bug: Rangechecker.Check passes even when bits int is 1 less than bit-length of v variable in case when bits is odd

[ultrainstinct30]

Description

After creating a new instance of Rangechecker by using rangecheck.New(api), calling Check function using this instance passes even when the input bits is 1 less than the actual bit-length of v when bits is odd.

Expected Behavior

Check should fail

Actual Behavior

Check passes, implying v has bit-length bits

Steps to Reproduce

import (
	"testing"

	"github.com/consensys/gnark-crypto/ecc"
	"github.com/consensys/gnark/frontend"
	"github.com/consensys/gnark/frontend/cs/r1cs"
	"github.com/consensys/gnark/std/rangecheck"
	"github.com/consensys/gnark/test"
)

type TestRangeCheckCircuit struct {
	I1 frontend.Variable
	N  int
}

func (circuit *TestRangeCheckCircuit) Define(api frontend.API) error {
	rangeChecker := rangecheck.New(api)
	rangeChecker.Check(circuit.I1, circuit.N)
	return nil
}

func TestRangeCheck_Odd(t *testing.T) {
	assert := test.NewAssert(t)

	type testData struct {
		i1 uint64
		n  int
	}

	tests := []testData{}
	for i := 128; i < 256; i++ { //[128, 256) all have bit-length 8
		tests = append(tests, testData{i1: uint64(i), n: 7})
	}

	for _, t_i := range tests {
		circuit := TestRangeCheckCircuit{
			N: t_i.n,
		}
		r1cs, err := frontend.Compile(ecc.BN254.ScalarField(), r1cs.NewBuilder, &circuit)
		if err != nil {
			t.Fatal("Error in compiling circuit: ", err)
		}
		var witness TestRangeCheckCircuit
		witness.I1 = t_i.i1
		w, err := frontend.NewWitness(&witness, ecc.BN254.ScalarField())
		if err != nil {
			t.Fatal("Error in witness: ", err, "\n test: ", t_i)
		}
		err = r1cs.IsSolved(w)
		if err != nil {
			t.Fatal("Circuit not solved: ", err, "\n test: ", t_i)
		}

		assert.CheckCircuit(&circuit, test.WithValidAssignment(&witness), test.WithCurves(ecc.BN254))
	}
}

func TestRangeCheck_Even(t *testing.T) {
	assert := test.NewAssert(t)

	type testData struct {
		i1 uint64
		n  int
	}

	tests := []testData{}
	for i := 64; i < 128; i++ { //[64, 128) all have bit-length 7
		tests = append(tests, testData{i1: uint64(i), n: 6})
	}

	for _, t_i := range tests {
		circuit := TestRangeCheckCircuit{
			N: t_i.n,
		}
		r1cs, err := frontend.Compile(ecc.BN254.ScalarField(), r1cs.NewBuilder, &circuit)
		if err != nil {
			t.Fatal("Error in compiling circuit: ", err)
		}
		var witness TestRangeCheckCircuit
		witness.I1 = t_i.i1
		w, err := frontend.NewWitness(&witness, ecc.BN254.ScalarField())
		if err != nil {
			t.Fatal("Error in witness: ", err, "\n test: ", t_i)
		}
		err = r1cs.IsSolved(w)
		if err != nil {
			t.Fatal("Circuit not solved: ", err, "\n test: ", t_i)
		}

		assert.CheckCircuit(&circuit, test.WithValidAssignment(&witness), test.WithCurves(ecc.BN254))
	}
}

TestRangeCheck_Odd output: ok
TestRangeCheck_Even output: Circuit not solved: constraint #0 is not satisfied: 1 ⋅ 0 != 64 test: {64 6}

Your Environment

• gnark version used: v0.9.1
• gnark-crypto version used: v0.12.2-0.20231013160410-1f65e75b6dfb
• go version: go1.21.3 linux/amd64
• Operating System and version: Ubuntu 20.04 WSL 2

[ivokub]

Thank you for the report. I will have a look at it promptly.