/********************************************************************************************
* SIDH: an efficient supersingular isogeny cryptography library
*
* Abstract: elliptic curve and isogeny functions
*********************************************************************************************/

void xDBL(const point_proj_t P, point_proj_t Q, const f2elm_t *A24plus, const f2elm_t *C24) { // Doubling of a Montgomery point in projective coordinates (X:Z).
	                                                                                               // Input: projective Montgomery x-coordinates P = (X1:Z1), where x1=X1/Z1 and Montgomery curve constants A+2C and 4C.
	                                                                                               // Output: projective Montgomery x-coordinates Q = 2*P = (X2:Z2).
	f2elm_t _t0, _t1;
	f2elm_t *t0=&_t0, *t1=&_t1;

	fp2sub(&P->X, &P->Z, t0);       // t0 = X1-Z1
	fp2add(&P->X, &P->Z, t1);       // t1 = X1+Z1
	fp2sqr_mont(t0, t0);          // t0 = (X1-Z1)^2
	fp2sqr_mont(t1, t1);          // t1 = (X1+Z1)^2
	fp2mul_mont(C24, t0, &Q->Z);   // Z2 = C24*(X1-Z1)^2
	fp2mul_mont(t1, &Q->Z, &Q->X);  // X2 = C24*(X1-Z1)^2*(X1+Z1)^2
	fp2sub(t1, t0, t1);           // t1 = (X1+Z1)^2-(X1-Z1)^2
	fp2mul_mont(A24plus, t1, t0); // t0 = A24plus*[(X1+Z1)^2-(X1-Z1)^2]
	fp2add(&Q->Z, t0, &Q->Z);       // Z2 = A24plus*[(X1+Z1)^2-(X1-Z1)^2] + C24*(X1-Z1)^2
	fp2mul_mont(&Q->Z, t1, &Q->Z);  // Z2 = [A24plus*[(X1+Z1)^2-(X1-Z1)^2] + C24*(X1-Z1)^2]*[(X1+Z1)^2-(X1-Z1)^2]
}

void xDBLe(const point_proj_t P, point_proj_t Q, const f2elm_t *A24plus, const f2elm_t *C24, const int e) { // Computes [2^e](X:Z) on Montgomery curve with projective constant via e repeated doublings.
	                                                                                                             // Input: projective Montgomery x-coordinates P = (XP:ZP), such that xP=XP/ZP and Montgomery curve constants A+2C and 4C.
	                                                                                                             // Output: projective Montgomery x-coordinates Q <- (2^e)*P.
	int i;

	copy_words((const digit_t *) P, (digit_t *) Q, 2 * 2 * NWORDS_FIELD);

	for (i = 0; i < e; i++) {
		xDBL(Q, Q, A24plus, C24);
	}
}

void get_4_isog(const point_proj_t P, f2elm_t *A24plus, f2elm_t *C24, f2elm_t *coeff) { // Computes the corresponding 4-isogeny of a projective Montgomery point (X4:Z4) of order 4.
	                                                                                         // Input:  projective point of order four P = (X4:Z4).
	                                                                                         // Output: the 4-isogenous Montgomery curve with projective coefficients A+2C/4C and the 3 coefficients
	                                                                                         //         that are used to evaluate the isogeny at a point in eval_4_isog().

	fp2sub(&P->X, &P->Z, &coeff[1]);         // coeff[1] = X4-Z4
	fp2add(&P->X, &P->Z, &coeff[2]);         // coeff[2] = X4+Z4
	fp2sqr_mont(&P->Z, &coeff[0]);          // coeff[0] = Z4^2
	fp2add(&coeff[0], &coeff[0], &coeff[0]); // coeff[0] = 2*Z4^2
	fp2sqr_mont(&coeff[0], C24);           // C24 = 4*Z4^4
	fp2add(&coeff[0], &coeff[0], &coeff[0]); // coeff[0] = 4*Z4^2
	fp2sqr_mont(&P->X, A24plus);           // A24plus = X4^2
	fp2add(A24plus, A24plus, A24plus);    // A24plus = 2*X4^2
	fp2sqr_mont(A24plus, A24plus);        // A24plus = 4*X4^4
}

