Use constant for 180/π in to_degrees

The current `f32|f64.to_degrees` implementation uses a division to calculate 180/π, which causes a loss of precision. Using a constant is still not perfect (implementing a maximally-precise algorithm would come with a high performance cost), but improves precision with a minimal change.

diff --git a/src/libcore/num/f32.rs b/src/libcore/num/f32.rs
index 207df84d080f2856d5407a15181443f288b9a8c6..3586fa5442fb44a1712882cbc8864570fa5ea04a 100644 (file)
--- a/src/libcore/num/f32.rs
+++ b/src/libcore/num/f32.rs
@@ -239,7 +239,9 @@ fn powi(self, n: i32) -> f32 {
     /// Converts to degrees, assuming the number is in radians.
     #[inline]
     fn to_degrees(self) -> f32 {
-        self * (180.0f32 / consts::PI)
+        // Use a constant for better precision.
+        const PIS_IN_180: f32 = 57.2957795130823208767981548141051703_f32;
+        self * PIS_IN_180
     }
 
     /// Converts to radians, assuming the number is in degrees.

diff --git a/src/libcore/num/f64.rs b/src/libcore/num/f64.rs
index 9206132e8b46ff8be695108e516e5986a16cd6a0..64c0d508b388cf6b41b8a3df3e0567af55092822 100644 (file)
--- a/src/libcore/num/f64.rs
+++ b/src/libcore/num/f64.rs
@@ -237,6 +237,9 @@ fn powi(self, n: i32) -> f64 {
     /// Converts to degrees, assuming the number is in radians.
     #[inline]
     fn to_degrees(self) -> f64 {
+        // The division here is correctly rounded with respect to the true
+        // value of 180/π. (This differs from f32, where a constant must be
+        // used to ensure a correctly rounded result.)
         self * (180.0f64 / consts::PI)
     }
 

diff --git a/src/libstd/f32.rs b/src/libstd/f32.rs
index 9810dede61821e03c80c3405e7f32b2c28744146..ecf68f29d6f1f7c80c4324d1c71073b09a8a019c 100644 (file)
--- a/src/libstd/f32.rs
+++ b/src/libstd/f32.rs
@@ -1531,6 +1531,7 @@ fn test_to_degrees() {
         assert!(nan.to_degrees().is_nan());
         assert_eq!(inf.to_degrees(), inf);
         assert_eq!(neg_inf.to_degrees(), neg_inf);
+        assert_eq!(1_f32.to_degrees(), 57.2957795130823208767981548141051703);
     }
 
     #[test]