From 539caf7ad9cb2ca1456a60def745a46eb0688695 Mon Sep 17 00:00:00 2001
From: Corwin <corwin@kuiper.dev>
Date: Sun, 8 Oct 2023 14:22:50 +0100
Subject: [PATCH 1/5] add test showing inaccuracy

---
 agb-fixnum/src/lib.rs | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/agb-fixnum/src/lib.rs b/agb-fixnum/src/lib.rs
index 3ba3a47f..ba20b183 100644
--- a/agb-fixnum/src/lib.rs
+++ b/agb-fixnum/src/lib.rs
@@ -1234,6 +1234,12 @@ mod tests {
         test_base::<11>();
     }
 
+    #[test]
+    fn check_cos_accuracy() {
+        let n: Num<i32, 8> = Num::new(1) / 32;
+        assert_eq!(n.cos(), num!(0.9808));
+    }
+
     #[test]
     fn test_numbers() {
         // test addition

From 548dd9ff673b4f4a119a92f4127ac47e3827aca6 Mon Sep 17 00:00:00 2001
From: Corwin <corwin@kuiper.dev>
Date: Sun, 8 Oct 2023 14:23:04 +0100
Subject: [PATCH 2/5] fix cos function

---
 agb-fixnum/src/lib.rs | 13 +++----------
 1 file changed, 3 insertions(+), 10 deletions(-)

diff --git a/agb-fixnum/src/lib.rs b/agb-fixnum/src/lib.rs
index ba20b183..19e5a704 100644
--- a/agb-fixnum/src/lib.rs
+++ b/agb-fixnum/src/lib.rs
@@ -533,17 +533,10 @@ impl<I: FixedWidthSignedInteger, const N: usize> Num<I, N> {
     /// ```
     #[must_use]
     pub fn cos(self) -> Self {
-        let one: Self = I::one().into();
         let mut x = self;
-        let four: I = 4.into();
-        let two: I = 2.into();
-        let sixteen: I = 16.into();
-        let nine: I = 9.into();
-        let forty: I = 40.into();
-
-        x -= one / four + (x + one / four).floor();
-        x *= (x.abs() - one / two) * sixteen;
-        x += x * (x.abs() - one) * (nine / forty);
+        x -= num!(0.25) + (x + num!(0.25)).floor();
+        x *= (x.abs() - num!(0.5)) * num!(16.);
+        x += x * (x.abs() - num!(1.)) * num!(0.225);
         x
     }
 

From e894367c52b8629332d84665149076825b766f0b Mon Sep 17 00:00:00 2001
From: Corwin <corwin@kuiper.dev>
Date: Sun, 8 Oct 2023 14:56:32 +0100
Subject: [PATCH 3/5] use proper implementation of cos to check against

---
 agb-fixnum/src/lib.rs | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/agb-fixnum/src/lib.rs b/agb-fixnum/src/lib.rs
index 19e5a704..ea8d4adf 100644
--- a/agb-fixnum/src/lib.rs
+++ b/agb-fixnum/src/lib.rs
@@ -1230,7 +1230,10 @@ mod tests {
     #[test]
     fn check_cos_accuracy() {
         let n: Num<i32, 8> = Num::new(1) / 32;
-        assert_eq!(n.cos(), num!(0.9808));
+        assert_eq!(
+            n.cos(),
+            Num::from_f64((2. * core::f64::consts::PI / 32.).cos())
+        );
     }
 
     #[test]

From 71e680f3655d511494013aeac9798e520b5834d6 Mon Sep 17 00:00:00 2001
From: Corwin <corwin@kuiper.dev>
Date: Sun, 8 Oct 2023 15:04:57 +0100
Subject: [PATCH 4/5] update changelog

---
 CHANGELOG.md | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6b03400b..90bbd777 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- Fixed inaccuracy in cosine implementation caused by accidentally multiplying correction term by zero.
+
 ## [0.17.1] - 2023/10/05
 
 ### Fixed

From 09331d1cb5870fe2a2f8d12389439d372d4bccb4 Mon Sep 17 00:00:00 2001
From: Corwin <corwin@kuiper.dev>
Date: Mon, 9 Oct 2023 19:16:24 +0100
Subject: [PATCH 5/5] make 16 bit precision work

---
 agb-fixnum/src/lib.rs | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/agb-fixnum/src/lib.rs b/agb-fixnum/src/lib.rs
index ea8d4adf..8b0d1b0c 100644
--- a/agb-fixnum/src/lib.rs
+++ b/agb-fixnum/src/lib.rs
@@ -133,7 +133,7 @@ macro_rules! upcast_multiply_impl {
                         .wrapping_mul(b_frac)
                         .wrapping_add(b_floor.wrapping_mul(a_frac)),
                 )
-                .wrapping_add(a_frac.wrapping_mul(b_frac) >> n)
+                .wrapping_add(((a_frac as u32).wrapping_mul(b_frac as u32) >> n) as $T)
         }
     };
     ($T: ty, $Upcast: ty) => {
@@ -1236,6 +1236,17 @@ mod tests {
         );
     }
 
+    #[test]
+    fn check_16_bit_precision_i32() {
+        let a: Num<i32, 16> = num!(1.923);
+        let b = num!(2.723);
+
+        assert_eq!(
+            a * b,
+            Num::from_raw(((a.to_raw() as i64 * b.to_raw() as i64) >> 16) as i32)
+        )
+    }
+
     #[test]
     fn test_numbers() {
         // test addition