AK: Implement `acos<T>` correctly

This is a naive implementation based on the symmetry with `asin`.

Before, I'm not really sure what we were doing, but it was returning
wildly incorrect results.

2 files changed, 9 insertions, 1 deletions

diff --git a/AK/Math.h b/AK/Math.h
index 8424ec8f2f..d7aaf9ed11 100644
--- a/AK/Math.h
+++ b/AK/Math.h
@@ -250,7 +250,7 @@ constexpr T acos(T value)
     CONSTEXPR_STATE(acos, value);
 
     // FIXME: I am naive
-    return Pi<T> + asin(value);
+    return static_cast<T>(0.5) * Pi<T> - asin<T>(value);
 }
 
 template<FloatingPoint T>
diff --git a/Tests/LibM/test-math.cpp b/Tests/LibM/test-math.cpp
index 35bbab3bbe..6368aa293f 100644
--- a/Tests/LibM/test-math.cpp
+++ b/Tests/LibM/test-math.cpp
@@ -259,3 +259,11 @@ TEST_CASE(fmax_and_fmin)
     EXPECT(fmin(0, NAN) == 0);
     EXPECT(isnan(fmin(NAN, NAN)));
 }
+
+TEST_CASE(acos)
+{
+    EXPECT_APPROXIMATE(acos(-1), M_PI);
+    EXPECT_APPROXIMATE(acos(0), 0.5 * M_PI);
+    EXPECT_APPROXIMATE(acos(1), 0);
+    EXPECT(isnan(acos(1.1)));
+}