solver, solverrpc, csppsolver: Add RootFactors

The RootFactors function provides the result of the polynomial factorization,
including too few results or repeated roots, without erroring for these
conditions.

cmd/csppsolver/solver.go

@@ -36,23 +36,38 @@ type Args struct {
 
 type Result struct {
 	Roots        []*big.Int
+	Exponents    []int
 	RepeatedRoot *big.Int
 }
 
+func (*Solver) RootFactors(args Args, res *Result) error {
+	roots, exps, err := solver.RootFactors(args.A, args.F)
+	if err != nil {
+		return err
+	}
+	res.Roots = roots
+	res.Exponents = exps
+	return nil
+}
+
 type repeatedRoot interface {
 	RepeatedRoot() *big.Int
 }
 
 func (*Solver) Roots(args Args, res *Result) error {
-	roots, err := solver.Roots(args.A, args.F)
-	if rr, ok := err.(repeatedRoot); ok {
-		res.RepeatedRoot = rr.RepeatedRoot()
-		return nil // error set by client package
-	}
+	roots, exps, err := solver.RootFactors(args.A, args.F)
 	if err != nil {
 		return err
 	}
+	for i, exp := range exps {
+		if exp != 1 {
+			res.RepeatedRoot = roots[i]
+			return nil // error set by client package
+		}
+	}
+
 	res.Roots = roots
+	res.Exponents = exps
 	return nil
 }
 

solver/solver.go

@@ -35,16 +35,12 @@ func factorPoly(fac *C.fmpz_mod_poly_factor_struct, i uintptr) *C.fmpz_mod_poly_
 	return (*C.fmpz_mod_poly_struct)(unsafe.Pointer(uintptr(unsafe.Pointer(fac.poly)) + i*C.sizeof_fmpz_mod_poly_struct))
 }
 
-type repeatedRoot big.Int
-
-func (r *repeatedRoot) Error() string          { return "repeated roots" }
-func (r *repeatedRoot) RepeatedRoot() *big.Int { return (*big.Int)(r) }
-
-// Roots solves for len(a)-1 roots of the polynomial with coefficients a (mod F).
-// Repeated roots are considered an error for the purposes of unique slot assignment.
-func Roots(a []*big.Int, F *big.Int) ([]*big.Int, error) {
+// RootFactors returns the roots and their number of solutions in the
+// factorized polynomial.  Repeated roots are an error in the mixing protocol
+// but unlike the Roots function are not returned as an error here.
+func RootFactors(a []*big.Int, F *big.Int) ([]*big.Int, []int, error) {
 	if len(a) < 2 {
-		return nil, errors.New("too few coefficients")
+		return nil, nil, errors.New("too few coefficients")
 	}
 
 	var mod C.fmpz_t
@@ -80,6 +76,7 @@ func Roots(a []*big.Int, F *big.Int) ([]*big.Int, error) {
 	C.fmpz_mod_poly_factor(&factor[0], &poly[0], &modctx[0])
 
 	roots := make([]*big.Int, 0, len(a)-1)
+	exps := make([]int, 0, len(a)-1)
 	var m C.fmpz_t
 	C.fmpz_init(&m[0])
 	defer C.fmpz_clear(&m[0])
@@ -93,19 +90,36 @@ func Roots(a []*big.Int, F *big.Int) ([]*big.Int, error) {
 
 		b, ok := new(big.Int).SetString(str, base)
 		if !ok {
-			return nil, errors.New("failed to read fmpz")
+			return nil, nil, errors.New("failed to read fmpz")
 		}
 		b.Neg(b)
 		b.Mod(b, F)
 
