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SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
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rapidsnark
rapidsnark is a fast zkSNARK prover written in C++, that generates proofs for circuits created with circom and snarkjs.
The advantage of SMT over the general Merkle tree is that it not only allows us to provide an inclusion proof but also an exclusion proof. This means that we can prove that a tree does not contain a value for the given key. Thanks to this, we can prove any operation (insert, delete, update), which will be a very important property of our rollup. Fortunately, Circomlib (and CircomlibJS) includes an SMT implementation, so we do not need to implement this ourselves.
In a nutshell, that's how our rollup works. Doesn't sound too complicated, right? We'll see from the code that the devil is in the details. Let's take a look at the code. (Of course, all code is available on GitHub.)
I used the circom_tester library to check a circuit, which is a very effective solution for testing circuits since there is no need to compile the circuit for testing. The calculateWitness function calculates the witness which is then checked by checkConstraints.
To compile a circuit, a lot of RAM and computational capability is required. On my laptop (24G RAM, i7) the largest circuit I could compile was around 64 elements. For a larger batch-size circuit (say 256 elements), a huge amount of RAM is needed. The compile time was also around half to one hour, and the fan was really roaring. Thankfully, generating the proof for the batch was much faster, but still had to wait a few minutes. This time can probably be improved a lot by using rapidsnark for proof generation, and the technology will also probably improve a lot in the future, for example using GPU or ASICs-based proof generation.