From af5bbbb55db93cd815afe232bb7d588069d80edd Mon Sep 17 00:00:00 2001
From: Jean-Philippe Bossuat <jeanphilippe.bossuat@gmail.com>
Date: Tue, 8 Jul 2025 13:48:51 +0200
Subject: [PATCH] fixed modulus switching rounding

---
 core/src/blind_rotation/ccgi.rs | 20 ++------------------
 1 file changed, 2 insertions(+), 18 deletions(-)

diff --git a/core/src/blind_rotation/ccgi.rs b/core/src/blind_rotation/ccgi.rs
index 0d4f6fd..62ec4ac 100644
--- a/core/src/blind_rotation/ccgi.rs
+++ b/core/src/blind_rotation/ccgi.rs
@@ -438,7 +438,7 @@ pub(crate) fn negate_and_mod_switch_2n(n: usize, res: &mut [i64], lwe: &LWECiphe
     if basek > log2n {
         let diff: usize = basek - log2n;
         res.iter_mut().for_each(|x| {
-            *x = div_ceil_signed_by_pow2(x, diff);
+            *x = div_round_by_pow2(x, diff);
         })
     } else {
         let rem: usize = basek - (log2n % basek);
@@ -460,21 +460,5 @@ pub(crate) fn negate_and_mod_switch_2n(n: usize, res: &mut [i64], lwe: &LWECiphe
 
 #[inline(always)]
 fn div_round_by_pow2(x: &i64, k: usize) -> i64 {
-    if x >= &0 {
-        (x + (1 << (k - 1))) >> k
-    } else {
-        (x + (-1 << (k - 1))) >> k
-    }
-}
-
-// #[inline(always)]
-// fn div_floor_signed_by_pow2(x: &i64, k: usize) -> i64{
-// let bias: i64 = (1 << k) - 1;
-// (x + ((x >> 63) & bias)) >> k
-// }
-
-#[inline(always)]
-fn div_ceil_signed_by_pow2(x: &i64, k: usize) -> i64 {
-    let bias: i64 = (1 << k) - 1;
-    (x + ((x >> 63) & bias)) >> k
+    (x + (1 << (k - 1))) >> k
 }