Make spell absorption multiplicative (bug #4684)

CHANGELOG.md

@@ -143,6 +143,7 @@
     Bug #4674: Journal can be opened when settings window is open
     Bug #4677: Crash in ESM reader when NPC record has DNAM record without DODT one
     Bug #4678: Crash in ESP parser when SCVR has no variable names
+    Bug #4684: Spell Absorption is additive
     Bug #4685: Missing sound causes an exception inside Say command
     Bug #4689: Default creature soundgen entries are not used
     Feature #912: Editor: Add missing icons to UniversalId tables

apps/openmw/mwmechanics/spellcasting.cpp

@@ -324,6 +324,34 @@ namespace MWMechanics
         return true;
     }
 
+    class GetAbsorptionProbability : public MWMechanics::EffectSourceVisitor
+    {
+    public:
+        float mProbability;
+
+        GetAbsorptionProbability(const MWWorld::Ptr& actor)
+            : mProbability(0.f){}
+
+        virtual void visit (MWMechanics::EffectKey key,
+                                const std::string& sourceName, const std::string& sourceId, int casterActorId,
+                            float magnitude, float remainingTime = -1, float totalTime = -1)
+        {
+            if (key.mId == ESM::MagicEffect::SpellAbsorption)
+            {
+                if (mProbability == 0.f)
+                    mProbability = magnitude / 100;
+                else
+                {
+                    // If there are different sources of SpellAbsorption effect, multiply failing probability for all effects.
+                    // Real absorption probability will be the (1 - total fail chance) in this case.
+                    float failProbability = 1.f - mProbability;
+                    failProbability *= 1.f - magnitude / 100;
+                    mProbability = 1.f - failProbability;
+                }
+            }
+        }
+    };
+
     CastSpell::CastSpell(const MWWorld::Ptr &caster, const MWWorld::Ptr &target, const bool fromProjectile, const bool manualSpell)
         : mCaster(caster)
         , mTarget(target)
@@ -444,12 +472,18 @@ namespace MWMechanics
                 MWBase::Environment::get().getWindowManager()->setEnemy(target);
 
             // Try absorbing if it's a spell
-            // NOTE: Vanilla does this once per spell absorption effect source instead of adding the % from all sources together, not sure
-            // if that is worth replicating.
+            // NOTE: Vanilla does this once per spell absorption effect source instead of adding the % from all sources together, so use the same approach here
             bool absorbed = false;
             if (spell && caster != target && target.getClass().isActor())
             {
-                float absorb = target.getClass().getCreatureStats(target).getMagicEffects().get(ESM::MagicEffect::SpellAbsorption).getMagnitude();
+                GetAbsorptionProbability check(target);
+                MWMechanics::CreatureStats& stats = target.getClass().getCreatureStats(target);
+                stats.getActiveSpells().visitEffectSources(check);
+                stats.getSpells().visitEffectSources(check);
+                if (target.getClass().hasInventoryStore(target))
+                    target.getClass().getInventoryStore(target).visitEffectSources(check);
+
+                int absorb = check.mProbability * 100;
                 absorbed = (Misc::Rng::roll0to99() < absorb);
                 if (absorbed)
                 {