From 66bd7ee3a3d83306614d1096a06ec1e284b51680 Mon Sep 17 00:00:00 2001
From: Tavian Barnes <tavianator@tavianator.com>
Date: Sat, 27 Jun 2020 17:55:18 -0400
Subject: cos: Add a TryFrom<T> impl for AngularDistance<T>

---
 src/cos.rs | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/src/cos.rs b/src/cos.rs
index 99a08f4..e7f0a87 100644
--- a/src/cos.rs
+++ b/src/cos.rs
@@ -7,6 +7,7 @@ use num_traits::real::Real;
 use num_traits::{one, zero};
 
 use std::cmp::Ordering;
+use std::convert::TryFrom;
 
 /// Compute the [cosine *similarity*] between two points.
 ///
@@ -397,6 +398,7 @@ where
 ///
 ///     # use acap::distance::Distance;
 ///     # use acap::cos::AngularDistance;
+///     # use std::convert::TryFrom;
 ///     let zero = AngularDistance::from_cos(1.0);
 ///     let pi_2 = AngularDistance::from_cos(0.0);
 ///     let pi = AngularDistance::from_cos(-1.0);
@@ -425,8 +427,25 @@ impl<T: PartialOrd> PartialOrd for AngularDistance<T> {
     }
 }
 
+/// Error type for failed conversions from angles outside of `$[0, \pi]$` to [`AngularDistance`].
+#[derive(Debug)]
+pub struct InvalidAngleError;
+
 macro_rules! impl_distance {
-    ($f:ty) => {
+    ($f:ident) => {
+        impl TryFrom<$f> for AngularDistance<$f> {
+            type Error = InvalidAngleError;
+
+            #[inline]
+            fn try_from(value: $f) -> Result<Self, Self::Error> {
+                if value >= 0.0 && value <= std::$f::consts::PI {
+                    Ok(Self(value.cos()))
+                } else {
+                    Err(InvalidAngleError)
+                }
+            }
+        }
+
         impl From<AngularDistance<$f>> for $f {
             #[inline]
             fn from(value: AngularDistance<$f>) -> $f {
-- 
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