From: Mark <markjordanthom@gmail.com>
Date: Sun, 29 Oct 2023 00:13:42 +0000 (-0600)
Subject: fix signed modulus bug (#2134)
X-Git-Tag: remove~3
X-Git-Url: https://git.sagredo.dev/?a=commitdiff_plain;h=ae7cf15c7044fc3295aef9ad9d17433169691955;p=scryer-prolog.git

fix signed modulus bug (#2134)
---

diff --git a/src/machine/arithmetic_ops.rs b/src/machine/arithmetic_ops.rs
index 92e9c599..20c503a9 100644
--- a/src/machine/arithmetic_ops.rs
+++ b/src/machine/arithmetic_ops.rs
@@ -1,4 +1,4 @@
-use dashu::base::{Abs, Gcd, UnsignedAbs};
+use dashu::base::{Abs, Gcd, Signed, UnsignedAbs};
 use dashu::integer::IBig;
 use dashu::integer::fast_div::ConstDivisor;
 use divrem::*;
@@ -852,7 +852,18 @@ pub(crate) fn modulus(x: Number, y: Number, arena: &mut Arena) -> Result<Number,
     fn ibig_rem_floor(n1: &Integer, n2: &Integer) -> Integer {
         let ring = ConstDivisor::new(n2.unsigned_abs());
         let n1 = n1.clone();
-        IBig::from(ring.reduce(n1).residue())
+
+        if n2.is_negative() {
+            let unsigned_result = IBig::from(ring.reduce(n1).residue());
+
+            if unsigned_result.is_zero() {
+                unsigned_result
+            } else {
+                unsigned_result + n2
+            }
+        } else {
+            IBig::from(ring.reduce(n1).residue())
+        }
     }
 
     match (x, y) {