Add a damage formula that combines other formulas

src/game/combat.ts

@@ -165,6 +165,24 @@ export interface DamageFormula {
   explain (user: Creature): LogEntry;
 }
 
+export class CompositeDamageFormula implements DamageFormula {
+  constructor (private formulas: DamageFormula[]) {
+
+  }
+
+  calc (user: Creature, target: Creature): Damage {
+    return this.formulas.reduce((total: Damage, next: DamageFormula) => total.combine(next.calc(user, target)), new Damage())
+  }
+
+  describe (user: Creature, target: Creature): LogEntry {
+    return new LogLines(...this.formulas.map(formula => formula.describe(user, target)))
+  }
+
+  explain (user: Creature): LogEntry {
+    return new LogLines(...this.formulas.map(formula => formula.explain(user)))
+  }
+}
+
 /**
  * Simply returns the damage it was given.
  */