Damage as a percentage of HP, which the case did not read

src/NosCore.GameObject/Services/BattleService/HitQueue.cs

@@ -132,7 +132,8 @@ private async Task TryApplyHit(HitRequest request)
                 return;
             }
 
-            var newHp = target.Hp - damage.Damage;
+            // Damage proportional to HP rather than to the stats, on top of the blow.
+            var newHp = target.Hp - damage.Damage - PercentageHpLoss(target, request.Skill.BCards);
             var overkill = 0;
             var killed = false;
             if (newHp <= 0)
@@ -191,6 +192,39 @@ await ApplySpecialActionsAsync(request.Skill.BCards, request.Origin, target)
         }
     }
 
+    /// <summary>
+    /// Type 37 subtype 31: "Decreases the opponent's HP by %s%%." 112 of the 122 declarations on
+    /// skills are this subtype, and the case was not read at all.
+    /// </summary>
+    /// <remarks>
+    /// The percentage is taken from maximum HP. The file says only "HP by %s%%"; off current HP
+    /// the loss would halve and halve again without ever finishing anything, which is not what a
+    /// skill declaring 90% is for.
+    ///
+    /// Subtype 32 hits the caster instead and no skill declares it, so it is left out.
+    ///
+    /// The loss can kill: it is part of the same subtraction as the blow, so the ordinary death
+    /// path handles it and nothing says the effect must leave its victim standing.
+    /// </remarks>
+    private static int PercentageHpLoss(IAliveEntity target, IReadOnlyList<BCardDto> bCards)
+    {
+        var loss = 0;
+        for (var i = 0; i < bCards.Count; i++)
+        {
+            var bCard = bCards[i];
+            if ((BCardType.CardType)bCard.Type != BCardType.CardType.RecoveryAndDamagePercent
+                || bCard.SubType != (byte)AdditionalTypes.RecoveryAndDamagePercent.DecreaseEnemyHp
+                || bCard.FirstData <= 0)
+            {
+                continue;
+            }
+
+            loss += target.MaxHp * bCard.FirstData / 100;
+        }
+
+        return loss;
+    }
+
     private static void FlipIsAlive(IAliveEntity entity, bool alive)
     {
         switch (entity)

test/NosCore.GameObject.Tests/Services/BattleService/HitQueueTests.cs

@@ -4,6 +4,7 @@
 // |_|\__|\__/ |___/ \__/\__/|_|_\___|
 //
 
+using NosCore.Data.Enumerations.Buff;
 using System;
 using System.Collections.Concurrent;
 using System.Collections.Generic;
@@ -144,6 +145,115 @@ public async Task MissHitsDoNotAffectHpOrHitList()
             Assert.AreEqual(0, target.HitList.Count);
         }
 
