From 1818074018347776954a7d0e689af84d9a92cfaa Mon Sep 17 00:00:00 2001
From: Ihnatus <ignatus31oct@mail.ru>
Date: Thu, 23 Mar 2023 22:05:31 +0300
Subject: [PATCH] A generic simple function to calculate effect expected value

See #47671

Signed-off-by: Ihnatus <ignatus31oct@mail.ru>
---
 common/effects.c | 28 ++++++++++++++++++++++++++++
 common/effects.h | 14 ++++++++++++++
 2 files changed, 42 insertions(+)

diff --git a/common/effects.c b/common/effects.c
index 6dc58b0a15..1e94e20434 100644
--- a/common/effects.c
+++ b/common/effects.c
@@ -1194,6 +1194,34 @@ void get_target_effects_limits(int *minv, int *maxv,
   }
 }
 
+/**********************************************************************//**
+  Returns the expected value of the effect of given type for given context,
+  calculating value weighted with probability for each individual effect
+  with given callback using arbitrary additional data passed to it.
+
+  The way of using multipliers is left on the callback.
+**************************************************************************/
+double get_effect_expected_value(const struct req_context *context,
+                                 const struct player *other_player,
+                                 enum effect_type effect_type,
+                                 eft_value_filter_cb weighter,
+                                 void *data, int n_data)
+{
+  double sum = 0.;
+
+  fc_assert_ret_val(weighter, 0.);
+  if (context == NULL) {
+    context = req_context_empty();
+  }
+
+  /* Loop over all effects of this type. */
+  effect_list_iterate(get_effects(effect_type), peffect) {
+    sum += weighter(peffect, context, other_player, data, n_data);
+  } effect_list_iterate_end;
+
+  return sum;
+}
+
 /**********************************************************************//**
   Returns the effect bonus for the whole world.
 **************************************************************************/
diff --git a/common/effects.h b/common/effects.h
index dad701a9e3..763eeb399d 100644
--- a/common/effects.h
+++ b/common/effects.h
@@ -381,6 +381,15 @@ struct effect {
   } rulesave;
 };
 
+/* A callback type that takes an individual effect and a context for it
+ * and tells its weighted (by probability or something) value.
+ */
+typedef double
+   (*eft_value_filter_cb)(const struct effect *eft,
+                          const struct req_context *context,
+                          const struct player *other_player,
+                          void *data, int n_data);
+
 /* An effect_list is a list of effects. */
 #define SPECLIST_TAG effect
 #define SPECLIST_TYPE struct effect
@@ -487,6 +496,11 @@ void get_target_effects_limits(int *minv, int *maxv,
                                enum effect_type effect_type,
                                req_tester_cb tester,
                                void *data, int n_data);
+double get_effect_expected_value(const struct req_context *context,
+                                 const struct player *other_player,
+                                 enum effect_type effect_type,
+                                 eft_value_filter_cb weighter,
+                                 void *data, int n_data);
 
 bool building_has_effect(const struct impr_type *pimprove,
 			 enum effect_type effect_type);
-- 
2.37.2