add example tests for damage

src/changing-level.allium

@@ -0,0 +1,120 @@
+-- allium: 3
+
+-- allium: changing-level
+
+------------------------------------------------------------
+-- Value Types
+------------------------------------------------------------
+
+value Faction {
+    name: String
+}
+
+value Health {
+    value: Integer
+    requires: value >= 0
+}
+
+value Level {
+    value: Integer
+    requires: value >= 1 and value <= 10
+}
+
+value Damage {
+    value: Integer
+    requires: value >= 0
+}
+
+------------------------------------------------------------
+-- Enumerations
+------------------------------------------------------------
+
+enum Status {
+    alive | dead
+}
+
+------------------------------------------------------------
+-- Entities
+------------------------------------------------------------
+
+entity Character {
+    name: String
+    health: Health
+    status: Status
+    level: Level
+    factions: Set<Faction>
+    totalDamageTaken: Damage
+    factionsJoined: Set<Faction>
+}
+
+------------------------------------------------------------
+-- Rules
+------------------------------------------------------------
+
+rule DamageIsAccumulated {
+    when: Character.dealDamage(attacker, target, damage)
+    requires: target.status = alive
+    ensures: target.totalDamageTaken.value = old(target.totalDamageTaken.value) + damage.value
+}
+
+rule LevelUpFromDamage {
+    when: Character.dealDamage(attacker, target, damage)
+    requires: target.status = alive
+    requires: old(target.totalDamageTaken.value) + damage.value >= 1000 * (target.level.value + 1) * (target.level.value + 2) / 2
+    ensures: target.level.value = target.level.value + 1
+    ensures: target.totalDamageTaken.value = old(target.totalDamageTaken.value) + damage.value
+}
+
+rule LevelUpFromFaction {
+    when: Character.joinFaction(character, faction)
+    requires: character.status = alive
+    requires: old(character.factionsJoined.size) + 1 >= 3 * (character.level.value + 1)
+    ensures: character.level.value = character.level.value + 1
+    ensures: character.factionsJoined = old(character.factionsJoined) + {faction}
+}
+
+rule MaxLevelCappedOnDamage {
+    when: Character.dealDamage(attacker, target, damage)
+    requires: target.status = alive
+    requires: target.level.value = 10
+    ensures: target.level.value = 10
+}
+
+rule MaxLevelCappedOnFaction {
+    when: Character.joinFaction(character, faction)
+    requires: character.status = alive
+    requires: character.level.value = 10
+    ensures: character.level.value = 10
+}
+
+rule DeadCannotLevelUpFromDamage {
+    when: Character.dealDamage(attacker, target, damage)
+    requires: target.status = dead
+    ensures: target.level.value = old(target.level.value)
+    ensures: target.totalDamageTaken.value = old(target.totalDamageTaken.value)
+}
+
+rule DeadCannotLevelUpFromFaction {
+    when: Character.joinFaction(character, faction)
+    requires: character.status = dead
+    ensures: character.level.value = old(character.level.value)
+}
+
+------------------------------------------------------------
+-- Invariants
+------------------------------------------------------------
+
+invariant LevelBounded {
+    for c in Characters:
+        c.level.value >= 1 and c.level.value <= 10
+}
+
+invariant DamageTotalNonNegative {
+    for c in Characters:
+        c.totalDamageTaken.value >= 0
+}
+
+invariant FactionsJoinedNonNegative {
+    for c in Characters:
+        c.factionsJoined.size >= 0
+}

