feat: wip gas metering

packages/contracts/contracts/L1/abstracts/DepositFeed.sol

@@ -2,6 +2,7 @@
 pragma solidity 0.8.10;
 
 import { IDepositFeed } from "./IDepositFeed.sol";
+import { GasMetering } from "./GasMetering.sol";
 
 /* Library Imports */
 import { AddressAliasHelper } from "@eth-optimism/contracts/standards/AddressAliasHelper.sol";
@@ -10,7 +11,7 @@ import { AddressAliasHelper } from "@eth-optimism/contracts/standards/AddressAli
  * @title DepositFeed
  * @notice Implements the logic for depositing from L1 to L2.
  */
-abstract contract DepositFeed is IDepositFeed {
+abstract contract DepositFeed is IDepositFeed, GasMetering {
     /**********
      * Errors *
      **********/
@@ -57,7 +58,7 @@ abstract contract DepositFeed is IDepositFeed {
         uint64 _gasLimit,
         bool _isCreation,
         bytes memory _data
-    ) public payable {
+    ) public payable gasMetered {
         if (_isCreation && _to != address(0)) {
             revert NonZeroCreationTarget();
         }

packages/contracts/contracts/L1/abstracts/GasMetering.sol

@@ -0,0 +1,87 @@
+//SPDX-License-Identifier: MIT
+pragma solidity 0.8.10;
+
+abstract contract GasMetering {
+
+    // TODO: i think solc is smart enough to pack
+    // things into a single slot when they are
+    // smaller than uint256 and next to each other
+    // but not sure what will happen if there
+    // are getters for them or if they are public
+    // https://github.com/cronus-finance/cronus-core/blob/0ad76ef2498766954f2494700418160732f69c24/contracts/CronusPair.sol#L23
+
+    uint256 internal storage1;
+    uint256 internal storage2;
+
+    function getBaseFee() public view returns (uint128) {
+        (uint128 basefee , ,) = getState();
+        return basefee;
+    }
+
+    function getBlockNumber() public view returns (uint64) {
+        (, uint64 blockNumber, ) = getState();
+        return blockNumber;
+    }
+
+    function getBoughtGas() public view returns (uint64) {
+        (, , uint64 boughtGas) = getState();
+        return boughtGas;
+    }
+
+    function getTargetGasLimit() public view returns (uint64) {
+        (uint64 targetGasLimit, ) = getGasLimits();
+        return targetGasLimit;
+    }
+
+    function getSanityGasLimit() public view returns (uint64) {
+        (, uint64 sanityGasLimit) = getGasLimits();
+        return sanityGasLimit;
+    }
+
+    function getState() internal view returns (uint128 a, uint64 b, uint64 c) {
+        uint256 s = storage1;
+        assembly {
+            a := shr(128, s)
+            b := and(0xffffffffffffffff, shr(64, s))
+            c := and(0xffffffffffffffff, s)
+        }
+        return (a, b, c);
+    }
+
+    function getGasLimits() internal view returns (uint64 a, uint64 b) {
+        uint256 s = storage2;
+        assembly {
+            a := and(0xffffffffffffffff, s)
+            b := and(0xffffffffffffffff, shr(64, s))
+        }
+    }
+
+    modifier gasMetered() {
+        (uint128 baseFee, uint64 blockNumber, uint64 boughtGas) = getState();
+        (uint64 gasTarget, uint64 gasSanity) = getGasLimits();
+
+        // TODO: spec is inconsistent
+        /*
+        if blockNumber != block.number {
+            if boughtGas == gas_target {
+                new_basefee := curr_basefee
+            } else if curr_bought_gas > gas_target {
+                gas_delta     := curr_bought_gas - gas_target
+                basefee_delta := gas_delta * curr_basefee // gas_target // BASE_FEE_MAX_CHANGE_DENOMINATOR
+                basefee_delta := max(basefee_delta, 1) # TODO: Why does 1559 have this asymmetry?
+                new_basefee   := curr_basefee + basefee_delta
+            } else {
+                gas_delta     := gas_target - curr_bought_gas
+                basefee_delta := gas_delta * curr_basefee // gas_target // BASE_FEE_MAX_CHANGE_DENOMINATOR
+                // Fun fact, geth doesn't let the new_basefee get below 0.
+                new_basefee   := curr_basefee - basefee_delta
+            }
+            curr_basefee := new_basefee
+            curr_number := block.number
+            curr_bought_gas := 0
+            pack_and_store(curr_basefee, curr_number, curr_bought_gas)
+        }
+        */
+        _;
+    }
+}