Fix calculation of inverted Resistances when having a source of Pen (#2177)

The calculation for inverted res was handled incorrectly where it was applying an average reduction to the resistance value and then we were applying pen on top of that
We now calculated the average res based on the chance to invert and the impact of pen

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

Author: LocalIdentity

spec/System/TestSkills_spec.lua

@@ -798,6 +798,20 @@ describe("TestSkills", function()
 		assert.True(build.calcsTab.mainEnv.enemyDB:Sum("BASE", nil, "FireResist") < fireResistWithoutPotentExposure)
 	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")
+
+		assert.are.equals(1.25, build.calcsTab.calcsOutput.FireEffMult)
+		local breakdownText = table.concat(build.calcsTab.calcsEnv.player.breakdown.FireEffMult, "\n")
+		assert.truthy(breakdownText:match("inverted hit"))
+		assert.truthy(breakdownText:match("weighted average"))
+	end)
+
 	it("Test granted skills with exposure stats make exposure configurable", function()
 		build.skillsTab:PasteSocketGroup("Fireball 20/0  1")
 		local spec = build.spec

src/Modules/CalcBreakdown.lua

@@ -110,20 +110,29 @@ function breakdown.area(base, areaMod, total, incBreakpoint, moreBreakpoint, red
 	return out
 end
 
-function breakdown.effMult(damageType, resist, pen, taken, mult, takenMore, sourceRes, useRes, invertChance, minPen)
+function breakdown.effMult(damageType, resist, pen, taken, mult, takenMore, sourceRes, useRes, invertChance, minPen, effectiveResist)
 	local out = { }
 	local resistForm = (damageType == "Physical") and "physical damage reduction" or "resistance"
 	local resistLabel = resistForm
-
-	if invertChance and invertChance ~= 0 then
-		resistLabel = "average inverted "..resistForm
+	minPen = minPen or 0
+	local calcPenResist = function(resist)
+		return resist > minPen and m_max(resist - pen, minPen) or resist
 	end
+	effectiveResist = effectiveResist or calcPenResist(resist)
+
 	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 = calcPenResist(resist)
+		local invertedResist = calcPenResist(-resist)
+		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
@@ -145,7 +154,7 @@ function breakdown.effMult(damageType, resist, pen, taken, mult, takenMore, sour
 	if useRes then
 		breakdown.multiChain(out, {
 			label = "Effective DPS modifier:",
-			{ "%.2f ^8(%s)", 1 - (math.max(resist - pen,0)) / 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

@@ -4157,11 +4157,7 @@ function calcs.offence(env, actor, activeSkill)
 							end
 						end
 
-						local invertChance = m_max(m_min(skillModList:Sum("CHANCE", cfg, "HitsInvertEleResChance"), 1), 0)
-						if isElemental[damageType] and invertChance > 0 then
-							-- resist = (1 - invertChance) * resist + invertChance * (-1 * resist)
-							resist = resist - 2 * invertChance * resist
-						end
+						local invertChance = isElemental[damageType] and m_max(m_min(skillModList:Sum("CHANCE", cfg, "HitsInvertEleResChance"), 1), 0) 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")
@@ -4174,23 +4170,33 @@ function calcs.offence(env, actor, activeSkill)
 						end
 						local effMult = (1 + takenInc / 100) * takenMore
 						local useRes = useThisResist(damageType)
+						local effectiveResist = resist
+						local calcPenResist = function(resist)
+							return resist > minPen and m_max(resist - pen, minPen) or resist
+						end
 						if skillModList:Flag(cfg, isElemental[damageType] and "CannotElePenIgnore" or nil) then
-							effMult = effMult * (1 - resist / 100)
+							effectiveResist = (isElemental[damageType] and invertChance > 0) and (resist - 2 * invertChance * resist) or resist
+							effMult = effMult * (1 - effectiveResist / 100)
 						elseif useRes then
-							effMult = effMult * (1 - (resist > minPen and m_max(resist - pen, minPen) or resist) / 100)
+							if isElemental[damageType] and invertChance > 0 then
+								effectiveResist = calcPenResist(resist) * (1 - invertChance) + calcPenResist(-resist) * invertChance
+							else
+								effectiveResist = calcPenResist(resist)
+							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
-						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, minPen)
-						elseif pass == 2 and breakdown and (effMult ~= 1 or (resist - pen) < minPen or sourceRes ~= damageType) then
+							breakdown[damageType.."EffMult"] = breakdown.effMult(damageType, resist, 0, takenInc, effMult, takenMore, sourceRes, useRes, invertChance, minPen, effectiveResist)
+						elseif pass == 2 and breakdown and (effMult ~= 1 or (resist - pen) < minPen 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, minPen)
+							breakdown[damageType.."EffMult"] = breakdown.effMult(damageType, resist, pen, takenInc, effMult, takenMore, sourceRes, useRes, invertChance, minPen, effectiveResist)
 						end
 					end
 					if pass == 2 and breakdown then