From 3eab4acbd8d3fb2fea1be36af154f84b0f4a0b61 Mon Sep 17 00:00:00 2001
From: Henry de Valence <hdevalence@hdevalence.ca>
Date: Sun, 12 Mar 2017 23:18:31 -0700
Subject: [PATCH] Add p constant for subtraction

---
 src/constants.rs |  3 +++
 src/field.rs     | 19 ++++++++++++++-----
 2 files changed, 17 insertions(+), 5 deletions(-)

diff --git a/src/constants.rs b/src/constants.rs
index 1dd931f..487d557 100644
--- a/src/constants.rs
+++ b/src/constants.rs
@@ -26,6 +26,9 @@ use curve::CompressedEdwardsY;
 use curve::EdwardsBasepointTable;
 use scalar::Scalar;
 
+#[cfg(feature="radix_51")]
+pub const p: FieldElement = FieldElement([2251799813685229, 2251799813685247, 2251799813685247, 2251799813685247, 2251799813685247]);
+
 #[cfg(feature="radix_25_5")]
 pub const d: FieldElement       = FieldElement([
     -10913610,  13857413, -15372611,   6949391,    114729,
diff --git a/src/field.rs b/src/field.rs
index b2bde0c..8b448c9 100644
--- a/src/field.rs
+++ b/src/field.rs
@@ -244,11 +244,11 @@ impl<'b> SubAssign<&'b FieldElement> for FieldElement {
     fn sub_assign(&mut self, _rhs: &'b FieldElement) {
         // To avoid underflow, first add p
         // XXX how many copies should we add to preserve headroom?
-        self.0[0] += 2251799813685229;
-        self.0[1] += 2251799813685247;
-        self.0[2] += 2251799813685247;
-        self.0[3] += 2251799813685247;
-        self.0[4] += 2251799813685247;
+        self.0[0] += constants::p.0[0];
+        self.0[1] += constants::p.0[1];
+        self.0[2] += constants::p.0[2];
+        self.0[3] += constants::p.0[3];
+        self.0[4] += constants::p.0[4];
         // then subtract _rhs
         self.0[0] -= _rhs.0[0];
         self.0[1] -= _rhs.0[1];
@@ -338,6 +338,15 @@ impl FieldElement {
             self[i] = -self[i];
         }
     }
+    #[cfg(feature="radix_51")]
+    pub fn negate(&mut self) {
+        // XXX how many copies of p
+        self.0[0] = constants::p.0[0] - self.0[0];
+        self.0[1] = constants::p.0[1] - self.0[1];
+        self.0[2] = constants::p.0[2] - self.0[2];
+        self.0[3] = constants::p.0[3] - self.0[3];
+        self.0[4] = constants::p.0[4] - self.0[4];
+    }
 
     /// Construct the additive identity
     #[cfg(feature="radix_25_5")]