diff --git a/core/src/com/riiablo/attributes/AttributesUpdater.java b/core/src/com/riiablo/attributes/AttributesUpdater.java
new file mode 100644
index 00000000..c7e08b21
--- /dev/null
+++ b/core/src/com/riiablo/attributes/AttributesUpdater.java
@@ -0,0 +1,126 @@
+package com.riiablo.attributes;
+
+import com.riiablo.codec.excel.CharStats;
+import com.riiablo.codec.excel.ItemStatCost;
+import com.riiablo.logger.LogManager;
+import com.riiablo.logger.Logger;
+import com.riiablo.logger.MDC;
+
+public class AttributesUpdater {
+  private static final Logger log = LogManager.getLogger(AttributesUpdater.class);
+
+  public Attributes update(Attributes attrs, Attributes opAttrs) {
+    log.traceEntry("update(attrs: {}, opAttrs: {})", attrs, opAttrs);
+    return update(attrs, opAttrs, null);
+  }
+
+  public Attributes update(Attributes attrs, Attributes opAttrs, CharStats.Entry charStats) {
+    log.traceEntry("update(attrs: {}, opAttrs: {}, charStats: {})", attrs, opAttrs, charStats);
+    final StatList list = attrs.list();
+    if (list.isEmpty()) return attrs;
+    final StatListGetter base = attrs.base();
+    final StatListBuilder agg = attrs.aggregate().builder();
+    final StatListBuilder rem = attrs.remaining().builder();
+    for (StatListGetter stats : list.listIterator()) {
+      update(opAttrs, charStats, stats, base, agg, rem);
+    }
+
+    return attrs;
+  }
+
+  private static void update(
+      final Attributes opAttrs,
+      final CharStats.Entry charStats,
+      final StatListGetter stats,
+      final StatListGetter base,
+      final StatListBuilder agg,
+      final StatListBuilder rem) {
+    for (StatGetter stat : stats) {
+      final ItemStatCost.Entry entry = stat.entry();
+      try {
+        MDC.put("updateStat", stat.id());
+        if (entry.op > 0) {
+          final int ops = op(opAttrs, charStats, agg, stat);
+          if (ops == 0) {
+            log.trace("Propagating stat({})", stat.debugString());
+            rem.add(stat);
+          }
+        } else if (!base.contains(stat)) {
+          log.trace("Propagating stat({})", stat.debugString());
+          rem.add(stat);
+        } else {
+          log.trace("Aggregating stat({})", stat.debugString());
+          agg.add(stat);
+        }
+      } finally {
+        MDC.remove("updateStat");
+      }
+    }
+  }
+
+  private static int op(
+      final Attributes opAttrs,
+      final CharStats.Entry charStats,
+      final StatListBuilder agg,
+      final StatGetter stat) {
+    final ItemStatCost.Entry entry = stat.entry();
+    final int op = entry.op;
+    final int op_param = entry.op_param;
+    final int op_base = op_param > 0
+        ? opAttrs.aggregate().get(Stat.index(entry.op_base)).value()
+        : 1;
+    int ops = 0;
+    for (String op_stat : entry.op_stat) {
+      if (op_stat.isEmpty()) break;
+      final short statId = Stat.index(op_stat);
+      final StatGetter opStat = agg.get(statId);
+      if (opStat != null) {
+        final int opValue = op(charStats, agg, stat, opStat, op, op_base, op_param);
+        opStat.add(opValue);
+        ops++;
+      }
+    }
+
+    return ops;
+  }
+
+  /** @see StatFormatter#op(StatGetter, Attributes) */
+  private static int op(
+      final CharStats.Entry charStats,
+      final StatListBuilder agg,
+      final StatGetter stat,
+      final StatGetter opStat,
+      final int op,
+      final int op_base,
+      final int op_param) {
+    switch (op) {
+      case 1: return (stat.value() * opStat.value()) / 100;
+      case 2: return Fixed.intBitsToFloatFloor(stat.value() * op_base, op_param);
+      case 3: return Fixed.intBitsToFloatFloor(stat.value() * op_base, op_param) * opStat.value() / 100;
+      case 4: return Fixed.intBitsToFloatFloor(stat.value() * op_base, op_param);
+      case 5: return Fixed.intBitsToFloatFloor(stat.value() * op_base, op_param) * opStat.value() / 100;
+      case 6: return 0; // by-time
+      case 7: return 0; // by-time percent
+      case 8:
+        if (charStats == null) return 0;
+        agg.add(stat);
+        //mod.set(stat.id);
+        return stat.value() * charStats.ManaPerMagic; // max mana
+      case 9:
+        if (charStats == null) return 0;
+        if (opStat.id() == Stat.maxhp) { // only increment vit on maxhp op
+          agg.add(stat);
+          //mod.set(stat.id);
+        }
+        return stat.value() // max hitpoints or stamina
+            * (opStat.id() == Stat.maxhp
+                ? charStats.LifePerVitality
+                : charStats.StaminaPerVitality);
+      case 10: return 0; // no-op
+      case 11: return (stat.value() * opStat.value()) / 100; // TODO: modify field value? used with item_maxhp_percent and item_maxmana_percent
+      case 12: return 0; // no-op
+      case 13: return (stat.value() * opStat.value()) / 100;
+      default: throw new AssertionError("Unsupported op: " + op + " for " + stat);
+    }
+  }
+}
diff --git a/core/test/com/riiablo/attributes/AttributesTest.java b/core/test/com/riiablo/attributes/AttributesTest.java
new file mode 100644
index 00000000..0ad9a8fd
--- /dev/null
+++ b/core/test/com/riiablo/attributes/AttributesTest.java
@@ -0,0 +1,48 @@
+package com.riiablo.attributes;
+
+import org.junit.AfterClass;
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+import com.badlogic.gdx.ApplicationAdapter;
+import com.badlogic.gdx.Gdx;
+import com.badlogic.gdx.backends.headless.HeadlessApplication;
+
+import com.riiablo.Files;
+import com.riiablo.Riiablo;
+import com.riiablo.codec.StringTBLs;
+import com.riiablo.logger.Level;
+import com.riiablo.logger.LogManager;
+import com.riiablo.mpq.MPQFileHandleResolver;
+
+public class AttributesTest {
+  @BeforeClass
+  public static void setup() {
+    Gdx.app = new HeadlessApplication(new ApplicationAdapter() {});
+    Riiablo.home = Gdx.files.absolute("C:\\Program Files (x86)\\Steam\\steamapps\\common\\Diablo II");
+    Riiablo.mpqs = new MPQFileHandleResolver();
+    Riiablo.string = new StringTBLs(Riiablo.mpqs);
+    Riiablo.files = new Files();
+  }
+
+  @AfterClass
+  public static void teardown() {
+    Gdx.app.exit();
+  }
+
+  @BeforeClass
+  public static void before() {
+    LogManager.setLevel("com.riiablo.attributes", Level.TRACE);
+  }
+
+  @Test
+  public void reset_does_not_reset_base() {
+    AggregateAttributes attrs = Attributes.aggregateAttributes();
+    attrs.base().builder().put(Stat.strength, 5).put(Stat.vitality, 15);
+    attrs.reset();
+
+    for (StatGetter stat : attrs) {
+      System.out.println(stat.debugString());
+    }
+  }
+}
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