refactor: plonk uses constraint/ and couple of fixes closes #467

debug/debug.go

@@ -75,5 +75,8 @@ func writeStack(sbb *strings.Builder, forceClean ...bool) {
 		if strings.HasSuffix(function, "Define") {
 			break
 		}
+		if strings.HasSuffix(function, "callDeferred") {
+			break
+		}
 	}
 }

debug/symbol_table.go

@@ -76,6 +76,9 @@ func (st *SymbolTable) CollectStack() []int {
 		if strings.HasSuffix(function, "Define") {
 			break
 		}
+		if strings.HasSuffix(function, "callDeferred") {
+			break
+		}
 	}
 	return r
 }

debug_test.go

@@ -131,7 +131,7 @@ func TestTraceNotEqual(t *testing.T) {
 	{
 		_, err := getPlonkTrace(&circuit, &witness)
 		assert.Error(err)
-		assert.Contains(err.Error(), "constraint #1 is not satisfied: [assertIsEqual] 1 + -66 == 0")
+		assert.Contains(err.Error(), "constraint #1 is not satisfied: [assertIsEqual] 1 == 66")
 		assert.Contains(err.Error(), "(*notEqualTrace).Define")
 		assert.Contains(err.Error(), "debug_test.go:")
 	}

frontend/builder.go

@@ -49,12 +49,10 @@ type Compiler interface {
 	// FieldBitLen returns the number of bits needed to represent an element in the scalar field
 	FieldBitLen() int
 
-	// Commit returns a commitment to the given variables, to be used as initial randomness in
-	// Fiat-Shamir when the statement to prove is particularly large.
-	// TODO cite paper
-	// ! Experimental
-	// TENTATIVE: Functions regarding fiat-shamir-ed proofs over enormous statements  TODO finalize
-	Commit(...Variable) (Variable, error)
+	// Defer is called after circuit.Define() and before Compile(). This method
+	// allows for the circuits to register callbacks which finalize batching
+	// operations etc. Unlike Go defer, it is not locally scoped.
+	Defer(cb func(api API) error)
 }
 
 // Builder represents a constraint system builder
@@ -73,3 +71,10 @@ type Builder interface {
 	// called inside circuit.Define()
 	SecretVariable(schema.LeafInfo) Variable
 }
+
+// Committer allows to commit to the variables and returns the commitment. The
+// commitment can be used as a challenge using Fiat-Shamir heuristic.
+type Committer interface {
+	// Commit commits to the variables and returns the commitment.
+	Commit(toCommit ...Variable) (commitment Variable, err error)
+}

frontend/compile.go

@@ -9,6 +9,7 @@ import (
 	"github.com/consensys/gnark/constraint"
 	"github.com/consensys/gnark/debug"
 	"github.com/consensys/gnark/frontend/schema"
+	"github.com/consensys/gnark/internal/circuitdefer"
 	"github.com/consensys/gnark/logger"
 )
 
@@ -122,10 +123,22 @@ func parseCircuit(builder Builder, circuit Circuit) (err error) {
 	if err = circuit.Define(builder); err != nil {
 		return fmt.Errorf("define circuit: %w", err)
 	}
+	if err = callDeferred(builder); err != nil {
+		return fmt.Errorf("")
+	}
 
 	return
 }
 
+func callDeferred(builder Builder) error {
+	for i, cb := range circuitdefer.GetAll[func(API) error](builder) {
+		if err := cb(builder); err != nil {
+			return fmt.Errorf("defer fn %d: %w", i, err)
+		}
+	}
+	return nil
+}
+
 // CompileOption defines option for altering the behaviour of the Compile
 // method. See the descriptions of the functions returning instances of this
 // type for available options.

frontend/cs/r1cs/builder.go

@@ -29,6 +29,7 @@ import (
 	"github.com/consensys/gnark/frontend"
 	"github.com/consensys/gnark/frontend/internal/expr"
 	"github.com/consensys/gnark/frontend/schema"
+	"github.com/consensys/gnark/internal/circuitdefer"
 	"github.com/consensys/gnark/internal/kvstore"
 	"github.com/consensys/gnark/internal/tinyfield"
 	"github.com/consensys/gnark/internal/utils"
@@ -45,27 +46,27 @@ import (
 )
 
 // NewBuilder returns a new R1CS builder which implements frontend.API.
+// Additionally, this builder also implements [frontend.Committer].
 func NewBuilder(field *big.Int, config frontend.CompileConfig) (frontend.Builder, error) {
 	return newBuilder(field, config), nil
 }
 
 type builder struct {
-	cs constraint.R1CS
-
+	cs     constraint.R1CS
 	config frontend.CompileConfig
+	kvstore.Store
 
 	// map for recording boolean constrained variables (to not constrain them twice)
 	mtBooleans map[uint64][]expr.LinearExpression
 
-	q    *big.Int
 	tOne constraint.Coeff
-	heap minHeap // helps merge k sorted linear expressions
+
+	// helps merge k sorted linear expressions
+	heap minHeap
 
 	// buffers used to do in place api.MAC
 	mbuf1 expr.LinearExpression
 	mbuf2 expr.LinearExpression
-
-	kvstore.Store
 }
 
 // initialCapacity has quite some impact on frontend performance, especially on large circuits size
@@ -114,11 +115,6 @@ func newBuilder(field *big.Int, config frontend.CompileConfig) *builder {
 	builder.tOne = builder.cs.One()
 	builder.cs.AddPublicVariable("1")
 
-	builder.q = builder.cs.Field()
-	if builder.q.Cmp(field) != 0 {
-		panic("invalid modulus on cs impl") // sanity check
-	}
-
 	return &builder
 }
 
@@ -452,3 +448,7 @@ func (builder *builder) compress(le expr.LinearExpression) expr.LinearExpression
 	builder.cs.AddConstraint(builder.newR1C(le, one, t))
 	return t
 }
+
+func (builder *builder) Defer(cb func(frontend.API) error) {
+	circuitdefer.Put(builder, cb)
+}

frontend/cs/r1cs/r1cs_test.go

@@ -135,3 +135,28 @@ func BenchmarkReduce(b *testing.B) {
 		}
 	})
 }
+
+type EmptyCircuit struct {
+	X  frontend.Variable
+	cb func(frontend.API) error
+}
+
+func (c *EmptyCircuit) Define(api frontend.API) error {
+	api.AssertIsEqual(c.X, 0)
+	api.Compiler().Defer(c.cb)
+	return nil
+}
+
+func TestPreCompileHook(t *testing.T) {
+	var called bool
+	c := &EmptyCircuit{
+		cb: func(a frontend.API) error { called = true; return nil },
+	}
+	_, err := frontend.Compile(ecc.BN254.ScalarField(), NewBuilder, c)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !called {
+		t.Error("callback not called")
+	}
+}