global root index check (#106)

packages/contracts/contracts/optimistic-ethereum/OVM/verification/OVM_FraudVerifier.sol

@@ -173,9 +173,9 @@ contract OVM_FraudVerifier is Lib_AddressResolver, OVM_FraudContributor, iOVM_Fr
             "State transition process must be completed prior to finalization."
         );
 
-        require(
-            _postStateRootProof.index == _preStateRootProof.index + 1,
-            "Invalid post-state root index."
+        require (
+            _postStateRootBatchHeader.prevTotalElements + _postStateRootProof.index == _preStateRootBatchHeader.prevTotalElements + _preStateRootProof.index + 1,
+            "Post-state root global index must equal to the pre state root global index plus one."
         );
 
         require(

packages/contracts/test/contracts/OVM/verification/OVM_FraudVerifier.spec.ts

@@ -274,7 +274,7 @@ describe('OVM_FraudVerifier', () => {
               DUMMY_BATCH_HEADERS[0],
               batchProof
             )
-          ).to.be.revertedWith('Invalid post-state root index.')
+          ).to.be.revertedWith('Post-state root global index must equal to the pre state root global index plus one.')
         })
       })