src/changing-level.spec.ts

@@ -0,0 +1,126 @@
+import fc from 'fast-check';
+import { describe, it, expect } from 'vitest';
+import { Character } from './characters/Character.ts';
+import { Level } from './value-objects/Level.ts';
+import { Damage } from './value-objects/Damage.ts';
+import { Faction } from './factions/Faction.ts';
+
+describe('ChangingLevel', () => {
+  describe('MaxLevelCappedOnDamage (example)', () => {
+    it('L10 takes 99999 damage → stays L10', () => {
+      const attacker = Character.create({ name: 'dragons', level: Level.create(1) });
+      const target = Character.create({ name: 'hero', level: Level.create(10) });
+      const result = attacker.dealDamage(target, Damage.create(99999));
+      expect(result.level.value).toBe(10);
+    });
+  });
+
+  describe('MaxLevelCappedOnFaction (example)', () => {
+    it('L10 joins 100 factions → stays L10', () => {
+      const hero = Character.create({ name: 'hero', level: Level.create(10) });
+      let char = hero;
+      for (let i = 0; i < 100; i++) {
+        char = char.joinFaction(Faction.create(`faction-${i}`));
+      }
+      expect(char.level.value).toBe(10);
+    });
+  });
+
+  describe('DeadCannotLevelUpFromDamage (example)', () => {
+    it('dead char takes 1000 damage → stays same level', () => {
+      const attacker = Character.create({ name: 'attacker', level: Level.create(1) });
+      const target = Character.create({ name: 'hero', level: Level.create(1) });
+      const dead = attacker.dealDamage(target, Damage.create(10000));
+      const result = attacker.dealDamage(dead, Damage.create(1000));
+      expect(result.level.value).toBe(1);
+    });
+  });
+
+  describe('DeadCannotLevelUpFromFaction (example)', () => {
+    it('dead char joins 5 factions → stays same level', () => {
+      const attacker = Character.create({ name: 'attacker', level: Level.create(1) });
+      const target = Character.create({ name: 'hero', level: Level.create(1) });
+      const dead = attacker.dealDamage(target, Damage.create(10000));
+      let char = dead;
+      for (let i = 0; i < 5; i++) {
+        char = char.joinFaction(Faction.create(`faction-${i}`));
+      }
+      expect(char.level.value).toBe(1);
+    });
+  });
+
+  describe('LevelUpFromDamage (property)', () => {
+    it('property: cumulative damage triggers level-up at threshold', () => {
+      fc.assert(
+        fc.property(
+          fc.integer({ min: 1, max: 9 }),
+          fc.integer({ min: 1, max: 10000 }),
+          fc.integer({ min: 1, max: 10000 }),
+          (level, dmg1, dmg2) => {
+            const currentLevel = Level.create(level);
+            const targetLevel = Level.create(level + 1);
+            const threshold = 1000 * (level + 1) * (level + 2) / 2;
+            const total = dmg1 + dmg2;
+
+            // If total damage meets threshold, level should increase
+            if (total >= threshold) {
+              const attacker = Character.create({ name: 'a', level: Level.create(1) });
+              const target = Character.create({ name: 't', level: currentLevel });
+              const afterFirst = attacker.dealDamage(target, Damage.create(dmg1));
+              const afterSecond = attacker.dealDamage(afterFirst, Damage.create(dmg2));
+              return afterSecond.level.value === Math.min(10, level + 1);
+            }
+            return true; // skip non-applicable cases
+          },
+        ),
+      );
+    });
+  });
+
+  describe('LevelUpFromFaction (property)', () => {
+    it('property: faction count triggers level-up at threshold', () => {
+      fc.assert(
+        fc.property(
+          fc.integer({ min: 1, max: 9 }),
+          fc.integer({ min: 1, max: 100 }),
+          fc.integer({ min: 1, max: 100 }),
+          (level, join1, join2) => {
+            const threshold = 3 * (level + 1);
+            const total = join1 + join2;
+
+            if (total >= threshold) {
+              const hero = Character.create({ name: 'hero', level: Level.create(level) });
+              let char = hero;
+              for (let i = 0; i < total; i++) {
+                char = char.joinFaction(Faction.create(`faction-${i}`));
+              }
+              return char.level.value === Math.min(10, level + 1);
+            }
+            return true; // skip non-applicable cases
+          },
+        ),
+      );
+    });
+  });
+
+  describe('DamageAccumulation (property)', () => {
+    it('property: totalDamageTaken accumulates across multiple damage events', () => {
+      fc.assert(
+        fc.property(
+          fc.array(fc.integer({ min: 1, max: 5000 }), { minLength: 2, maxLength: 10 }),
+          (damageEvents) => {
+            const expectedTotal = damageEvents.reduce((sum, d) => sum + d, 0);
+            const attacker = Character.create({ name: 'a', level: Level.create(1) });
+            const target = Character.create({ name: 't', level: Level.create(1) });
+
+            let current = target;
+            for (const dmg of damageEvents) {
+              current = attacker.dealDamage(current, Damage.create(dmg));
+            }
+            return current.totalDamageTaken.value === expectedTotal;
+          },
+        ),
+      );
+    });
+  });
+});

