Minimum requirement for a circuit

[fosgate29]

Hi, I build a pretty simple cirtuit, but it won't work. I get a runtime error: index out of range [0] with length 0.
My struct is

type circuitv5 struct {
	AcceptedQuote frontend.Variable `gnark:",public"` 
	QuoteFromA    frontend.Variable `gnark:",private"` 
}

and my circuit has only this line:
cs.AssertIsEqual(circuit.AcceptedQuote, circuit.QuoteFromA)

When I setup, it fails with the error message above.

But if I add a multiplication, it works:
cs.Mul(circuit.AcceptedQuote, circuit.QuoteFromA).

So if there is Mul before the Assert, it works. Is it a must to have a computation before? Or am I doing something wrong or missunderstood it?

Thanks.
here is my repo in case you would like to run it: https://github.com/fosgate29/financialcircuit/blob/master/bond5.go

[gbotrel]

Hi, indeed this popped up before, and this should throw a clean error message ( #112 ).

A circuit that has no computational constraints make little sense (do you have something we missed in mind? ) --> it means the proof is stating something like x == y and to verify the proof, one must supply... y.

Note that some assertions (like AssertIsLessOrEqual) will add computational constraints. AssertIsEqual does not.

[fosgate29]

Thank you. I will give it another try. Thanks.

[fosgate29]

Actually we were trying to build this circuit:

type bondCircuitv struct {
	AcceptedQuote frontend.Variable `gnark:",public"`  
	QuoteFromA     frontend.Variable `gnark:",private"` 
	QuoteFromB    frontend.Variable `gnark:",private"` 
	QuoteFromC    frontend.Variable `gnark:",private"`
}

And with this logic to get the best quote:

	if(QuoteFromA < QuoteFromB){
		if(QuoteFromA < QuoteC){
			Result = QuoteFromA
		}
		else{
			Result = QuoteFromC
		}
	}
	else if(QuoteFromB < QuoteFromC){
		Result = QuoteFromB
	}
	else{
		Result = QuoteFromC
	} 
	
	cs.AssertIsEqual(cs.AcceptedQuote, Result)

But we are not able to compare QuoteFromA, QuoteFromB and QuoteFromC and get as a result the best quote.

[fosgate29]

I built this one:

type bondCircuitv5 struct {
	//Bid 92.63
	AcceptedQuote frontend.Variable `gnark:",public"` // 92
	QuoteFromA  frontend.Variable `gnark:",private"` // 92
	QuoteFromB  frontend.Variable `gnark:",private"` // 93
	QuoteFromC  frontend.Variable `gnark:",private"` // 95
	WinnerQuote frontend.Variable `gnark:",private"` // 92
	Quote1      frontend.Variable `gnark:",private"` // 93
	Quote2      frontend.Variable `gnark:",private"` // 95
}

func (circuit *bondCircuitv5) Define(curveID ecc.ID, cs *frontend.ConstraintSystem) error {

	//Make sure Winner Quote is the smallest one
	cs.AssertIsLessOrEqual(circuit.WinnerQuote, circuit.Quote1)
	cs.AssertIsLessOrEqual(circuit.WinnerQuote, circuit.Quote2)
	cs.AssertIsEqual(circuit.WinnerQuote, circuit.AcceptedQuote)

	//If winner quote is from A, B or C, one of the subtraction is going to return zero
	subA := cs.Sub(circuit.QuoteFromA, circuit.WinnerQuote)
	subB := cs.Sub(circuit.QuoteFromB, circuit.WinnerQuote)
	subC := cs.Sub(circuit.QuoteFromC, circuit.WinnerQuote)

	outputA := cs.IsZero(subA, curveID) // 1 - iszero - true
	outputB := cs.IsZero(subB, curveID) // 0 false
	outputC := cs.IsZero(subC, curveID) // 0 false

	// outputA || outputB || outputC == 1
	result_temp := cs.Or(outputA, outputB)
	result := cs.Or(result_temp, outputC)

	one := cs.Constant(1)
	cs.AssertIsEqual(result, one) //

I need to add a Signature to make sure A, B and C knows their quotes are inside the circuit. PubKey from A, B and C is going to be public parameters.

I post here when it is ready.