }
 }

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