From 653d8b2e260e8b51fe372ccf4095928dcd120c95 Mon Sep 17 00:00:00 2001
From: Pieter Wuille <pieter@wuille.net>
Date: Wed, 3 Apr 2024 10:50:22 -0400
Subject: [PATCH] Avoid overflowing shift by special casing inverse of 1

---
 src/int_utils.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/int_utils.h b/src/int_utils.h
index bd70c90..2b3d8cb 100644
--- a/src/int_utils.h
+++ b/src/int_utils.h
@@ -194,6 +194,7 @@ class BitsInt {
     }
 
     static constexpr inline bool IsZero(I a) { return a == 0; }
+    static constexpr inline bool IsOne(I a) { return a == 1; }
     static constexpr inline I Mask(I val) { return val & MASK; }
     static constexpr inline I Shift(I val, int bits) { return ((val << bits) & MASK); }
     static constexpr inline I UnsafeShift(I val, int bits) { return (val << bits); }
@@ -252,7 +253,7 @@ template<typename I, int N, typename L, typename F> inline constexpr I GFMul(con
 template<typename I, typename F, int BITS, uint32_t MOD>
 inline I InvExtGCD(I x)
 {
-    if (F::IsZero(x)) return x;
+    if (F::IsZero(x) || F::IsOne(x)) return x;
     I t(0), newt(1);
     I r(MOD), newr = x;
     int rlen = BITS + 1, newrlen = F::Bits(newr, BITS);