src/characters/Character.ts

@@ -6,6 +6,7 @@
  */
 import { Health } from '../value-objects/Health.ts';
 import { Level } from '../value-objects/Level.ts';
+import type { Damage } from '../value-objects/Damage.ts';
 import type { Status } from '../value-objects/Status.ts';
 import { StatusAlive, StatusDead } from '../value-objects/Status.ts';
 import type { CharacterState } from './CharacterState.ts';
@@ -176,24 +177,22 @@ export class Character {
    * Deal damage to another character. Returns a new Character with updated state.
    * Does not mutate the attacker or the original target reference.
    */
-  dealDamage(target: Character, damage: number): Character {
+  dealDamage(target: Character, damage: Damage): Character {
     // Self-damage is forbidden — use reference equality, not name
     if (this === target) return target;
     // Allies cannot deal damage to each other
     if (this.isAllyOf(target)) return target;
     // Dead characters cannot take damage
     if (target.status.kind === 'dead') return target;
-    // Negative damage is invalid
-    if (damage < 0) throw new Error(`Damage must be non-negative, got ${damage}`);
     // Level-based damage modifier
     const levelDiff = this.level.diff(target.level); // = this.level - target.level
-    let actualDamage = damage;
+    let actualDamage = damage.value;
     if (levelDiff <= -5) {
       // Target is ≥5 levels above → damage reduced by 50%
-      actualDamage = Math.floor(damage * 0.5);
+      actualDamage = Math.floor(damage.value * 0.5);
     } else if (levelDiff >= 5) {
       // Target is ≥5 levels below → damage increased by 50%
-      actualDamage = Math.floor(damage * 1.5);
+      actualDamage = Math.floor(damage.value * 1.5);
     }
     // Reduce health by the (possibly modified) damage amount
     const newHealth = target.health.sub(actualDamage);

src/characters/CharacterState.ts

@@ -8,6 +8,7 @@ import type { Health } from '../value-objects/Health.ts';
 import type { Level } from '../value-objects/Level.ts';
 import type { Status } from '../value-objects/Status.ts';
 import type { Faction } from '../factions/Faction.ts';
+import type { Damage } from '../value-objects/Damage.ts';
 
 export type CharacterState = {
   readonly name: string;
@@ -15,4 +16,6 @@ export type CharacterState = {
   readonly status: Status;
   readonly level: Level;
   readonly factions: ReadonlySet<Faction>;
+  readonly totalDamageTaken: Damage;
+  readonly factionsJoined: ReadonlySet<Faction>;
 };

src/damage-and-health.spec.ts

@@ -2,6 +2,7 @@ import fc from 'fast-check';
 import { describe, it } from 'vitest';
 import { Character } from './characters/Character.ts';
 import { Level } from './value-objects/Level.ts';
+import { Damage } from './value-objects/Damage.ts';
 
 describe('DamageAndHealth', () => {
   describe('DamageReducesHealth', () => {
@@ -18,7 +19,7 @@ describe('DamageAndHealth', () => {
               health,
             });
             const expected = Math.max(0, health - damage);
-            const result = attacker.dealDamage(target, damage);
+            const result = attacker.dealDamage(target, Damage.create(damage));
             return result.health.value === expected;
           },
         ),
@@ -39,7 +40,7 @@ describe('DamageAndHealth', () => {
               level: Level.create(1),
               health,
             });
-            const result = attacker.dealDamage(target, damage);
+            const result = attacker.dealDamage(target, Damage.create(damage));
             return result.health.value >= 0;
           },
         ),
@@ -60,7 +61,7 @@ describe('DamageAndHealth', () => {
               level: Level.create(1),
               health,
             });
-            const result = attacker.dealDamage(target, damage);
+            const result = attacker.dealDamage(target, Damage.create(damage));
             const expected = Math.max(0, health - damage);
             if (expected === 0) {
               return result.status.kind === 'dead';
@@ -82,7 +83,7 @@ describe('DamageAndHealth', () => {
             const c = Character.createWithHealth({ name: 'hero', level: Level.create(1), health });
             const healthBefore = c.health.value;
             const statusBefore = c.status.kind;
-            const result = c.dealDamage(c, damage);
+            const result = c.dealDamage(c, Damage.create(damage));
             // Should return the same reference
             return (
               result === c &&
@@ -102,11 +103,11 @@ describe('DamageAndHealth', () => {
           const attacker = Character.create({ name: 'attacker', level: Level.create(1) });
           const target = Character.create({ name: 'target', level: Level.create(1) });
           // Kill the target first — capture the returned (dead) character
-          const deadTarget = attacker.dealDamage(target, 10000);
+          const deadTarget = attacker.dealDamage(target, Damage.create(10000));
           const healthBefore = deadTarget.health.value;
           const statusBefore = deadTarget.status.kind;
           // Then try to deal more damage to the dead character
-          const result = attacker.dealDamage(deadTarget, damage);
+          const result = attacker.dealDamage(deadTarget, Damage.create(damage));
           return (
             result === deadTarget &&
             result.health.value === healthBefore &&
@@ -118,14 +119,12 @@ describe('DamageAndHealth', () => {
   });
 
