[Fix] start sequences at 1 (#39)

* fix: sequence number starts at 1

* docs: more of them

* fix: snapshot

Author: prestwich

.gas-snapshot

@@ -1,6 +1,6 @@
-ZenithTest:test_badSequence() (gas: 77339)
-ZenithTest:test_badSignature() (gas: 66856)
-ZenithTest:test_blockExpired() (gas: 55471)
-ZenithTest:test_notSequencer() (gas: 58576)
-ZenithTest:test_onePerBlock() (gas: 123118)
-ZenithTest:test_submitBlock() (gas: 90078)
+ZenithTest:test_badSequence() (gas: 60049)
+ZenithTest:test_badSignature() (gas: 66665)
+ZenithTest:test_blockExpired() (gas: 55344)
+ZenithTest:test_notSequencer() (gas: 58385)
+ZenithTest:test_onePerBlock() (gas: 105869)
+ZenithTest:test_submitBlock() (gas: 90173)

src/Zenith.sol

@@ -22,8 +22,9 @@ contract Zenith is Passage {
     }
 
     /// @notice The sequence number of the next block that can be submitted for a given rollup chainId.
-    /// rollupChainId => nextSequence number
-    mapping(uint256 => uint256) public nextSequence;
+    /// @dev Because sequences must start at 1, accessors must add 1 to this number.
+    /// rollupChainId => (nextSequence - 1)
+    mapping(uint256 => uint256) sequences;
 
     /// @notice The host block number that a block was last submitted at for a given rollup chainId.
     /// rollupChainId => host blockNumber that block was last submitted at
@@ -82,6 +83,22 @@ contract Zenith is Passage {
         sequencerAdmin = _sequencerAdmin;
     }
 
+    /// @notice Returns the next sequence number.
+    /// @dev Because sequences must start at 1, we add 1 to the mapping value.
+    /// @param _rollupChainId - the chainId of the rollup chain. Any chainId is accepted by the contract.
+    /// @return The next sequence number.
+    function nextSequence(uint256 _rollupChainId) public view returns (uint256) {
+        return sequences[_rollupChainId] + 1;
+    }
+
+    /// @notice Increments the sequence number and returns it.
+    /// @dev Because sequences must start at 1, we add 1 to the mapping value.
+    /// @param _rollupChainId - the chainId of the rollup chain. Any chainId is accepted by the contract.
+    /// @return The next sequence number.
+    function incrementSequence(uint256 _rollupChainId) internal returns (uint256) {
+        return ++sequences[_rollupChainId];
+    }
+
     /// @notice Add a sequencer to the permissioned sequencer list.
     /// @param sequencer - the address of the sequencer to add.
     /// @custom:emits SequencerSet if the sequencer is added.
@@ -134,7 +151,7 @@ contract Zenith is Passage {
 
     function _submitBlock(BlockHeader memory header, bytes32 blockDataHash, uint8 v, bytes32 r, bytes32 s) internal {
         // assert that the sequence number is valid and increment it
-        uint256 _nextSequence = nextSequence[header.rollupChainId]++;
+        uint256 _nextSequence = incrementSequence(header.rollupChainId);
         if (_nextSequence != header.sequence) revert BadSequence(_nextSequence);
 
         // assert that confirmBy time has not passed

test/Zenith.t.sol

@@ -32,7 +32,7 @@ contract ZenithTest is Test {
 
         // set default block values
         header.rollupChainId = block.chainid + 1;
-        header.sequence = 0; // first block has index
+        header.sequence = 1; // first block has index 1
         header.confirmBy = block.timestamp + 10 minutes;
         header.gasLimit = 30_000_000;
         header.rewardAddress = address(this);
@@ -46,13 +46,13 @@ contract ZenithTest is Test {
     // cannot submit block with incorrect sequence number
     function test_badSequence() public {
         // change to incorrect sequence number
-        header.sequence = 1;
+        header.sequence = 100;
         commit = target.blockCommitment(header, blockDataHash);
 
         // sign block commitmenet with sequencer key
         (uint8 v, bytes32 r, bytes32 s) = vm.sign(sequencerKey, commit);
 
-        vm.expectRevert(abi.encodeWithSelector(Zenith.BadSequence.selector, 0));
+        vm.expectRevert(abi.encodeWithSelector(Zenith.BadSequence.selector, 1));
         target.submitBlock(header, blockDataHash, v, r, s, blockData);
     }