From e34c31bf02eb0f0ff4dd43ae72e0eae53f2ac519 Mon Sep 17 00:00:00 2001
From: varkor <github@varkor.com>
Date: Thu, 1 Feb 2018 18:35:51 +0000
Subject: [PATCH] =?UTF-8?q?Use=20constant=20for=20180/=CF=80=20in=20to=5Fd?=
 =?UTF-8?q?egrees?=
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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.
---
 src/libcore/num/f32.rs | 4 +++-
 src/libcore/num/f64.rs | 3 +++
 src/libstd/f32.rs      | 1 +
 3 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/src/libcore/num/f32.rs b/src/libcore/num/f32.rs
index 207df84d080..3586fa5442f 100644
--- 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 9206132e8b4..64c0d508b38 100644
--- 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 9810dede618..ecf68f29d6f 100644
--- 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]
-- 
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