   describe('NegativeDamageForbidden', () => {
-    it('property: negative damage throws an error', () => {
+    it('property: negative damage throws an error via Damage.create', () => {
       fc.assert(
         fc.property(fc.integer({ min: -10000, max: -1 }), (negativeDamage) => {
-          const attacker = Character.create({ name: 'attacker', level: Level.create(1) });
-          const target = Character.create({ name: 'target', level: Level.create(1) });
           let threw = false;
           try {
-            attacker.dealDamage(target, negativeDamage);
+            Damage.create(negativeDamage);
           } catch {
             threw = true;
           }

src/factions.spec.ts

@@ -3,6 +3,7 @@ import { describe, it } from 'vitest';
 import { Character } from './characters/Character.ts';
 import { Faction } from './factions/Faction.ts';
 import { Level } from './value-objects/Level.ts';
+import { Damage } from './value-objects/Damage.ts';
 
 describe('Factions', () => {
   const hero = () => Character.create({ name: 'hero', level: Level.create(1) });
@@ -189,7 +190,7 @@ describe('Factions', () => {
           (factionName) => {
             const attacker = Character.create({ name: 'attacker', level: Level.create(1) });
             const hero = Character.create({ name: 'hero', level: Level.create(1) });
-            const deadHero = attacker.dealDamage(hero, 10000);
+            const deadHero = attacker.dealDamage(hero, Damage.create(10000));
             const f = Faction.create(factionName);
             const result = deadHero.joinFaction(f);
             return result === deadHero && result.factions.size === 0;
@@ -209,7 +210,7 @@ describe('Factions', () => {
             const hero = Character.create({ name: 'hero', level: Level.create(1) });
             const f = Faction.create(factionName);
             const withFaction = hero.joinFaction(f);
-            const deadHero = attacker.dealDamage(withFaction, 10000);
+            const deadHero = attacker.dealDamage(withFaction, Damage.create(10000));
             const result = deadHero.leaveFaction(f);
             return result === deadHero && result.factions.has(f);
           },
@@ -302,7 +303,7 @@ describe('Factions', () => {
           const target = ally().joinFaction(faction);
           const healthBefore = target.health.value;
           const statusBefore = target.status.kind;
-          const result = attacker.dealDamage(target, 500);
+          const result = attacker.dealDamage(target, Damage.create(500));
           return result.health.value === healthBefore && result.status.kind === statusBefore;
         }),
       );
@@ -314,7 +315,7 @@ describe('Factions', () => {
           const faction = Faction.create('guard');
           const attacker = hero().joinFaction(faction);
           const target = ally().joinFaction(faction);
-          const result = attacker.dealDamage(target, 500);
+          const result = attacker.dealDamage(target, Damage.create(500));
           return result === target;
         }),
       );
@@ -326,7 +327,7 @@ describe('Factions', () => {
           const attacker = hero();
           const target = enemy();
           const healthBefore = target.health.value;
-          const result = attacker.dealDamage(target, 100);
+          const result = attacker.dealDamage(target, Damage.create(100));
           return result.health.value === healthBefore - 100;
         }),
       );
@@ -414,7 +415,7 @@ describe('Factions', () => {
           const target = ally().joinFaction(faction);
           // Kill the target with a non-ally attacker
           const killer = enemy();
-          const deadTarget = killer.dealDamage(target, 10000);
+          const deadTarget = killer.dealDamage(target, Damage.create(10000));
           const healthBefore = deadTarget.health.value;
           const statusBefore = deadTarget.status.kind;
           const result = healer.healAlly(deadTarget, 500);

src/healing.spec.ts

@@ -2,6 +2,7 @@ import fc from 'fast-check';
 import { describe, it } from 'vitest';
 import { Character } from './characters/Character.ts';
 import { Level } from './value-objects/Level.ts';
+import { Damage } from './value-objects/Damage.ts';
 