void eval_4_isog(point_proj_t P, f2elm_t *coeff) { // Evaluates the isogeny at the point (X:Z) in the domain of the isogeny, given a 4-isogeny phi defined
	                                                      // by the 3 coefficients in coeff (computed in the function get_4_isog()).
	                                                      // Inputs: the coefficients defining the isogeny, and the projective point P = (X:Z).
	                                                      // Output: the projective point P = phi(P) = (X:Z) in the codomain.
	f2elm_t _t0, _t1;
	f2elm_t *t0=&_t0, *t1=&_t1;

	fp2add(&P->X, &P->Z, t0);          // t0 = X+Z
	fp2sub(&P->X, &P->Z, t1);          // t1 = X-Z
	fp2mul_mont(t0, &coeff[1], &P->X); // X = (X+Z)*coeff[1]
	fp2mul_mont(t1, &coeff[2], &P->Z); // Z = (X-Z)*coeff[2]
	fp2mul_mont(t0, t1, t0);         // t0 = (X+Z)*(X-Z)
	fp2mul_mont(t0, &coeff[0], t0);   // t0 = coeff[0]*(X+Z)*(X-Z)
	fp2add(&P->X, &P->Z, t1);          // t1 = (X-Z)*coeff[2] + (X+Z)*coeff[1]
	fp2sub(&P->X, &P->Z, &P->Z);        // Z = (X-Z)*coeff[2] - (X+Z)*coeff[1]
	fp2sqr_mont(t1, t1);             // t1 = [(X-Z)*coeff[2] + (X+Z)*coeff[1]]^2
	fp2sqr_mont(&P->Z, &P->Z);         // Z = [(X-Z)*coeff[2] - (X+Z)*coeff[1]]^2
	fp2add(t1, t0, &P->X);            // X = coeff[0]*(X+Z)*(X-Z) + [(X-Z)*coeff[2] + (X+Z)*coeff[1]]^2
	fp2sub(&P->Z, t0, t0);            // t0 = [(X-Z)*coeff[2] - (X+Z)*coeff[1]]^2 - coeff[0]*(X+Z)*(X-Z)
	fp2mul_mont(&P->X, t1, &P->X);     // Xfinal
	fp2mul_mont(&P->Z, t0, &P->Z);     // Zfinal
}

void xTPL(const point_proj_t P, point_proj_t Q, const f2elm_t *A24minus, const f2elm_t *A24plus) { // Tripling of a Montgomery point in projective coordinates (X:Z).
	                                                                                                    // Input: projective Montgomery x-coordinates P = (X:Z), where x=X/Z and Montgomery curve constants A24plus = A+2C and A24minus = A-2C.
	                                                                                                    // Output: projective Montgomery x-coordinates Q = 3*P = (X3:Z3).
	f2elm_t _t0, _t1, _t2, _t3, _t4, _t5, _t6;
    f2elm_t *t0=&_t0, *t1=&_t1, *t2=&_t2, *t3=&_t3, *t4=&_t4, *t5=&_t5, *t6=&_t6;

	fp2sub(&P->X, &P->Z, t0);        // t0 = X-Z
	fp2sqr_mont(t0, t2);           // t2 = (X-Z)^2
	fp2add(&P->X, &P->Z, t1);        // t1 = X+Z
	fp2sqr_mont(t1, t3);           // t3 = (X+Z)^2
	fp2add(t0, t1, t4);            // t4 = 2*X
	fp2sub(t1, t0, t0);            // t0 = 2*Z
	fp2sqr_mont(t4, t1);           // t1 = 4*X^2
	fp2sub(t1, t3, t1);            // t1 = 4*X^2 - (X+Z)^2
	fp2sub(t1, t2, t1);            // t1 = 4*X^2 - (X+Z)^2 - (X-Z)^2
	fp2mul_mont(t3, A24plus, t5);  // t5 = A24plus*(X+Z)^2
	fp2mul_mont(t3, t5, t3);       // t3 = A24plus*(X+Z)^3
	fp2mul_mont(A24minus, t2, t6); // t6 = A24minus*(X-Z)^2
	fp2mul_mont(t2, t6, t2);       // t2 = A24minus*(X-Z)^3
	fp2sub(t2, t3, t3);            // t3 = A24minus*(X-Z)^3 - coeff*(X+Z)^3
	fp2sub(t5, t6, t2);            // t2 = A24plus*(X+Z)^2 - A24minus*(X-Z)^2
	fp2mul_mont(t1, t2, t1);       // t1 = [4*X^2 - (X+Z)^2 - (X-Z)^2]*[A24plus*(X+Z)^2 - A24minus*(X-Z)^2]
	fp2add(t3, t1, t2);            // t2 = [4*X^2 - (X+Z)^2 - (X-Z)^2]*[A24plus*(X+Z)^2 - A24minus*(X-Z)^2] + A24minus*(X-Z)^3 - coeff*(X+Z)^3
	fp2sqr_mont(t2, t2);           // t2 = t2^2
	fp2mul_mont(t4, t2, &Q->X);     // X3 = 2*X*t2
	fp2sub(t3, t1, t1);            // t1 = A24minus*(X-Z)^3 - A24plus*(X+Z)^3 - [4*X^2 - (X+Z)^2 - (X-Z)^2]*[A24plus*(X+Z)^2 - A24minus*(X-Z)^2]
	fp2sqr_mont(t1, t1);           // t1 = t1^2
	fp2mul_mont(t0, t1, &Q->Z);     // Z3 = 2*Z*t1
}