+        // --- Type 37 subtype 31, "Decreases the opponent's HP by %s%%" ----------------------
+        //
+        // 112 of the 122 declarations on skills are this subtype, and the case was not read at
+        // all. The percentage comes off MAXIMUM HP - our assumption, stated in the code, because
+        // the file says only "HP by %s%%". These pin it: read off current HP every number below
+        // changes, and nothing would raise.
+
+        private static BCardDto PercentOfHp(short percent) => new()
+        {
+            Type = (byte)BCardType.CardType.RecoveryAndDamagePercent,
+            SubType = (byte)AdditionalTypes.RecoveryAndDamagePercent.DecreaseEnemyHp,
+            FirstData = percent
+        };
+
+        private static HitQueue QueueDealing(int ordinaryDamage)
+        {
+            var calc = new Mock<IDamageCalculator>();
+            calc.Setup(c => c.Calculate(It.IsAny<CombatStats>(), It.IsAny<CombatStats>(), It.IsAny<SkillInfo>()))
+                .Returns(new DamageResult(ordinaryDamage, SuPacketHitMode.SuccessAttack));
+            var stats = new Mock<IBattleStatsProvider>();
+            stats.Setup(s => s.GetStats(It.IsAny<IAliveEntity>())).Returns(new CombatStats());
+            return new HitQueue(calc.Object, stats.Object, new Mock<IBuffService>().Object,
+                new Mock<IRegenerationService>().Object, new Mock<ILogger<HitQueue>>().Object);
+        }
+
+        [TestMethod]
+        public async Task ThePercentageComesOffTheMaximumAndNotTheCurrentHp()
+        {
+            // Half of the maximum is 500. Off the current 600 it would be 300, and the target
+            // would end at 290 instead of 90.
+            var target = new FakeBattleEntity { Hp = 600, MaxHp = 1000 };
+            var attacker = new FakeBattleEntity();
+
+            await QueueDealing(10).EnqueueAsync(Request(attacker, target) with
+            {
+                Skill = MakeSkill() with { BCards = new[] { PercentOfHp(50) } }
+            });
+
+            Assert.AreEqual(90, target.Hp);
+        }
+
+        [TestMethod]
+        public async Task WithoutTheEffectOnlyTheOrdinaryDamageLands()
+        {
+            var target = new FakeBattleEntity { Hp = 600, MaxHp = 1000 };
+            var attacker = new FakeBattleEntity();
+
+            await QueueDealing(10).EnqueueAsync(Request(attacker, target) with { Skill = MakeSkill() });
+
+            Assert.AreEqual(590, target.Hp);
+        }
+
+        // Two slots on one skill add up: the loop must not stop at the first.
+        [TestMethod]
+        public async Task TwoDeclarationsOnOneSkillBothCount()
+        {
+            var target = new FakeBattleEntity { Hp = 1000, MaxHp = 1000 };
+            var attacker = new FakeBattleEntity();
+
+            // One point of ordinary damage, not zero: a blow that rolls zero is already a miss
+            // by the guard above, and the percentage rides along with a blow that landed.
+            await QueueDealing(1).EnqueueAsync(Request(attacker, target) with
+            {
+                Skill = MakeSkill() with { BCards = new[] { PercentOfHp(20), PercentOfHp(30) } }
+            });
+
+            Assert.AreEqual(499, target.Hp);
+        }
+
+        // It goes through the ordinary death rather than being floored short of it.
+        [TestMethod]
+        public async Task ThePercentageDamageCanKillAndTheDeathIsTheOrdinaryOne()
+        {
+            var target = new FakeBattleEntity { Hp = 200, MaxHp = 1000 };
+            var attacker = new FakeBattleEntity();
+
+            var outcome = await QueueDealing(1).EnqueueAsync(Request(attacker, target) with
+            {
+                Skill = MakeSkill() with { BCards = new[] { PercentOfHp(90) } }
+            });
+
+            Assert.AreEqual(0, target.Hp);
+            Assert.IsTrue(outcome.Killed);
+            Assert.IsFalse(target.IsAlive);
+        }
+
+        // Subtype 32 hits the caster, and no skill in the file declares it. Reading it here would
+        // take the HP off the wrong entity.
+        [TestMethod]
+        public async Task TheSelfInflictedSubtypeIsNotReadAsIfItHitTheTarget()
+        {
+            var target = new FakeBattleEntity { Hp = 1000, MaxHp = 1000 };
+            var attacker = new FakeBattleEntity();
+            var selfInflicted = new BCardDto
+            {
+                Type = (byte)BCardType.CardType.RecoveryAndDamagePercent,
+                SubType = (byte)AdditionalTypes.RecoveryAndDamagePercent.DecreaseSelfHp,
+                FirstData = 50
+            };
+
+            await QueueDealing(1).EnqueueAsync(Request(attacker, target) with
+            {
+                Skill = MakeSkill() with { BCards = new[] { selfInflicted } }
+            });
+
+            // Only the ordinary point of damage: the self-inflicted subtype took nothing here.
+            Assert.AreEqual(999, target.Hp);
+        }
+
         private static HitRequest Request(IAliveEntity attacker, IAliveEntity target) => new(
             Origin: attacker,
             Target: target,