 describe('Healing', () => {
   describe('SelfHealIncreasesHealth', () => {
@@ -88,7 +89,7 @@ describe('Healing', () => {
           const attacker = Character.create({ name: 'attacker', level: Level.create(1) });
           const hero = Character.create({ name: 'hero', level: Level.create(1) });
           // Kill the hero first using a different attacker
-          const deadHero = attacker.dealDamage(hero, 10000);
+          const deadHero = attacker.dealDamage(hero, Damage.create(10000));
           const healthBefore = deadHero.health.value;
           const statusBefore = deadHero.status.kind;
           // Try to heal the dead character

src/levels.spec.ts

@@ -2,6 +2,7 @@ import fc from 'fast-check';
 import { describe, it } from 'vitest';
 import { Character } from './characters/Character.ts';
 import { Level } from './value-objects/Level.ts';
+import { Damage } from './value-objects/Damage.ts';
 
 describe('Levels', () => {
   describe('CloseLevelNoModifier', () => {
@@ -21,7 +22,7 @@ describe('Levels', () => {
               level: Level.create(targetLevel),
               health: 1000,
             });
-            const result = attacker.dealDamage(target, baseDamage);
+            const result = attacker.dealDamage(target, Damage.create(baseDamage));
             return result.health.value === Math.max(0, 1000 - baseDamage);
           },
         ),
@@ -47,7 +48,7 @@ describe('Levels', () => {
               health: 1000,
             });
             const expectedDamage = Math.floor(baseDamage * 0.5);
-            const result = attacker.dealDamage(target, baseDamage);
+            const result = attacker.dealDamage(target, Damage.create(baseDamage));
             return result.health.value === Math.max(0, 1000 - expectedDamage);
           },
         ),
@@ -66,7 +67,7 @@ describe('Levels', () => {
             health: 1000,
           });
           const expectedDamage = Math.floor(oddDamage * 0.5);
-          const result = attacker.dealDamage(target, oddDamage);
+          const result = attacker.dealDamage(target, Damage.create(oddDamage));
           return result.health.value === Math.max(0, 1000 - expectedDamage);
         }),
       );
@@ -91,7 +92,7 @@ describe('Levels', () => {
               health: 1000,
             });
             const expectedDamage = Math.floor(baseDamage * 1.5);
-            const result = attacker.dealDamage(target, baseDamage);
+            const result = attacker.dealDamage(target, Damage.create(baseDamage));
             return result.health.value === Math.max(0, 1000 - expectedDamage);
           },
         ),
@@ -111,7 +112,7 @@ describe('Levels', () => {
             health: 1000,
           });
           const expectedDamage = Math.floor(damage * 0.5);
-          const result = attacker.dealDamage(target, damage);
+          const result = attacker.dealDamage(target, Damage.create(damage));
           return result.health.value === Math.max(0, 1000 - expectedDamage);
         }),
       );
@@ -128,7 +129,7 @@ describe('Levels', () => {
             health: 1000,
           });
           const expectedDamage = Math.floor(damage * 1.5);
-          const result = attacker.dealDamage(target, damage);
+          const result = attacker.dealDamage(target, Damage.create(damage));
           return result.health.value === Math.max(0, 1000 - expectedDamage);
         }),
       );
@@ -144,7 +145,7 @@ describe('Levels', () => {
             level: Level.create(5),
             health: 1000,
           });
-          const result = attacker.dealDamage(target, damage);
+          const result = attacker.dealDamage(target, Damage.create(damage));
           return result.health.value === Math.max(0, 1000 - damage);
         }),
       );
@@ -160,7 +161,7 @@ describe('Levels', () => {
             level: Level.create(1),
             health: 1000,
           });
-          const result = attacker.dealDamage(target, damage);
+          const result = attacker.dealDamage(target, Damage.create(damage));
           return result.health.value === Math.max(0, 1000 - damage);
         }),
       );

src/magical-objects.spec.ts

@@ -2,6 +2,7 @@ import fc from 'fast-check';
 import { describe, it } from 'vitest';
 import { Character } from './characters/Character.ts';
 import { Level } from './value-objects/Level.ts';
+import { Damage } from './value-objects/Damage.ts';
 import { MagicalWeapon } from './magical-objects/MagicalWeapon.ts';
 import { HealingObject } from './magical-objects/HealingObject.ts';
 
