[PAN-2966] - Implement EIP-2200 - Net Gas Metering Revised (#1743)

Note that EIP-1706 was already implemented elsewhere and this is
just the gas calculation updates.

ethereum/core/src/main/java/tech/pegasys/pantheon/ethereum/mainnet/IstanbulGasCalculator.java

@@ -12,16 +12,27 @@
  */
 package tech.pegasys.pantheon.ethereum.mainnet;
 
+import tech.pegasys.pantheon.ethereum.core.Account;
 import tech.pegasys.pantheon.ethereum.core.Gas;
 import tech.pegasys.pantheon.ethereum.core.Transaction;
 import tech.pegasys.pantheon.util.bytes.BytesValue;
+import tech.pegasys.pantheon.util.uint.UInt256;
 
 public class IstanbulGasCalculator extends ConstantinopleFixGasCalculator {
 
   private static final Gas TX_DATA_ZERO_COST = Gas.of(4L);
   private static final Gas TX_DATA_NON_ZERO_COST = Gas.of(16L);
   private static final Gas TX_BASE_COST = Gas.of(21_000L);
 
+  private static final Gas SLOAD_GAS = Gas.of(800);
+  private static final Gas SSTORE_SET_GAS = Gas.of(20_000);
+  private static final Gas SSTORE_RESET_GAS = Gas.of(5_000);
+  private static final Gas SSTORE_CLEARS_SCHEDULE = Gas.of(15_000);
+
+  private static final Gas SSTORE_SET_GAS_LESS_SLOAD_GAS = SSTORE_SET_GAS.minus(SLOAD_GAS);
+  private static final Gas SSTORE_RESET_GAS_LESS_SLOAD_GAS = SSTORE_RESET_GAS.minus(SLOAD_GAS);
+  private static final Gas NEGATIVE_SSTORE_CLEARS_SCHEDULE = Gas.ZERO.minus(SSTORE_CLEARS_SCHEDULE);
+
   @Override
   public Gas transactionIntrinsicGasCost(final Transaction transaction) {
     final BytesValue payload = transaction.getPayload();
@@ -45,4 +56,62 @@ public Gas transactionIntrinsicGasCost(final Transaction transaction) {
 
     return cost;
   }
+
+  @Override
+  // As per https://eips.ethereum.org/EIPS/eip-2200
+  public Gas calculateStorageCost(
+      final Account account, final UInt256 key, final UInt256 newValue) {
+
+    final UInt256 currentValue = account.getStorageValue(key);
+    if (currentValue.equals(newValue)) {
+      return SLOAD_GAS;
+    } else {
+      final UInt256 originalValue = account.getOriginalStorageValue(key);
+      if (originalValue.equals(currentValue)) {
+        return originalValue.isZero() ? SSTORE_SET_GAS : SSTORE_RESET_GAS;
+      } else {
+        return SLOAD_GAS;
+      }
+    }
+  }
+
+  @Override
+  // As per https://eips.ethereum.org/EIPS/eip-2200
+  public Gas calculateStorageRefundAmount(
+      final Account account, final UInt256 key, final UInt256 newValue) {
+
+    final UInt256 currentValue = account.getStorageValue(key);
+    if (currentValue.equals(newValue)) {
+      return Gas.ZERO;
+    } else {
+      final UInt256 originalValue = account.getOriginalStorageValue(key);
+      if (originalValue.equals(currentValue)) {
+        if (originalValue.isZero()) {
+          return Gas.ZERO;
+        } else if (newValue.isZero()) {
+          return SSTORE_CLEARS_SCHEDULE;
+        } else {
+          return Gas.ZERO;
+        }
+      } else {
+        Gas refund = Gas.ZERO;
+        if (!originalValue.isZero()) {
+          if (currentValue.isZero()) {
+            refund = NEGATIVE_SSTORE_CLEARS_SCHEDULE;
+          } else if (newValue.isZero()) {
+            refund = SSTORE_CLEARS_SCHEDULE;
+          }
+        }
+
+        if (originalValue.equals(newValue)) {
+          refund =
+              refund.plus(
+                  originalValue.isZero()
+                      ? SSTORE_SET_GAS_LESS_SLOAD_GAS
+                      : SSTORE_RESET_GAS_LESS_SLOAD_GAS);
+        }
+        return refund;
+      }
+    }
+  }
 }