provide open reconstruct circuit interface from witness stream

[mdj33]

hi,
seems project not provide the interface to reconstruct parameters from witness stream against to WriteFullTo and WritePublicTo interface.
func WritePublicTo(w io.Writer, curveID ecc.ID, publicWitness frontend.Circuit) (int64, error) {

just like the FromPublicAssignment, but it is internal interface.

		if err := w.FromPublicAssignment(publicWitness); err != nil {
			return err
		}

for example:

type Circuit struct {
 	    X frontend.Variable
	    Y frontend.Variable `gnark:",public"`
 	    Z frontend.Variable `gnark:",public"`
 	}

public witness stream like "`0000000200000000000000000000000000000000000000000000000000000000000000230000000000000000000000000000000000000000000000000000000000000189"

we hope a open interface to construct the Circuit's specific Y and Z variable from the stream.

[gbotrel]

Hello, current implementation forces the user to use groth16.ReadAndProve (resp plonk.ReadAndProve) that parses the witness. (see https://docs.gnark.consensys.net/en/latest/HowTo/serialize/#ordering )

Is that sufficient for your use case?

[mdj33]

hi gbotrel,
thanks for your response. it is not sufficient for our case only to readAndProve or readAndVerify. some times, in smart contract, we also are care the public inputs to verify or record. so it is necessary to parse the public input to its original struct.
now I just work around it by implemented LimitReadFrom() locally and reflect my circuit to assign them :)

[gbotrel]

@mdj33 latest commit on develop introduces witness.ReadPublicFrom and witness.ReadFullFrom which assign values to a provided witness (as big.Int values) from a io.Reader.