1m gas to do an update isn't bad at all

contracts/MIPSMemory.sol

@@ -16,23 +16,23 @@ contract MIPSMemory {
   }
 
   // one per owner (at a time)
-  mapping(address => uint64[25]) public largePreimages;
+  mapping(address => uint64[25]) public largePreimage;
 
   function AddLargePreimageInit() public {
     Lib_Keccak256.CTX memory c;
     Lib_Keccak256.keccak_init(c);
-    largePreimages[msg.sender] = c.A;
+    largePreimage[msg.sender] = c.A;
   }
 
   function AddLargePreimageUpdate(uint64[17] calldata data) public {
     // sha3_process_block
     Lib_Keccak256.CTX memory c;
-    c.A = largePreimages[msg.sender];
+    c.A = largePreimage[msg.sender];
     for (uint i = 0; i < 17; i++) {
       c.A[i] ^= data[i];
     }
     Lib_Keccak256.sha3_permutation(c);
-    largePreimages[msg.sender] = c.A;
+    largePreimage[msg.sender] = c.A;
   }
 
   function AddLargePreimageFinal(uint64[17] calldata data) public {

hardhat.config.js

 /**
  * @type import('hardhat/config').HardhatUserConfig
  */
+
+require("@nomiclabs/hardhat-ethers");
+require("hardhat-gas-reporter");
+
 module.exports = {
   solidity: {
     version: "0.7.3",

package.json

 {
   "dependencies": {
+    "@nomiclabs/hardhat-ethers": "^2.0.2",
     "@types/node": "^16.9.2",
+    "chai": "^4.3.4",
     "ethers": "^5.4.7",
     "hardhat": "^2.6.4",
+    "hardhat-gas-reporter": "^1.0.4",
     "rlp": "^2.2.6",
     "typescript": "^4.4.3"
   }

test/libkeccak.js

+const { expect } = require("chai");
+
+describe("MIPSMemory contract", function () {
+  it("Keccak should work", async function () {
+    const [owner] = await ethers.getSigners();
+
+    const MIPSMemory = await ethers.getContractFactory("MIPSMemory");
+    const mm = await MIPSMemory.deploy();
+    console.log("deployed at", mm.address, "by", owner.address);
+
+    await mm.AddLargePreimageInit();
+    console.log("preimage initted");
+
+    var a = [0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0];
+    await mm.AddLargePreimageUpdate(a);
+    console.log("preimage updated");
+
+    var tst = await mm.largePreimage(owner.address, 0);
+    console.log(tst);
+  });
+});
\ No newline at end of file