track active effects on other players via synced effect particles

Accessor mixins expose LivingEntity's DATA_EFFECT_PARTICLES and
ColorParticleOption's color; DamageSyncTracker maps the synced colors
to resistance/slowness/strength (turtle master = resistance + slowness)
for the nametag effect icons.

src/main/java/cc/fros/mixin/entity/ColorParticleOptionAccessor.java

@@ -0,0 +1,12 @@
+package cc.fros.mixin.entity;
+
+import net.minecraft.core.particles.ColorParticleOption;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.gen.Accessor;
+
+@Mixin(ColorParticleOption.class)
+public interface ColorParticleOptionAccessor {
+
+    @Accessor("color")
+    int fros$getColor();
+}

src/main/java/cc/fros/mixin/entity/LivingEntityEffectParticlesAccessor.java

@@ -0,0 +1,18 @@
+package cc.fros.mixin.entity;
+
+import net.minecraft.core.particles.ParticleOptions;
+import net.minecraft.network.syncher.EntityDataAccessor;
+import net.minecraft.world.entity.LivingEntity;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.gen.Accessor;
+
+import java.util.List;
+
+@Mixin(LivingEntity.class)
+public interface LivingEntityEffectParticlesAccessor {
+
+    @Accessor("DATA_EFFECT_PARTICLES")
+    static EntityDataAccessor<List<ParticleOptions>> fros$getDataEffectParticles() {
+        throw new AssertionError();
+    }
+}

src/main/java/cc/fros/util/DamageSyncTracker.java

@@ -0,0 +1,54 @@
+package cc.fros.util;
+
+import cc.fros.mixin.entity.ColorParticleOptionAccessor;
+import cc.fros.mixin.entity.LivingEntityEffectParticlesAccessor;
+import net.minecraft.core.particles.ColorParticleOption;
+import net.minecraft.core.particles.ParticleOptions;
+import net.minecraft.world.effect.MobEffects;
+import net.minecraft.world.entity.LivingEntity;
+
+import java.util.List;
+
+/**
+ * Detects active effects on other players from the effect particle data the
+ * server syncs with them, ported from homovore's {@code DamageSyncTracker}.
+ */
+public final class DamageSyncTracker {
+
+    private DamageSyncTracker() {
+        throw new AssertionError();
+    }
+
+    private static final int RESISTANCE_COLOR = MobEffects.RESISTANCE.value().getColor() & 0xFFFFFF;
+    private static final int SLOWNESS_COLOR = MobEffects.SLOWNESS.value().getColor() & 0xFFFFFF;
+    private static final int STRENGTH_COLOR = MobEffects.STRENGTH.value().getColor() & 0xFFFFFF;
+
+    private static boolean hasEffectColor(LivingEntity entity, int rgb) {
+        List<ParticleOptions> particles = entity.getEntityData()
+                .get(LivingEntityEffectParticlesAccessor.fros$getDataEffectParticles());
+        if (particles.isEmpty()) return false;
+        for (int i = 0, n = particles.size(); i < n; i++) {
+            if (particles.get(i) instanceof ColorParticleOption color
+                    && (((ColorParticleOptionAccessor) (Object) color).fros$getColor() & 0xFFFFFF) == rgb) {
+                return true;
+            }
+        }
+        return false;
+    }
+
+    public static boolean hasResistance(LivingEntity entity) {
+        return hasEffectColor(entity, RESISTANCE_COLOR);
+    }
+
+    public static boolean hasSlowness(LivingEntity entity) {
+        return hasEffectColor(entity, SLOWNESS_COLOR);
+    }
+
+    public static boolean hasStrength(LivingEntity entity) {
+        return hasEffectColor(entity, STRENGTH_COLOR);
+    }
+
+    public static boolean isTurtleMaster(LivingEntity entity) {
+        return hasResistance(entity) && hasSlowness(entity);
+    }
+}

src/main/resources/fros.mixins.json

@@ -7,6 +7,8 @@
     "network.MixinConnection"
   ],
   "client": [
+    "entity.ColorParticleOptionAccessor",
+    "entity.LivingEntityEffectParticlesAccessor",
     "entity.MixinAbstractClientPlayer",
     "entity.MixinClientPlayerEntity",
     "input.MixinKeyboardHandler",