[pob2-port] Fix calculation of inverted Resistances when having a source of Penetration (#9898)

* Apply changes from PathOfBuildingCommunity/PathOfBuilding-PoE2#2177

* Fix merge

---------

Co-authored-by: LocalIdentity <LocalIdentity@users.noreply.github.com>
Co-authored-by: LocalIdentity <localidentity2@gmail.com>

Author: 3 people

spec/System/TestSkills_spec.lua

@@ -150,6 +150,22 @@ describe("TestAttacks", function()
 
 		assert.True(preAdrenalineMaxStages < build.calcsTab.mainEnv.player.activeSkillList[1].skillModList:Sum("BASE", nil, "Multiplier:BlightMaxStages"))
 	end)
+
+	it("averages inverted elemental resistance after penetration", function()
+		build.skillsTab:PasteSocketGroup("Fireball 20/0  1")
+		build.configTab.input.enemyIsBoss = "None"
+		build.configTab.input.enemyFireResist = 50
+		build.configTab.input.customMods = "Hits have 50% chance to treat Enemy Monster Elemental Resistance values as inverted\nDamage Penetrates 50% of Enemy Fire Resistance"
+		build.configTab:BuildModList()
+		runCallback("OnFrame")
+
+		-- Unlike PoE 2, PoE 1 penetration can lower resistance below zero:
+		-- 50% of hits use 0% resistance and 50% use -100% resistance.
+		assert.are.equals(1.5, build.calcsTab.calcsOutput.FireEffMult)
+		local breakdownText = table.concat(build.calcsTab.calcsEnv.player.breakdown.FireEffMult, "\n")
+		assert.is_truthy(breakdownText:match("inverted hit"))
+		assert.is_truthy(breakdownText:match("weighted average"))
+	end)
 	it("Test cost efficiency modifiers", function()
 		-- Test Mana Cost Efficiency
 		build.skillsTab:PasteSocketGroup("Ball Lightning 1/0  1\n")
@@ -212,4 +228,4 @@ describe("TestAttacks", function()
 		local finalCost = build.calcsTab.mainOutput.ManaCost
 		assert.are.equals(7, round(finalCost))
 	end)
-end)
+end)

src/Modules/CalcBreakdown.lua

@@ -108,20 +108,25 @@ function breakdown.area(base, areaMod, total, incBreakpoint, moreBreakpoint, red
 	return out
 end
 
-function breakdown.effMult(damageType, resist, pen, taken, mult, takenMore, sourceRes, useRes, invertChance)
+function breakdown.effMult(damageType, resist, pen, taken, mult, takenMore, sourceRes, useRes, invertChance, effectiveResist)
 	local out = { }
 	local resistForm = (damageType == "Physical") and "physical damage reduction" or "resistance"
 	local resistLabel = resistForm
+	effectiveResist = effectiveResist or resist - pen
 
-	if invertChance and invertChance ~= 0 then
-		resistLabel = "average inverted "..resistForm
-	end
 	if sourceRes and sourceRes ~= damageType then
 		t_insert(out, s_format("Enemy %s: %d%% ^8(%s)", resistLabel, resist, sourceRes))
 	elseif resist ~= 0 then
 		t_insert(out, s_format("Enemy %s: %d%%", resistLabel, resist))
 	end
-	if pen ~= 0 or not useRes then
+	if invertChance and invertChance ~= 0 and useRes then
+		local normalResist = resist - pen
+		local invertedResist = -resist - pen
+		t_insert(out, "Effective resistance:")
+		t_insert(out, s_format("%g%% ^8(non-inverted hit after penetration)", normalResist))
+		t_insert(out, s_format("%g%% ^8(inverted hit after penetration)", invertedResist))
+		t_insert(out, s_format("= %g%% ^8(weighted average from %.0f%% inversion chance)", effectiveResist, invertChance * 100))
+	elseif pen ~= 0 or not useRes then
 		t_insert(out, "Effective resistance:")
 		t_insert(out, s_format("%d%% ^8(resistance)", resist))
 		if pen < 0 then
@@ -139,7 +144,7 @@ function breakdown.effMult(damageType, resist, pen, taken, mult, takenMore, sour
 	if useRes then
 		breakdown.multiChain(out, {
 			label = "Effective DPS modifier:",
-			{ "%.2f ^8(%s)", 1 - (resist - pen) / 100, resistForm },
+			{ "%.2f ^8(%s)", 1 - effectiveResist / 100, resistForm },
 			{ "%.2f ^8(increased/reduced damage taken)", 1 + taken / 100 },
 			{ "%.2f ^8(more/less damage taken)", takenMore },
 			total = s_format("= %.3f", mult),