void xTPLe(const point_proj_t P, point_proj_t Q, const f2elm_t *A24minus, const f2elm_t *A24plus, const int e) { // Computes [3^e](X:Z) on Montgomery curve with projective constant via e repeated triplings.
	                                                                                                                  // Input: projective Montgomery x-coordinates P = (XP:ZP), such that xP=XP/ZP and Montgomery curve constants A24plus = A+2C and A24minus = A-2C.
	                                                                                                                  // Output: projective Montgomery x-coordinates Q <- (3^e)*P.
	int i;

	copy_words((const digit_t *) P, (digit_t *) Q, 2 * 2 * NWORDS_FIELD);

	for (i = 0; i < e; i++) {
		xTPL(Q, Q, A24minus, A24plus);
	}
}

void get_3_isog(const point_proj_t P, f2elm_t *A24minus, f2elm_t *A24plus, f2elm_t *coeff) { // Computes the corresponding 3-isogeny of a projective Montgomery point (X3:Z3) of order 3.
	                                                                                              // Input:  projective point of order three P = (X3:Z3).
	                                                                                              // Output: the 3-isogenous Montgomery curve with projective coefficient A/C.
    f2elm_t _t0, _t1, _t2, _t3, _t4;
    f2elm_t *t0=&_t0, *t1=&_t1, *t2=&_t2, *t3=&_t3, *t4=&_t4;

	fp2sub(&P->X, &P->Z, &coeff[0]);   // coeff0 = X-Z
	fp2sqr_mont(&coeff[0], t0);      // t0 = (X-Z)^2
	fp2add(&P->X, &P->Z, &coeff[1]);   // coeff1 = X+Z
	fp2sqr_mont(&coeff[1], t1);      // t1 = (X+Z)^2
	fp2add(t0, t1, t2);             // t2 = (X+Z)^2 + (X-Z)^2
	fp2add(&coeff[0], &coeff[1], t3); // t3 = 2*X
	fp2sqr_mont(t3, t3);            // t3 = 4*X^2
	fp2sub(t3, t2, t3);             // t3 = 4*X^2 - (X+Z)^2 - (X-Z)^2
	fp2add(t1, t3, t2);             // t2 = 4*X^2 - (X-Z)^2
	fp2add(t3, t0, t3);             // t3 = 4*X^2 - (X+Z)^2
	fp2add(t0, t3, t4);             // t4 = 4*X^2 - (X+Z)^2 + (X-Z)^2
	fp2add(t4, t4, t4);             // t4 = 2(4*X^2 - (X+Z)^2 + (X-Z)^2)
	fp2add(t1, t4, t4);             // t4 = 8*X^2 - (X+Z)^2 + 2*(X-Z)^2
	fp2mul_mont(t2, t4, A24minus);  // A24minus = [4*X^2 - (X-Z)^2]*[8*X^2 - (X+Z)^2 + 2*(X-Z)^2]
	fp2add(t1, t2, t4);             // t4 = 4*X^2 + (X+Z)^2 - (X-Z)^2
	fp2add(t4, t4, t4);             // t4 = 2(4*X^2 + (X+Z)^2 - (X-Z)^2)
	fp2add(t0, t4, t4);             // t4 = 8*X^2 + 2*(X+Z)^2 - (X-Z)^2
	fp2mul_mont(t3, t4, A24plus);   // A24plus = [4*X^2 - (X+Z)^2]*[8*X^2 + 2*(X+Z)^2 - (X-Z)^2]
}

