[4844] CancunGasCalculator -> Fix Excess Data Gas Calculation (#5643)

* Fix computeExcessDataGas formula

Signed-off-by: Gabriel-Trintinalia <gabriel.trintinalia@consensys.net>

evm/src/main/java/org/hyperledger/besu/evm/gascalculator/CancunGasCalculator.java

@@ -62,11 +62,30 @@ public long dataGasUsed(final int blobCount) {
     return DATA_GAS_PER_BLOB * blobCount;
   }
 
+  /**
+   * Retrieves the target data gas per block.
+   *
+   * @return The target data gas per block.
+   */
+  public static long getTargetDataGasPerBlock() {
+    return TARGET_DATA_GAS_PER_BLOCK;
+  }
+
+  /**
+   * Computes the excess data gas for a given block based on the parent's excess data gas and data
+   * gas used. If the sum of parent's excess data gas and parent's data gas used is less than the
+   * target data gas per block, the excess data gas is calculated as 0. Otherwise, it is computed as
+   * the difference between the sum and the target data gas per block.
+   *
+   * @param parentExcessDataGas The excess data gas of the parent block.
+   * @param parentDataGasUsed The data gas used in the parent block.
+   * @return The excess data gas for the current block.
+   */
   @Override
   public long computeExcessDataGas(final long parentExcessDataGas, final long parentDataGasUsed) {
     if (parentExcessDataGas + parentDataGasUsed < TARGET_DATA_GAS_PER_BLOCK) {
       return 0L;
     }
-    return parentDataGasUsed + parentDataGasUsed - TARGET_DATA_GAS_PER_BLOCK;
+    return parentExcessDataGas + parentDataGasUsed - TARGET_DATA_GAS_PER_BLOCK;
   }
 }

evm/src/test/java/org/hyperledger/besu/evm/gascalculator/CancunGasCalculatorTest.java

@@ -0,0 +1,43 @@
+/*
+ * Copyright Hyperledger Besu contributors.
+ *
+ * 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 org.hyperledger.besu.evm.gascalculator;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import org.junit.jupiter.api.Test;
+
+public class CancunGasCalculatorTest {
+
+  private final GasCalculator gasCalculator = new CancunGasCalculator();
+
+  @Test
+  public void shouldCalculateExcessDataGasCorrectly() {
+
+    long target = CancunGasCalculator.getTargetDataGasPerBlock();
+    long excess = 1L;
+
+    assertThat(gasCalculator.computeExcessDataGas(0L, 0L)).isEqualTo(0);
+
+    assertThat(gasCalculator.computeExcessDataGas(target, 0L)).isEqualTo(0);
+
+    assertThat(gasCalculator.computeExcessDataGas(0L, target)).isEqualTo(0);
+
+    assertThat(gasCalculator.computeExcessDataGas(excess, target)).isEqualTo(excess);
+
+    assertThat(gasCalculator.computeExcessDataGas(target, excess)).isEqualTo(excess);
+
+    assertThat(gasCalculator.computeExcessDataGas(target, target)).isEqualTo(target);
+  }
+}