Approximate gas calc. (#57)

Author: dvc94ch

test/GasUtils.t.sol

@@ -11,7 +11,7 @@ import {GasSpender} from "./GasSpender.sol";
 import {Gateway} from "../src/Gateway.sol";
 import {GasUtils} from "../src/GasUtils.sol";
 import {IGmpReceiver} from "../src/interfaces/IGmpReceiver.sol";
-import {GmpMessage, Signature, TssKey, GmpStatus, PrimitiveUtils, Batch} from "../src/Primitives.sol";
+import {GmpMessage, Signature, TssKey, GmpStatus, PrimitiveUtils, Batch, MAX_PAYLOAD_SIZE} from "../src/Primitives.sol";
 
 uint256 constant secret = 0x42;
 uint256 constant nonce = 0x69;
@@ -49,7 +49,7 @@ contract GasUtilsTest is Test {
     /**
      * @dev Compare the estimated gas cost VS the actual gas cost of the `execute` method.
      */
-    function test_baseExecutionCost(uint16 messageSize, uint16 gasLimit) external {
+    function test_gas_calc_and_refund(uint16 messageSize, uint16 gasLimit) external {
         vm.txGasPrice(1);
         vm.assume(gasLimit >= 5000);
         vm.assume(messageSize <= (0x6000 - 32));
@@ -99,4 +99,14 @@ contract GasUtilsTest is Test {
         console.log("proxyOverheadGas", proxyOverheadGas);
         assertEq(balanceAfter - balanceBefore - baseGas - mGasUsed, 0, "Balance should not change");
     }
+
+    function test_lin_approx(uint16 messageSize) external pure {
+        vm.assume(messageSize <= MAX_PAYLOAD_SIZE);
+        uint256 calcGas = TestUtils.calcGas(messageSize);
+        uint256 approxGas = TestUtils.linApproxGas(messageSize);
+        assertGe(approxGas + 650, calcGas);
+        int256 error = int256(approxGas) - int256(calcGas);
+        uint256 absError = error >= 0 ? uint256(error) : uint256(-error);
+        assertLe(absError, 750); 
+    }
 }

test/TestUtils.sol

@@ -18,7 +18,8 @@ import {
     Batch,
     GatewayOp,
     Command,
-    GMP_VERSION
+    GMP_VERSION,
+    MAX_PAYLOAD_SIZE
 } from "../src/Primitives.sol";
 import "@openzeppelin/contracts/proxy/ERC1967/ERC1967Proxy.sol";
 
@@ -110,8 +111,8 @@ library TestUtils {
                 gateway: gateway.toSender(),
                 relativeGasPriceNumerator: 1,
                 relativeGasPriceDenominator: 1,
-                gasCoef0: 0,
-                gasCoef1: 1
+                gasCoef0: gasCoef0(),
+                gasCoef1: gasCoef1()
             })
         );
         vm.stopPrank();
@@ -148,14 +149,6 @@ library TestUtils {
         return TestUtils.sign(shard, hash, nonce);
     }
 
-    function countNonZeros(bytes memory data) internal pure returns (uint256 count) {
-        for (uint256 i = 0; i < data.length; i++) {
-            if (data[i] != 0x0) {
-                count += 1;
-            }
-        }
-    }
-
     function estimateBaseGas(uint256 messageSize) internal pure returns (uint256) {
         uint256 calldataSize = messageSize.align32() + 676; // selector + Signature + Batch
         return 21000 + calldataSize * 16; // assume every byte is a 1
@@ -188,4 +181,20 @@ library TestUtils {
             return gas;
         }
     }
+
+    function calcGas(uint16 messageSize) internal pure returns (uint256) {
+        return estimateGas(messageSize, 0) + estimateBaseGas(uint256(messageSize));
+    }
+
+    function gasCoef0() internal pure returns (uint256) {
+        return calcGas(0);
+    }
+
+    function gasCoef1() internal pure returns (uint256) {
+        return (calcGas(uint16(MAX_PAYLOAD_SIZE)) - calcGas(0)) / MAX_PAYLOAD_SIZE;
+    }
+
+    function linApproxGas(uint16 messageSize) internal pure returns (uint256) {
+        return gasCoef1() * messageSize + gasCoef0();
+    }
 }