minor refactor, 0 length doesn't work

contracts/MIPSMemory.sol

@@ -62,11 +62,7 @@ contract MIPSMemory {
 
     int offset = int(largePreimage[msg.sender].offset) - int(largePreimage[msg.sender].len);
     if (offset >= 0 && offset < 136) {
-      largePreimage[msg.sender].data =
-        uint32(uint8(dat[uint256(offset)+0])) << 24 |
-        uint32(uint8(dat[uint256(offset)+1])) << 16 |
-        uint32(uint8(dat[uint256(offset)+2])) << 8 |
-        uint32(uint8(dat[uint256(offset)+3])) << 0;
+      largePreimage[msg.sender].data = fbo(dat, uint(offset));
     }
     Lib_Keccak256.sha3_xor_input(c, dat);
     Lib_Keccak256.sha3_permutation(c);
@@ -77,7 +73,7 @@ contract MIPSMemory {
   function AddLargePreimageFinal(bytes calldata idat) public view returns (bytes32, uint, uint32) {
     require(idat.length < 136, "final must be less than 136");
     int offset = int(largePreimage[msg.sender].offset) - int(largePreimage[msg.sender].len);
-    require(offset < idat.length, "offset must be less than length");
+    require(offset < int(idat.length), "offset must be less than length");
     Lib_Keccak256.CTX memory c;
     c.A = largePreimageState[msg.sender];
 
@@ -90,12 +86,7 @@ contract MIPSMemory {
     uint len = largePreimage[msg.sender].len + idat.length;
     uint32 data = largePreimage[msg.sender].data;
     if (offset >= 0) {
-      // comes from this block
-      data = 
-        uint32(uint8(dat[uint256(offset)+0])) << 24 |
-        uint32(uint8(dat[uint256(offset)+1])) << 16 |
-        uint32(uint8(dat[uint256(offset)+2])) << 8 |
-        uint32(uint8(dat[uint256(offset)+3])) << 0;
+      data = fbo(dat, uint(offset));
     }
     dat[135] = bytes1(uint8(0x80));
     dat[idat.length] |= bytes1(uint8(0x1));
@@ -138,6 +129,14 @@ contract MIPSMemory {
     return ret;
   }
 
+  function fbo(bytes memory dat, uint offset) internal pure returns (uint32) {
+    uint32 ret = uint32(uint8(dat[offset+0])) << 24 |
+                 uint32(uint8(dat[offset+1])) << 16 |
+                 uint32(uint8(dat[offset+2])) << 8 |
+                 uint32(uint8(dat[offset+3]));
+    return ret;
+  }
+
   function WriteMemory(bytes32 stateHash, uint32 addr, uint32 value) public returns (bytes32) {
     require(addr & 3 == 0, "write memory must be 32-bit aligned");
     return Lib_MerkleTrie.update(tb(addr>>2), tb(value), stateHash);

test/libkeccak.js

@@ -18,7 +18,7 @@ describe("MIPSMemory contract", function () {
       console.log("test size", n)
       expect((await mm.AddLargePreimageFinal(test))[0]).to.equal(keccak256(test));
     }
-    await tl(0)
+    await tl(1)
     await tl(100)
     await tl(134)
     await tl(135)