@@ -85,7 +86,7 @@ describe('Magical Objects', () => {
             const weapon = MagicalWeapon.create({ damage, maxHealth: weaponHP, owner: attacker });
             // Kill the attacker using a separate killer
             const killer = Character.create({ name: 'boss', level: Level.create(1) });
-            const deadAttacker = killer.dealDamage(attacker, 10000);
+            const deadAttacker = killer.dealDamage(attacker, Damage.create(10000));
             const weaponHPBefore = weapon.health.value;
             const targetHealthBefore = target.health.value;
             const result = deadAttacker.useWeapon(weapon, target);
@@ -256,7 +257,7 @@ describe('Magical Objects', () => {
           const object = HealingObject.create({ maxHealth: objectHP, currentHealth: objectHP });
           // Kill the character using a separate killer
           const killer = Character.create({ name: 'boss', level: Level.create(1) });
-          const deadCharacter = killer.dealDamage(character, 10000);
+          const deadCharacter = killer.dealDamage(character, Damage.create(10000));
           const objectHPBefore = object.health.value;
           const characterHealthBefore = deadCharacter.health.value;
           const result = deadCharacter.useHealingObject(object, 100);

src/magical-objects/MagicalWeapon.ts

@@ -7,18 +7,19 @@
  */
 import { Character } from '../characters/Character.ts';
 import { Health } from '../value-objects/Health.ts';
+import { Damage } from '../value-objects/Damage.ts';
 import { MagicalObject } from './MagicalObject.ts';
 import type { DamageDealer } from './magical-object-types.ts';
 
 export class MagicalWeapon extends MagicalObject implements DamageDealer {
-  readonly #damage: number;
+  readonly #damage: Damage;
   readonly #owner: Character;
 
   private constructor(
     health: Health,
     maxHealth: Health,
     status: { readonly kind: 'alive' } | { readonly kind: 'destroyed' },
-    damage: number,
+    damage: Damage,
     owner: Character,
   ) {
     super(health, maxHealth, status);
@@ -36,17 +37,16 @@ export class MagicalWeapon extends MagicalObject implements DamageDealer {
     owner: Character;
   }): MagicalWeapon {
     if (maxHealth < 0) throw new Error('MaxHealth cannot be negative');
-    if (damage < 0) throw new Error('Damage cannot be negative');
     return new MagicalWeapon(
       Health.create(maxHealth),
       Health.create(maxHealth),
       { kind: 'alive' },
-      damage,
+      Damage.create(damage),
       owner,
     );
   }
 
-  get damage(): number {
+  get damage(): Damage {
     return this.#damage;
   }
 
@@ -61,7 +61,7 @@ export class MagicalWeapon extends MagicalObject implements DamageDealer {
       return { weapon: this, target };
     }
     // Deal fixed damage
-    const newTargetHealth = Math.max(0, target.health.value - this.#damage);
+    const newTargetHealth = Math.max(0, target.health.value - this.#damage.value);
     const newTargetStatus = newTargetHealth === 0 ? { kind: 'dead' as const } : target.status;
     const newTarget = Character.createWithHealthAndStatus({
       name: target.name,

src/value-objects/Damage.ts

@@ -0,0 +1,26 @@
+/**
+ * Damage value object — non-negative, immutable.
+ *
+ * Invariant enforced at construction: n >= 0
+ */
+export class Damage {
+  readonly #value: number;
+
+  private constructor(n: number) {
+    this.#value = n;
+  }
+
+  static create(n: number): Damage {
+    if (n < 0) throw new Error(`Damage must be non-negative, got ${n}`);
+    return new Damage(n);
+  }
+
+  get value(): number {
+    return this.#value;
+  }
+
+  /** Add another damage amount — cumulative damage is still a Damage. */
+  add(other: Damage): Damage {
+    return Damage.create(this.#value + other.value);
+  }
+}

src/value-objects/Level.ts

@@ -31,4 +31,12 @@ export class Level {
   diff(target: Level): number {
     return this.value - target.value;
   }
+
+  /** Damage threshold to reach the given level.
+   *  Level 1→2: 1000, Level 2→3: 3000, Level 3→4: 6000, etc.
+   *  Formula: 1000 * N * (N+1) / 2
+   */
+  static damageThresholdForLevel(level: number): number {
+    return 1000 * level * (level + 1) / 2;
+  }
 }