void eval_3_isog(point_proj_t Q, const f2elm_t *coeff) { // Computes the 3-isogeny R=phi(X:Z), given projective point (X3:Z3) of order 3 on a Montgomery curve and
	                                                            // a point P with 2 coefficients in coeff (computed in the function get_3_isog()).
	                                                            // Inputs: projective points P = (X3:Z3) and Q = (X:Z).
	                                                            // Output: the projective point Q <- phi(Q) = (X3:Z3).
    f2elm_t _t0, _t1, _t2;
    f2elm_t *t0=&_t0, *t1=&_t1, *t2=&_t2;

	fp2add(&Q->X, &Q->Z, t0);        // t0 = X+Z
	fp2sub(&Q->X, &Q->Z, t1);        // t1 = X-Z
	fp2mul_mont(t0, &coeff[0], t0); // t0 = coeff0*(X+Z)
	fp2mul_mont(t1, &coeff[1], t1); // t1 = coeff1*(X-Z)
	fp2add(t0, t1, t2);            // t2 = coeff0*(X+Z) + coeff1*(X-Z)
	fp2sub(t1, t0, t0);            // t0 = coeff1*(X-Z) - coeff0*(X+Z)
	fp2sqr_mont(t2, t2);           // t2 = [coeff0*(X+Z) + coeff1*(X-Z)]^2
	fp2sqr_mont(t0, t0);           // t0 = [coeff1*(X-Z) - coeff0*(X+Z)]^2
	fp2mul_mont(&Q->X, t2, &Q->X);   // X3final = X*[coeff0*(X+Z) + coeff1*(X-Z)]^2
	fp2mul_mont(&Q->Z, t0, &Q->Z);   // Z3final = Z*[coeff1*(X-Z) - coeff0*(X+Z)]^2
}

void inv_3_way(f2elm_t *z1, f2elm_t *z2, f2elm_t *z3) { // 3-way simultaneous inversion
	                                                        // Input:  z1,z2,z3
	                                                        // Output: 1/z1,1/z2,1/z3 (override inputs).
    f2elm_t _t0, _t1, _t2, _t3;
    f2elm_t *t0=&_t0, *t1=&_t1, *t2=&_t2, *t3=&_t3;

	fp2mul_mont(z1, z2, t0); // t0 = z1*z2
	fp2mul_mont(z3, t0, t1); // t1 = z1*z2*z3
	fp2inv_mont(t1);         // t1 = 1/(z1*z2*z3)
	fp2mul_mont(z3, t1, t2); // t2 = 1/(z1*z2)
	fp2mul_mont(t2, z2, t3); // t3 = 1/z1
	fp2mul_mont(t2, z1, z2); // z2 = 1/z2
	fp2mul_mont(t0, t1, z3); // z3 = 1/z3
	fp2copy(t3, z1);         // z1 = 1/z1
}

void get_A(const f2elm_t *xP, const f2elm_t *xQ, const f2elm_t *xR, f2elm_t *A) { // Given the x-coordinates of P, Q, and R, returns the value A corresponding to the Montgomery curve E_A: y^2=x^3+A*x^2+x such that R=Q-P on E_A.
	                                                                                 // Input:  the x-coordinates xP, xQ, and xR of the points P, Q and R.
	                                                                                 // Output: the coefficient A corresponding to the curve E_A: y^2=x^3+A*x^2+x.
    f2elm_t _t0, _t1, one = {0};
    f2elm_t *t0=&_t0, *t1=&_t1;

	fpcopy((const digit_t *) &Montgomery_one, one.e[0]);
	fp2add(xP, xQ, t1);      // t1 = xP+xQ
	fp2mul_mont(xP, xQ, t0); // t0 = xP*xQ
	fp2mul_mont(xR, t1, A);  // A = xR*t1
	fp2add(t0, A, A);        // A = A+t0
	fp2mul_mont(t0, xR, t0); // t0 = t0*xR
	fp2sub(A, &one, A);       // A = A-1
	fp2add(t0, t0, t0);      // t0 = t0+t0
	fp2add(t1, xR, t1);      // t1 = t1+xR
	fp2add(t0, t0, t0);      // t0 = t0+t0
	fp2sqr_mont(A, A);       // A = A^2
	fp2inv_mont(t0);         // t0 = 1/t0
	fp2mul_mont(A, t0, A);   // A = A*t0
	fp2sub(A, t1, A);        // Afinal = A-t1
}

