From fb08d9ff9e7264d91850a1042ab3d6db39044310 Mon Sep 17 00:00:00 2001
From: nicoo <nicoo@debian.org>
Date: Thu, 18 Jun 2020 13:01:55 +0200
Subject: [PATCH] factor::numeric::Montgomery::add: Deal with rare overflow
 case

---
 src/uu/factor/src/numeric.rs | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/src/uu/factor/src/numeric.rs b/src/uu/factor/src/numeric.rs
index c0f9d9f47..4e0cee072 100644
--- a/src/uu/factor/src/numeric.rs
+++ b/src/uu/factor/src/numeric.rs
@@ -123,7 +123,17 @@ impl Arithmetic for Montgomery {
     }
 
     fn add(&self, a: Self::I, b: Self::I) -> Self::I {
-        let r = a + b;
+        let (r, overflow) = a.overflowing_add(b);
+
+        // In case of overflow, a+b = 2⁶⁴ + r = (2⁶⁴ - n) + r (working mod n)
+        let r = if !overflow {
+            r
+        } else {
+            r + self.n.wrapping_neg()
+        };
+
+        // Normalise to [0; n[
+        let r = if r < self.n { r } else { r - self.n };
 
         // Check that r (reduced back to the usual representation) equals
         // a+b % n