fix: closes #509 api did not handle AssertIsLessOrEqual with constant as first param

frontend/cs/r1cs/api_assertions.go

@@ -17,12 +17,12 @@ limitations under the License.
 package r1cs
 
 import (
+	"fmt"
 	"math/big"
 
 	"github.com/consensys/gnark/debug"
 	"github.com/consensys/gnark/frontend"
 	"github.com/consensys/gnark/frontend/internal/expr"
-	"github.com/consensys/gnark/internal/utils"
 	"github.com/consensys/gnark/std/math/bits"
 )
 
@@ -80,18 +80,34 @@ func (builder *builder) AssertIsBoolean(i1 frontend.Variable) {
 //
 // derived from:
 // https://github.com/zcash/zips/blob/main/protocol/protocol.pdf
-func (builder *builder) AssertIsLessOrEqual(_v frontend.Variable, bound frontend.Variable) {
-	v := builder.toVariable(_v)
+func (builder *builder) AssertIsLessOrEqual(v frontend.Variable, bound frontend.Variable) {
+	cv, vConst := builder.constantValue(v)
+	cb, bConst := builder.constantValue(bound)
+
+	// both inputs are constants
+	if vConst && bConst {
+		bv, bb := builder.cs.ToBigInt(&cv), builder.cs.ToBigInt(&cb)
+		if bv.Cmp(bb) == 1 {
+			panic(fmt.Sprintf("AssertIsLessOrEqual: %s > %s", bv.String(), bb.String()))
+		}
+	}
 
-	if b, ok := bound.(expr.LinearExpression); ok {
-		assertIsSet(b)
-		builder.mustBeLessOrEqVar(v, b)
-	} else {
-		builder.mustBeLessOrEqCst(v, utils.FromInterface(bound))
+	// bound is constant
+	if bConst {
+		vv := builder.toVariable(v)
+		builder.mustBeLessOrEqCst(vv, builder.cs.ToBigInt(&cb))
+		return
 	}
+
+	builder.mustBeLessOrEqVar(v, bound)
 }
 
-func (builder *builder) mustBeLessOrEqVar(a, bound expr.LinearExpression) {
+func (builder *builder) mustBeLessOrEqVar(a, bound frontend.Variable) {
+	// here bound is NOT a constant,
+	// but a can be either constant or a wire.
+
+	_, aConst := builder.constantValue(a)
+
 	debug := builder.newDebugInfo("mustBeLessOrEq", a, " <= ", bound)
 
 	nbBits := builder.cs.FieldBitLen()
@@ -128,16 +144,21 @@ func (builder *builder) mustBeLessOrEqVar(a, bound expr.LinearExpression) {
 		// note if bound[i] == 1, this constraint is (1 - ai) * ai == 0
 		// → this is a boolean constraint
 		// if bound[i] == 0, t must be 0 or 1, thus ai must be 0 or 1 too
-		builder.MarkBoolean(aBits[i].(expr.LinearExpression)) // this does not create a constraint
 
-		added = append(added, builder.cs.AddConstraint(builder.newR1C(l, aBits[i], zero)))
+		if aConst {
+			// aBits[i] is a constant;
+			l = builder.Mul(l, aBits[i])
+			added = append(added, builder.cs.AddConstraint(builder.newR1C(l, zero, zero)))
+		} else {
+			added = append(added, builder.cs.AddConstraint(builder.newR1C(l, aBits[i], zero)))
+		}
 	}
 
 	builder.cs.AttachDebugInfo(debug, added)
 
 }
 
-func (builder *builder) mustBeLessOrEqCst(a expr.LinearExpression, bound big.Int) {
+func (builder *builder) mustBeLessOrEqCst(a expr.LinearExpression, bound *big.Int) {
 
 	nbBits := builder.cs.FieldBitLen()
 