void j_inv(const f2elm_t *A, const f2elm_t *C, f2elm_t *jinv) { // Computes the j-invariant of a Montgomery curve with projective constant.
	                                                                // Input: A,C in GF(p^2).
	                                                                // Output: j=256*(A^2-3*C^2)^3/(C^4*(A^2-4*C^2)), which is the j-invariant of the Montgomery curve B*y^2=x^3+(A/C)*x^2+x or (equivalently) j-invariant of B'*y^2=C*x^3+A*x^2+C*x.
    f2elm_t _t0, _t1;
    f2elm_t *t0=&_t0, *t1=&_t1;

	fp2sqr_mont(A, jinv);        // jinv = A^2
	fp2sqr_mont(C, t1);          // t1 = C^2
	fp2add(t1, t1, t0);          // t0 = t1+t1
	fp2sub(jinv, t0, t0);        // t0 = jinv-t0
	fp2sub(t0, t1, t0);          // t0 = t0-t1
	fp2sub(t0, t1, jinv);        // jinv = t0-t1
	fp2sqr_mont(t1, t1);         // t1 = t1^2
	fp2mul_mont(jinv, t1, jinv); // jinv = jinv*t1
	fp2add(t0, t0, t0);          // t0 = t0+t0
	fp2add(t0, t0, t0);          // t0 = t0+t0
	fp2sqr_mont(t0, t1);         // t1 = t0^2
	fp2mul_mont(t0, t1, t0);     // t0 = t0*t1
	fp2add(t0, t0, t0);          // t0 = t0+t0
	fp2add(t0, t0, t0);          // t0 = t0+t0
	fp2inv_mont(jinv);           // jinv = 1/jinv
	fp2mul_mont(jinv, t0, jinv); // jinv = t0*jinv
}

void xDBLADD(point_proj_t P, point_proj_t Q, const f2elm_t *xPQ, const f2elm_t *A24) { // Simultaneous doubling and differential addition.
	                                                                                        // Input: projective Montgomery points P=(XP:ZP) and Q=(XQ:ZQ) such that xP=XP/ZP and xQ=XQ/ZQ, affine difference xPQ=x(P-Q) and Montgomery curve constant A24=(A+2)/4.
	                                                                                        // Output: projective Montgomery points P <- 2*P = (X2P:Z2P) such that x(2P)=X2P/Z2P, and Q <- P+Q = (XQP:ZQP) such that = x(Q+P)=XQP/ZQP.
    f2elm_t _t0, _t1, _t2;
    f2elm_t *t0=&_t0, *t1=&_t1, *t2=&_t2;

	fp2add(&P->X, &P->Z, t0); // t0 = XP+ZP
	fp2sub(&P->X, &P->Z, t1); // t1 = XP-ZP
	fp2sqr_mont(t0, &P->X);  // XP = (XP+ZP)^2
	fp2sub(&Q->X, &Q->Z, t2); // t2 = XQ-ZQ
	fp2correction(t2);
	fp2add(&Q->X, &Q->Z, &Q->X);      // XQ = XQ+ZQ
	fp2mul_mont(t0, t2, t0);       // t0 = (XP+ZP)*(XQ-ZQ)
	fp2sqr_mont(t1, &P->Z);         // ZP = (XP-ZP)^2
	fp2mul_mont(t1, &Q->X, t1);     // t1 = (XP-ZP)*(XQ+ZQ)
	fp2sub(&P->X, &P->Z, t2);        // t2 = (XP+ZP)^2-(XP-ZP)^2
	fp2mul_mont(&P->X, &P->Z, &P->X); // XP = (XP+ZP)^2*(XP-ZP)^2
	fp2mul_mont(t2, A24, &Q->X);    // XQ = A24*[(XP+ZP)^2-(XP-ZP)^2]
	fp2sub(t0, t1, &Q->Z);          // ZQ = (XP+ZP)*(XQ-ZQ)-(XP-ZP)*(XQ+ZQ)
	fp2add(&Q->X, &P->Z, &P->Z);      // ZP = A24*[(XP+ZP)^2-(XP-ZP)^2]+(XP-ZP)^2
	fp2add(t0, t1, &Q->X);          // XQ = (XP+ZP)*(XQ-ZQ)+(XP-ZP)*(XQ+ZQ)
	fp2mul_mont(&P->Z, t2, &P->Z);   // ZP = [A24*[(XP+ZP)^2-(XP-ZP)^2]+(XP-ZP)^2]*[(XP+ZP)^2-(XP-ZP)^2]
	fp2sqr_mont(&Q->Z, &Q->Z);       // ZQ = [(XP+ZP)*(XQ-ZQ)-(XP-ZP)*(XQ+ZQ)]^2
	fp2sqr_mont(&Q->X, &Q->X);       // XQ = [(XP+ZP)*(XQ-ZQ)+(XP-ZP)*(XQ+ZQ)]^2
	fp2mul_mont(&Q->Z, xPQ, &Q->Z);  // ZQ = xPQ*[(XP+ZP)*(XQ-ZQ)-(XP-ZP)*(XQ+ZQ)]^2
}

