MiMCSponge in Solidity

In an effort to reduce the gas cost of the TornadoCash MerkleTreeWithHistory contract, I set out to include the MiMCSponge hasher contract directly into the MerkleTreeWithHistory contract (this was a recommendation that ABDK consulting made in their audit of the TornadoCash contracts). After extensive testing, this is the final version.

Install and Test

1. Simply clone the repo and then run yarn or npm install . in the root directory.
2. Run npx hardhat test from the root directory.

Final Results

The results of the unit tests are below. I removed some old contracts, but the previous results can be found in older commits. Noteably, the recommendation that Hari made to me was to change from the switch/case expression to CODECOPY instead, which reduced the gas cost a further 5k gas. In total, this is about 15k gas savings compared to the original contract!

MerkleTrees
✓ MerkleTreeWithHistoryV1.yul::initialize should work (91ms)
final gas used:  904133
✓ MerkleTreeWithHistoryV1.yul::insert should work (1001ms)
final gas used:  899793
✓ MerkleTreeWithHistoryV3.yul::insert should work (846ms)
✓ Both implementations should share the same last root

MiMCSponge
✓ MiMCHasherCircomlib should match js implementation
✓ MerkleTreeWithHistory.yul::MiMCSponge should match js implementation
✓ MerkleTreeWithHistoryV2.yul::MiMCSponge should match js implementation
Circomlibjs::MiMCSponge gas estimate:  38804
MerkleTreeWithHistory.yul::MiMCSponge gas estimate:  39142 relative to circomlib:  1.0087104422224513
MerkleTreeWithHistoryV2.yul::MiMCSponge gas estimate:  39040 relative to circomlib:  1.0060818472322441