fix: semantics of LinkedList (#544)

arithmetization/src/main/java/net/consensys/linea/zktracer/container/stacked/list/CountedList.java

@@ -100,6 +100,10 @@ public void replaceAll(UnaryOperator<E> operator) {
     super.replaceAll(operator);
   }
 
+  public E getLast() {
+    return this.get(this.size() - 1);
+  }
+
   int lineCount() {
     if (this.countDirty) {
       this.count = 0;

arithmetization/src/main/java/net/consensys/linea/zktracer/container/stacked/list/StackedList.java

@@ -51,19 +51,23 @@ public String toString() {
 
   @Override
   public void enter() {
-    this.lists.push(new CountedList<>());
+    this.lists.addLast(new CountedList<>());
   }
 
   public void enter(int initialCapacity) {
-    this.lists.push(new CountedList<>(initialCapacity));
+    this.lists.addLast(new CountedList<>(initialCapacity));
+  }
+
+  public E getLast() {
+    return this.lists.getLast().getLast();
   }
 
   @Override
   public void pop() {
     if (this.lists.isEmpty()) {
       throw new RuntimeException("asymmetric pop");
     }
-    this.totalSize -= this.lists.pop().size();
+    this.totalSize -= this.lists.removeLast().size();
   }
 
   @Override
@@ -117,7 +121,7 @@ public <T> T[] toArray(@NotNull T[] a) {
   @Override
   public boolean add(E e) {
     this.totalSize++;
-    return this.lists.get(this.lists.size() - 1).add(e);
+    return this.lists.getLast().add(e);
   }
 
   @Override

arithmetization/src/main/java/net/consensys/linea/zktracer/container/stacked/set/StackedSet.java

@@ -49,9 +49,15 @@ public void enter() {
   public void pop() {
     Set<E> set = this.sets.pop();
     for (E e : set) {
-      Integer count = occurrences.get(e);
-      if (count > 0) occurrences.put(e, count - 1);
-      else throw new IllegalStateException("asymmetric element removal !");
+      occurrences.computeIfPresent(
+          e,
+          (k, count) -> {
+            if (count > 0) {
+              return count - 1;
+            } else {
+              throw new IllegalStateException("asymmetric element removal !");
+            }
+          });
     }
   }
 

arithmetization/src/main/java/net/consensys/linea/zktracer/module/txn_data/BlockSnapshot.java

@@ -50,7 +50,7 @@ public class BlockSnapshot {
    * @return the latest {@link TransactionSnapshot}
    */
   TransactionSnapshot currentTx() {
-    return this.txs.get(this.txs.size() - 1);
+    return this.txs.getLast();
   }
 
   /**

arithmetization/src/test/java/net/consensys/linea/zktracer/containers/StackedListTests.java

@@ -0,0 +1,51 @@
+/*
+ * Copyright ConsenSys AG.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on
+ * an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the
+ * specific language governing permissions and limitations under the License.
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package net.consensys.linea.zktracer.containers;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import lombok.RequiredArgsConstructor;
+import net.consensys.linea.zktracer.container.ModuleOperation;
+import net.consensys.linea.zktracer.container.stacked.list.StackedList;
+import org.junit.jupiter.api.Test;
+
+public class StackedListTests {
+  @RequiredArgsConstructor
+  private class IntegerModuleOperation extends ModuleOperation {
+    private final int x;
+
+    @Override
+    protected int computeLineCount() {
+      return x;
+    }
+  }
+
+  @Test
+  void testAddedToFront() {
+    final StackedList<IntegerModuleOperation> state = new StackedList<>();
+
+    state.enter();
+    state.add(new IntegerModuleOperation(1));
+    assertThat(state.lineCount()).isEqualTo(1);
+
+    state.enter();
+    state.add(new IntegerModuleOperation(3));
+    assertThat(state.lineCount()).isEqualTo(4);
+
+    state.pop();
+    assertThat(state.lineCount()).isEqualTo(1);
+  }
+}