static void swap_points(point_proj_t P, point_proj_t Q, const digit_t option) { // Swap points.
	                                                                            // If option = 0 then P <- P and Q <- Q, else if option = 0xFF...FF then P <- Q and Q <- P
	digit_t temp;
	unsigned int i;

	for (i = 0; i < NWORDS_FIELD; i++) {
		temp = option & (P->X.e[0][i] ^ Q->X.e[0][i]);
		P->X.e[0][i] = temp ^ P->X.e[0][i];
		Q->X.e[0][i] = temp ^ Q->X.e[0][i];
		temp = option & (P->Z.e[0][i] ^ Q->Z.e[0][i]);
		P->Z.e[0][i] = temp ^ P->Z.e[0][i];
		Q->Z.e[0][i] = temp ^ Q->Z.e[0][i];
		temp = option & (P->X.e[1][i] ^ Q->X.e[1][i]);
		P->X.e[1][i] = temp ^ P->X.e[1][i];
		Q->X.e[1][i] = temp ^ Q->X.e[1][i];
		temp = option & (P->Z.e[1][i] ^ Q->Z.e[1][i]);
		P->Z.e[1][i] = temp ^ P->Z.e[1][i];
		Q->Z.e[1][i] = temp ^ Q->Z.e[1][i];
	}
}

void LADDER3PT(const f2elm_t *xP, const f2elm_t *xQ, const f2elm_t *xPQ, const digit_t *m, const unsigned int AliceOrBob, point_proj_t R, const f2elm_t *A) {
	point_proj_t R0 = {0}, R2 = {0};
    f2elm_t _A24 = {0};
    f2elm_t *A24=&_A24;
	digit_t mask;
	int i, nbits, bit, swap, prevbit = 0;

	if (AliceOrBob == ALICE) {
		nbits = OALICE_BITS;
	} else {
		nbits = OBOB_BITS - 1;
	}

	// Initializing constant
	fpcopy((const digit_t *) &Montgomery_one, A24->e[0]);
	fp2add(A24, A24, A24);
	fp2add(A, A24, A24);
	fp2div2(A24, A24);
	fp2div2(A24, A24); // A24 = (A+2)/4

	// Initializing points
	fp2copy(xQ, &R0->X);
	fpcopy((const digit_t *) &Montgomery_one, (digit_t *) R0->Z.e);
	fp2copy(xPQ, &R2->X);
	fpcopy((const digit_t *) &Montgomery_one, (digit_t *) R2->Z.e);
	fp2copy(xP, &R->X);
	fpcopy((const digit_t *) &Montgomery_one, (digit_t *) R->Z.e);
	fpzero((digit_t *) (R->Z.e)[1]);

	// Main loop
	for (i = 0; i < nbits; i++) {
		bit = (m[i >> LOG2RADIX] >> (i & (RADIX - 1))) & 1;
		swap = bit ^ prevbit;
		prevbit = bit;
		mask = 0 - (digit_t) swap;

		swap_points(R, R2, mask);
		xDBLADD(R0, R2, &R->X, A24);
		fp2mul_mont(&R2->X, &R->Z, &R2->X);
	}
	swap = 0 ^ prevbit;
	mask = 0 - (digit_t) swap;
	swap_points(R, R2, mask);
}