From 74049e039ee278a97984ffdaa8d1329cdced369e Mon Sep 17 00:00:00 2001
From: Henry de Valence <hdevalence@hdevalence.ca>
Date: Fri, 6 Jan 2017 12:34:57 -0500
Subject: [PATCH] Make FieldElement's conditional_assign a trait impl

---
 src/field.rs | 73 ++++++++++++++++++++++++++++------------------------
 1 file changed, 39 insertions(+), 34 deletions(-)

diff --git a/src/field.rs b/src/field.rs
index 5b7885f..4b9758c 100644
--- a/src/field.rs
+++ b/src/field.rs
@@ -25,6 +25,7 @@ use std::cmp::{Eq, PartialEq};
 use std::ops::Neg;
 
 use util::byte_is_nonzero;
+use util::CTAssignable;
 
 /// FieldElements are represented as an array of ten "Limbs", which are radix
 /// 25.5, that is, each Limb of a FieldElement alternates between being
@@ -142,6 +143,44 @@ impl<'a> Neg for &'a FieldElement {
     }
 }
 
+impl CTAssignable for FieldElement {
+    /// Conditionally assign another FieldElement to this one.
+    ///
+    /// If `choice == 0`, replace `self` with `self`:
+    ///
+    /// ```
+    /// # use curve25519_dalek::field::FieldElement;
+    /// # use curve25519_dalek::util::CTAssignable;
+    /// let f     = FieldElement([1,1,1,1,1,1,1,1,1,1]);
+    /// let g     = FieldElement([2,2,2,2,2,2,2,2,2,2]);
+    /// let mut h = FieldElement([1,1,1,1,1,1,1,1,1,1]);
+    /// h.conditional_assign(&g, 0);
+    /// assert!(h == f);
+    /// ```
+    ///
+    /// If `choice == 1`, replace `self` with `f`:
+    ///
+    /// ```
+    /// # use curve25519_dalek::field::FieldElement;
+    /// # use curve25519_dalek::util::CTAssignable;
+    /// # let f     = FieldElement([1,1,1,1,1,1,1,1,1,1]);
+    /// # let g     = FieldElement([2,2,2,2,2,2,2,2,2,2]);
+    /// # let mut h = FieldElement([1,1,1,1,1,1,1,1,1,1]);
+    /// h.conditional_assign(&g, 1);
+    /// assert!(h == g);
+    /// ```
+    ///
+    /// # Preconditions
+    ///
+    /// * `choice` in {0,1}
+    fn conditional_assign(&mut self, f: &FieldElement, choice: u8) {
+        let mask = -(choice as Limb);
+        for i in 0..10 {
+            self[i] ^= mask & (self[i] ^ f[i]);
+        }
+    }
+}
+
 /// Convert an array of (at least) three bytes into an i64.
 #[inline]
 #[allow(dead_code)]
@@ -179,40 +218,6 @@ impl FieldElement {
         FieldElement([ 1, 0, 0, 0, 0, 0, 0, 0, 0, 0 ])
     }
 
-    /// Conditionally assign another FieldElement to this one.
-    ///
-    /// If `choice == 0`, replace `self` with `self`:
-    ///
-    /// ```
-    /// # use curve25519_dalek::field::FieldElement;
-    /// let f     = FieldElement([1,1,1,1,1,1,1,1,1,1]);
-    /// let g     = FieldElement([2,2,2,2,2,2,2,2,2,2]);
-    /// let mut h = FieldElement([1,1,1,1,1,1,1,1,1,1]);
-    /// h.conditional_assign(&g, 0);
-    /// assert!(h == f);
-    /// ```
-    ///
-    /// If `choice == 1`, replace `self` with `f`:
-    ///
-    /// ```
-    /// # use curve25519_dalek::field::FieldElement;
-    /// # let f     = FieldElement([1,1,1,1,1,1,1,1,1,1]);
-    /// # let g     = FieldElement([2,2,2,2,2,2,2,2,2,2]);
-    /// # let mut h = FieldElement([1,1,1,1,1,1,1,1,1,1]);
-    /// h.conditional_assign(&g, 1);
-    /// assert!(h == g);
-    /// ```
-    ///
-    /// # Preconditions
-    ///
-    /// * `choice` in {0,1}
-    pub fn conditional_assign(&mut self, f: &FieldElement, choice: u8) {
-        let mask = -(choice as Limb);
-        for i in 0..10 {
-            self[i] ^= mask & (self[i] ^ f[i]);
-        }
-    }
-
     fn combine_coeffs(input: &[i64;10]) -> FieldElement { //FeCombine
         let mut c = [0i64;10];
         let mut h = input.clone();