From f3e1b51fec9d050481cd3740a86595ad32901a43 Mon Sep 17 00:00:00 2001
From: Tony Edgin <tony.edgin@gmail.com>
Date: Wed, 6 Nov 2024 14:24:52 -0700
Subject: [PATCH] Add support for rem_euclid for Signed types

---
 src/system.rs       | 19 +++++++++++++++++++
 src/tests/system.rs | 20 ++++++++++++++++++++
 2 files changed, 39 insertions(+)

diff --git a/src/system.rs b/src/system.rs
index 7da3cead..04a7546f 100644
--- a/src/system.rs
+++ b/src/system.rs
@@ -754,6 +754,25 @@ macro_rules! system {
                     value: self.value.min(other.value),
                 }
             }
+
+            /// Calculates the Euclidean remainder (least nonnegative remainder) of a quantity
+            /// modulo `modulus`. The return value should be in the interval [0, |modulus|)`, but
+            /// rarely, due to rounding error resulting from finite floating point precision, the
+            /// return value can be `|modulus|`.
+            #[must_use = "method returns a new number and does not mutate the original value"]
+            #[inline(always)]
+            pub fn rem_euclid(self, modulus: Self) -> Self
+            where
+                V: $crate::num::Signed,
+            {
+                // The Signed trait does not provide rem_euclid, so it must be implemented
+                let rem = self.value % modulus.value.abs();
+                Quantity {
+                    dimension: $crate::lib::marker::PhantomData,
+                    units: $crate::lib::marker::PhantomData,
+                    value: if rem.is_negative() { rem + modulus.value.abs() } else { rem },
+                }
+            }
         }
 
         // Explicitly definte floating point methods for float and complex storage types.
diff --git a/src/tests/system.rs b/src/tests/system.rs
index 99e48491..7ecd31f9 100644
--- a/src/tests/system.rs
+++ b/src/tests/system.rs
@@ -401,6 +401,26 @@ mod signed {
                 Test::eq(&Length::new::<meter>(-(*l).clone()),
                     &-Length::new::<meter>((*l).clone()))
             }
+
+            #[allow(trivial_casts)]
+            fn rem_euclid(v: A<V>) -> bool {
+                let two = V::one() + V::one();
+
+                // NB: The builtin v.rem_euclid() will sometimes return a finite result when v is
+                // infinite. The result should be undefined (NaN).
+                let exp_res = if v.is_infinite() { V::nan() } else { v.rem_euclid(two) };
+
+                let act_res = Length::new::<meter>(*v).rem_euclid(Length::new::<meter>(two)).value;
+
+                // Finite precision floating point can result in exp_res being -0.0. This messes up
+                // test comparison, since the implementation being tested will map a -0.0 to the
+                // modulus.
+                if exp_res.is_sign_negative() {
+                    Test::eq(&two, &act_res)
+                } else {
+                    Test::eq(&exp_res, &act_res)
+                }
+            }
         }
     }
 }