up3

server/item/enchantment/damage.go

@@ -0,0 +1,52 @@
+package enchantment
+
+import (
+	"github.com/df-mc/dragonfly/server/item"
+	"github.com/df-mc/dragonfly/server/world"
+	"math"
+)
+
+// AffectedDamageSource represents a world.DamageSource whose damage may be
+// affected by an enchantment. A world.DamageSource does not need to implement
+// AffectedDamageSource to let protection affect the damage. This happens
+// depending on the (world.DamageSource).ReducedByResistance() method.
+type AffectedDamageSource interface {
+	world.DamageSource
+	// AffectedByEnchantment specifies if a world.DamageSource is affected by
+	// the item.EnchantmentType passed.
+	AffectedByEnchantment(e item.EnchantmentType) bool
+}
+
+// DamageModifier is an item.EnchantmentType that can reduce damage through a
+// modifier if an AffectedDamageSource returns true for it.
+type DamageModifier interface {
+	Modifier() float64
+}
+
+// ProtectionFactor calculates the combined protection factor for a slice of
+// item.Enchantment. The factor depends on the world.DamageSource passed and is
+// in a range of [0, 0.8], where 0.8 means incoming damage would be reduced by
+// 80%.
+func ProtectionFactor(src world.DamageSource, enchantments []item.Enchantment) float64 {
+	f := 0.0
+	for _, e := range enchantments {
+		t := e.Type()
+		modifier, ok := t.(DamageModifier)
+		if !ok {
+			continue
+		}
+		reduced := false
+		if _, ok := t.(protection); ok && src.ReducedByResistance() {
+			// Special case for protection, because it applies to all damage
+			// sources by default, except those not reduced by resistance.
+			reduced = true
+		} else if asrc, ok := src.(AffectedDamageSource); ok && asrc.AffectedByEnchantment(t) {
+			reduced = true
+		}
+
+		if reduced {
+			f += float64(e.Level()) * modifier.Modifier()
+		}
+	}
+	return math.Min(f, 0.8)
+}