src/Modules/CalcOffence.lua

@@ -3367,14 +3367,8 @@ function calcs.offence(env, actor, activeSkill)
 							end
 						end
 						local invertChanceEle = m_max(m_min(skillModList:Sum("CHANCE", cfg, "HitsInvertEleResChance"), 1), 0)
-						if isElemental[damageType] and invertChanceEle > 0 then
-							-- resist = (1 - invertChanceEle) * resist + invertChanceEle * (-1 * resist)
-							resist = resist - 2 * invertChanceEle * resist
-						end
 						local invertChanceChaos = m_max(m_min(skillModList:Sum("CHANCE", cfg, "HitsInvertChaosResChance"), 1), 0)
-						if damageType == "Chaos" and invertChanceChaos > 0 then
-							resist = resist - 2 * invertChanceChaos * resist
-						end
+						local invertChance = (isElemental[damageType] and invertChanceEle) or (damageType == "Chaos" and invertChanceChaos) or 0
 						sourceRes = env.modDB:Flag(nil, "Enemy"..sourceRes.."ResistEqualToYours") and "Your "..sourceRes.." Resistance" or (env.partyMembers.modDB:Flag(nil, "Enemy"..sourceRes.."ResistEqualToYours") and "Party Member "..sourceRes.." Resistance" or sourceRes)
 						if skillFlags.projectile then
 							takenInc = takenInc + enemyDB:Sum("INC", nil, "ProjectileDamageTaken")
@@ -3388,24 +3382,30 @@ function calcs.offence(env, actor, activeSkill)
 						local effMult = (1 + takenInc / 100) * takenMore
 						local useResChance = useThisResist(damageType)
 						local useRes = useResChance > 0
+						local effectiveResist = resist
 						if skillModList:Flag(cfg, isElemental[damageType] and "CannotElePenIgnore" or nil) then
-							effMult = effMult * (1 - resist / 100)
+							effectiveResist = invertChance > 0 and (resist - 2 * invertChance * resist) or resist
+							effMult = effMult * (1 - effectiveResist / 100)
 						elseif useRes then
-							effMult = effMult * (1 - ((resist - pen) * useResChance) / 100)
+							if invertChance > 0 then
+								effectiveResist = ((resist - pen) * (1 - invertChance) + (-resist - pen) * invertChance) * useResChance
+							else
+								effectiveResist = (resist - pen) * useResChance
+							end
+							effMult = effMult * (1 - effectiveResist / 100)
 						end
 						damageTypeHitMin = damageTypeHitMin * effMult
 						damageTypeHitMax = damageTypeHitMax * effMult
 						damageTypeHitAvg = damageTypeHitAvg * effMult
 						if env.mode == "CALCS" then
 							output[damageType.."EffMult"] = effMult
 						end
-						local invertChance = (isElemental[damageType] and invertChanceEle) or (damageType == "Chaos" and invertChanceChaos) or 0
-						if pass == 2 and breakdown and (effMult ~= 1 or sourceRes ~= damageType) and skillModList:Flag(cfg, isElemental[damageType] and "CannotElePenIgnore" or nil) then
+						if pass == 2 and breakdown and (effMult ~= 1 or sourceRes ~= damageType or invertChance > 0) and skillModList:Flag(cfg, isElemental[damageType] and "CannotElePenIgnore" or nil) then
 							t_insert(breakdown[damageType], s_format("x %.3f ^8(effective DPS modifier)", effMult))
-							breakdown[damageType.."EffMult"] = breakdown.effMult(damageType, resist, 0, takenInc, effMult, takenMore, sourceRes, useRes, invertChance)
-						elseif pass == 2 and breakdown and (effMult ~= 1 or sourceRes ~= damageType) then
+							breakdown[damageType.."EffMult"] = breakdown.effMult(damageType, resist, 0, takenInc, effMult, takenMore, sourceRes, useRes, invertChance, effectiveResist)
+						elseif pass == 2 and breakdown and (effMult ~= 1 or sourceRes ~= damageType or invertChance > 0) then
 							t_insert(breakdown[damageType], s_format("x %.3f ^8(effective DPS modifier)", effMult))
-							breakdown[damageType.."EffMult"] = breakdown.effMult(damageType, resist, pen, takenInc, effMult, takenMore, sourceRes, useRes, invertChance)
+							breakdown[damageType.."EffMult"] = breakdown.effMult(damageType, resist, pen, takenInc, effMult, takenMore, sourceRes, useRes, invertChance, effectiveResist)
 						end
 					end
 					if pass == 2 and breakdown then