-		if factorExp(&factor[0], uintptr(i)) != 1 {
-			return nil, (*repeatedRoot)(b)
-		}
 		roots = append(roots, b)
+		exps = append(exps, int(factorExp(&factor[0], uintptr(i))))
+	}
+
+	return roots, exps, nil
+}
+
+type repeatedRoot big.Int
+
+func (r *repeatedRoot) Error() string          { return "repeated roots" }
+func (r *repeatedRoot) RepeatedRoot() *big.Int { return (*big.Int)(r) }
+
+// Roots solves for len(a)-1 roots of the polynomial with coefficients a (mod F).
+// Repeated roots are considered an error for the purposes of unique slot
+// assignment, and an error with method RepeatedRoot() *big.Int is returned.
+func Roots(a []*big.Int, F *big.Int) ([]*big.Int, error) {
+	roots, exps, err := RootFactors(a, F)
+	if err != nil {
+		return roots, err
 	}
 
-	if len(roots) != len(a)-1 {
-		return nil, errors.New("too few roots")
+	for i, exp := range exps {
+		if exp != 1 {
+			return nil, (*repeatedRoot)(roots[i])
+		}
 	}
 
 	return roots, nil

solver/solver_test.go

@@ -429,6 +429,33 @@ func TestRoots(t *testing.T) {
 	}
 }
 
+func TestRootFactors(t *testing.T) {
+	for i := range tests {
+		roots, exps, err := RootFactors(tests[i].coeffs, tests[i].field)
+		if err != nil {
+			t.Error(err)
+			continue
+		}
+		for i, exp := range exps {
+			if exp != 1 {
+				t.Errorf("repeated root %v at index %v", roots[i], i)
+				continue
+			}
+		}
+		if len(roots) != len(tests[i].messages) {
+			t.Error("wrong root count")
+			continue
+		}
+		sortBig(tests[i].messages)
+		sortBig(roots)
+		for j := range roots {
+			if roots[j].Cmp(tests[i].messages[j]) != 0 {
+				t.Error("recovered wrong message")
+			}
+		}
+	}
+}
+
 func BenchmarkRoots(b *testing.B) {
 	for i := range tests {
 		b.Run(fmt.Sprintf("%d", tests[i].n), func(b *testing.B) {

solverrpc/rpc.go

@@ -77,6 +77,31 @@ func StartSolver() error {
 	return onceErr
 }
 
+// RootFactors returns the roots and their number of solutions in the
+// factorized polynomial.  Repeated roots are an error in the mixing protocol
+// but unlike the Roots function are not returned as an error here.
+func RootFactors(a []*big.Int, F *big.Int) ([]*big.Int, []int, error) {
+	if err := StartSolver(); err != nil {
+		return nil, nil, err
+	}
+
+	var args struct {
+		A []*big.Int
+		F *big.Int
+	}
+	args.A = a
+	args.F = F
+	var result struct {
+		Roots     []*big.Int
+		Exponents []int
+	}
+	err := client.Call("Solver.RootFactors", args, &result)
+	if err != nil {
+		return nil, nil, err
+	}
+	return result.Roots, result.Exponents, nil
+}
+
 type repeatedRoot big.Int
 
 func (r *repeatedRoot) Error() string          { return "repeated roots" }

solverrpc/solver_test.go

@@ -429,6 +429,33 @@ func TestRoots(t *testing.T) {
 	}
 }
 
+func TestRootFactors(t *testing.T) {
+	for i := range tests {
+		roots, exps, err := RootFactors(tests[i].coeffs, tests[i].field)
+		if err != nil {
+			t.Error(err)
+			continue
+		}
+		for i, exp := range exps {
+			if exp != 1 {
+				t.Errorf("repeated root %v at index %v", roots[i], i)
+				continue
+			}
+		}
+		if len(roots) != len(tests[i].messages) {
+			t.Error("wrong root count")
+			continue
+		}
+		sortBig(tests[i].messages)
+		sortBig(roots)
+		for j := range roots {
+			if roots[j].Cmp(tests[i].messages[j]) != 0 {
+				t.Error("recovered wrong message")
+			}
+		}
+	}
+}
+
 func BenchmarkRoots(b *testing.B) {
 	for i := range tests {
 		b.Run(fmt.Sprintf("%d", tests[i].n), func(b *testing.B) {