From 08694593420d25c6ed0318572a5588fe3d2282a3 Mon Sep 17 00:00:00 2001
From: Peter Dettman <peter.dettman@gmail.com>
Date: Wed, 22 Dec 2021 14:20:01 +0700
Subject: [PATCH] Update comments in _gej_add_ge

---
 src/group_impl.h | 26 +++++++++++++-------------
 1 file changed, 13 insertions(+), 13 deletions(-)

diff --git a/src/group_impl.h b/src/group_impl.h
index e5db70fd3f..c9cc5fcbf0 100644
--- a/src/group_impl.h
+++ b/src/group_impl.h
@@ -493,7 +493,7 @@ static void secp256k1_gej_add_zinv_var(secp256k1_gej *r, const secp256k1_gej *a,
 
 
 static void secp256k1_gej_add_ge(secp256k1_gej *r, const secp256k1_gej *a, const secp256k1_ge *b) {
-    /* Operations: 7 mul, 5 sqr, 4 normalize, 21 mul_int/add/negate/cmov */
+    /* Operations: 7 mul, 5 sqr, 24 add/cmov/half/mul_int/negate/normalize_weak/normalizes_to_zero */
     static const secp256k1_fe fe_1 = SECP256K1_FE_CONST(0, 0, 0, 0, 0, 0, 0, 1);
     secp256k1_fe zz, u1, u2, s1, s2, t, tt, m, n, q, rr;
     secp256k1_fe m_alt, rr_alt;
@@ -517,9 +517,9 @@ static void secp256k1_gej_add_ge(secp256k1_gej *r, const secp256k1_gej *a, const
      *    M = S1+S2
      *    Q = T*M^2
      *    R = T^2-U1*U2
-     *    X3 = 4*(R^2-Q)
-     *    Y3 = 4*(R*(3*Q-2*R^2)-M^4)
-     *    Z3 = 2*M*Z
+     *    X3 = R^2-Q
+     *    Y3 = (R*(3*Q-2*R^2)-M^4)/2
+     *    Z3 = M*Z
      *  (Note that the paper uses xi = Xi / Zi and yi = Yi / Zi instead.)
      *
      *  This formula has the benefit of being the same for both addition
@@ -591,17 +591,17 @@ static void secp256k1_gej_add_ge(secp256k1_gej *r, const secp256k1_gej *a, const
     secp256k1_fe_sqr(&n, &n);
     secp256k1_fe_cmov(&n, &m, degenerate);              /* n = M^3 * Malt (2) */
     secp256k1_fe_sqr(&t, &rr_alt);                      /* t = Ralt^2 (1) */
-    secp256k1_fe_mul(&r->z, &a->z, &m_alt);             /* r->z = Malt*Z (1) */
+    secp256k1_fe_mul(&r->z, &a->z, &m_alt);             /* r->z = Z3 = Malt*Z (1) */
     infinity = secp256k1_fe_normalizes_to_zero(&r->z) & ~a->infinity;
     secp256k1_fe_negate(&q, &q, 1);                     /* q = -Q (2) */
-    secp256k1_fe_add(&t, &q);                           /* t = Ralt^2-Q (3) */
-    r->x = t;                                           /* r->x = Ralt^2-Q (3) */
-    secp256k1_fe_mul_int(&t, 2);                        /* t = 2*x3 (6) */
-    secp256k1_fe_add(&t, &q);                           /* t = 2*x3 - Q: (8) */
-    secp256k1_fe_mul(&t, &t, &rr_alt);                  /* t = Ralt*(2*x3 - Q) (1) */
-    secp256k1_fe_add(&t, &n);                           /* t = Ralt*(2*x3 - Q) + M^3*Malt (3) */
-    secp256k1_fe_negate(&r->y, &t, 3);                  /* r->y = Ralt*(Q - 2*x3) - M^3*Malt (4) */
-    secp256k1_fe_half(&r->y);                           /* r->y = (Ralt*(Q - 2*x3) - M^3*Malt)/2 (3) */
+    secp256k1_fe_add(&t, &q);                           /* t = Ralt^2 - Q (3) */
+    r->x = t;                                           /* r->x = X3 = Ralt^2 - Q (3) */
+    secp256k1_fe_mul_int(&t, 2);                        /* t = 2*X3 (6) */
+    secp256k1_fe_add(&t, &q);                           /* t = 2*X3 - Q (8) */
+    secp256k1_fe_mul(&t, &t, &rr_alt);                  /* t = Ralt*(2*X3 - Q) (1) */
+    secp256k1_fe_add(&t, &n);                           /* t = Ralt*(2*X3 - Q) + M^3*Malt (3) */
+    secp256k1_fe_negate(&r->y, &t, 3);                  /* r->y = Ralt*(Q - 2*X3) - M^3*Malt (4) */
+    secp256k1_fe_half(&r->y);                           /* r->y = Y3 = (Ralt*(Q - 2*X3) - M^3*Malt)/2 (3) */
 
     /** In case a->infinity == 1, replace r with (b->x, b->y, 1). */
     secp256k1_fe_cmov(&r->x, &b->x, a->infinity);