@@ -187,7 +208,6 @@ func (builder *builder) mustBeLessOrEqCst(a expr.LinearExpression, bound big.Int
 			l = builder.Sub(l, aBits[i])
 
 			added = append(added, builder.cs.AddConstraint(builder.newR1C(l, aBits[i], builder.cstZero())))
-			builder.MarkBoolean(aBits[i].(expr.LinearExpression))
 		} else {
 			builder.AssertIsBoolean(aBits[i])
 		}

frontend/cs/scs/api_assertions.go

@@ -110,13 +110,13 @@ func (builder *builder) AssertIsBoolean(i1 frontend.Variable) {
 func (builder *builder) AssertIsLessOrEqual(v frontend.Variable, bound frontend.Variable) {
 	switch b := bound.(type) {
 	case expr.Term:
-		builder.mustBeLessOrEqVar(v.(expr.Term), b)
+		builder.mustBeLessOrEqVar(v, b)
 	default:
-		builder.mustBeLessOrEqCst(v.(expr.Term), utils.FromInterface(b))
+		builder.mustBeLessOrEqCst(v, utils.FromInterface(b))
 	}
 }
 
-func (builder *builder) mustBeLessOrEqVar(a expr.Term, bound expr.Term) {
+func (builder *builder) mustBeLessOrEqVar(a frontend.Variable, bound expr.Term) {
 
 	debug := builder.newDebugInfo("mustBeLessOrEq", a, " <= ", bound)
 
@@ -147,18 +147,28 @@ func (builder *builder) mustBeLessOrEqVar(a expr.Term, bound expr.Term) {
 		// note if bound[i] == 1, this constraint is (1 - ai) * ai == 0
 		// → this is a boolean constraint
 		// if bound[i] == 0, t must be 0 or 1, thus ai must be 0 or 1 too
-		builder.MarkBoolean(aBits[i].(expr.Term)) // this does not create a constraint
 
-		builder.addPlonkConstraint(sparseR1C{
-			xa: l.VID,
-			xb: aBits[i].(expr.Term).VID,
-			qM: l.Coeff,
-		}, debug)
+		if ai, ok := builder.constantValue(aBits[i]); ok {
+			// a is constant; ensure l == 0
+			builder.cs.Mul(&l.Coeff, &ai)
+			builder.addPlonkConstraint(sparseR1C{
+				xa: l.VID,
+				qL: l.Coeff,
+			}, debug)
+		} else {
+			// l * a[i] == 0
+			builder.addPlonkConstraint(sparseR1C{
+				xa: l.VID,
+				xb: aBits[i].(expr.Term).VID,
+				qM: l.Coeff,
+			}, debug)
+		}
+
 	}
 
 }
 
-func (builder *builder) mustBeLessOrEqCst(a expr.Term, bound big.Int) {
+func (builder *builder) mustBeLessOrEqCst(a frontend.Variable, bound big.Int) {
 
 	nbBits := builder.cs.FieldBitLen()
 
@@ -170,6 +180,14 @@ func (builder *builder) mustBeLessOrEqCst(a expr.Term, bound big.Int) {
 		panic("AssertIsLessOrEqual: bound is too large, constraint will never be satisfied")
 	}
 
+	if ca, ok := builder.constantValue(a); ok {
+		// a is constant, compare the big int values
+		ba := builder.cs.ToBigInt(&ca)
+		if ba.Cmp(&bound) == 1 {
+			panic(fmt.Sprintf("AssertIsLessOrEqual: %s > %s", ba.String(), bound.String()))
+		}
+	}
+
 	// debug info
 	debug := builder.newDebugInfo("mustBeLessOrEq", a, " <= ", bound)
 

frontend/cs/scs/builder.go

@@ -136,6 +136,12 @@ func (builder *builder) addMulGate(a, b, c expr.Term, debug ...constraint.DebugI
 
 // addPlonkConstraint adds a sparseR1C to the underlying constraint system
 func (builder *builder) addPlonkConstraint(c sparseR1C, debug ...constraint.DebugInfo) {
+	if !c.qM.IsZero() && (c.xa == 0 || c.xb == 0) {
+		// TODO this is internal but not easy to detect; if qM is set, but one or both of xa / xb is not,
+		// since wireID == 0 is a valid wire, it may trigger unexpected behavior.
+		log := logger.Logger()
+		log.Warn().Msg("adding a plonk constraint with qM set but xa or xb == 0 (wire 0)")
+	}
 	L := builder.cs.MakeTerm(&c.qL, c.xa)
 	R := builder.cs.MakeTerm(&c.qR, c.xb)
 	O := builder.cs.MakeTerm(&c.qO, c.xc)