Circom witness generators
-------------------------

This piece of software takes the computation graph files generated by 
[`circom-witnesscalc`](https://github.com/iden3/circom-witnesscalc),
and either interprets or compiles them to various algebra backends.

### Implementation status

Compiler (in Haskell):

- [x] parsing the graph files
- [x] naive interpreter
- [ ] constantine backend
- [ ] zikkurat backend
- [ ] arkworks backend

Nim witness generator:

- [x] parsing the graph files
- [ ] generating the witness
- [ ] proper error handling

### Testing & correctness

I haven't yet done any proper testing, apart from "works for our purposes".

### Circuit optimizations

NOTE: you _have to_ run `circom` with the `--O2` options, otherwise the 
witness format will be most probably incompatible with the the one generated
by `circom-witnesscalc`.

### Graph file format

`circom-witnesscalc` produces binary files encoding a computation graph.

This has the following format:

- magic header: "wtns.graph.001" (14 bytes)
- number of nodes (8 bytes little endian)
- list of nodes, in protobuf serialized format. Each prefixed by a `varint` length
- `GraphMetaData`, in protobuf 
- 8 bytes offset (yes, _at the very end_...), pointing to the start of `GraphMetaData`

Node format:

- varint length prefix
- protobuf tag-byte (lower 3 bits are `0x02`, upper bits are node type 1..5)
- varint length 
- several record fields
    - protobuf tag-byte (lower 3 bits are 0x00, upper bits are field index 1,2,3,4)
    - value is varint word32, except for ConstantNode when it's a little-endian bytes
      (wrapped a few times, because protobuf is being protobuf...)