From 39b0274a92f119a763196cb38a74367cc0377639 Mon Sep 17 00:00:00 2001
From: Hanno Braun <hello@hannobraun.com>
Date: Thu, 6 Jun 2024 19:50:54 +0200
Subject: [PATCH] Build circle-cycle out of multiple arcs

See the updated documentation for motivation.
---
 crates/fj-core/src/operations/build/cycle.rs | 42 +++++++++++++++++---
 1 file changed, 37 insertions(+), 5 deletions(-)

diff --git a/crates/fj-core/src/operations/build/cycle.rs b/crates/fj-core/src/operations/build/cycle.rs
index ca5b47f61..b36a3eee6 100644
--- a/crates/fj-core/src/operations/build/cycle.rs
+++ b/crates/fj-core/src/operations/build/cycle.rs
@@ -1,8 +1,8 @@
-use fj_math::{Point, Scalar};
+use fj_math::{Point, Scalar, Vector};
 use itertools::Itertools;
 
 use crate::{
-    operations::{build::BuildHalfEdge, update::UpdateCycle},
+    operations::{build::BuildHalfEdge, insert::Insert, update::UpdateCycle},
     storage::Handle,
     topology::{Cycle, HalfEdge, Surface},
     Core,
@@ -19,15 +19,47 @@ pub trait BuildCycle {
         Cycle::new([])
     }
 
-    /// Build a circle
+    /// # Build a circle
+    ///
+    /// This circle is built out of 4 distinct arcs.
+    ///
+    /// ## Implementation Note
+    ///
+    /// The cycle can't be built out of a single half-edge. That would be
+    /// invalid although there's not validation check to document and enforce
+    /// that yet. Please refer to the following issue for more information:
+    /// <https://github.com/hannobraun/fornjot/issues/2374>
+    ///
+    /// The cycle is built out of 4 arcs specifically, because that's easier to
+    /// implement than three, and building it out of two creates geometry that
+    /// the cycle winding code can't handle right now. The following issue has
+    /// more information on the cycle winding problems:
+    /// <https://github.com/hannobraun/fornjot/issues/2130>
     fn circle(
         center: impl Into<Point<2>>,
         radius: impl Into<Scalar>,
         surface: Handle<Surface>,
         core: &mut Core,
     ) -> Cycle {
-        let circle = HalfEdge::circle(center, radius, surface, core);
-        Cycle::empty().add_half_edges([circle], core)
+        let center = center.into();
+        let radius = radius.into();
+
+        let radius_right = Vector::from([radius, Scalar::ZERO]);
+        let radius_up = Vector::from([Scalar::ZERO, radius]);
+
+        let a = center + radius_right;
+        let b = center + radius_up;
+        let c = center - radius_right;
+        let d = center - radius_up;
+
+        let angle = Scalar::TAU / 4.;
+
+        let ab = HalfEdge::arc(a, b, angle, surface.clone(), core).insert(core);
+        let bc = HalfEdge::arc(b, c, angle, surface.clone(), core).insert(core);
+        let cd = HalfEdge::arc(c, d, angle, surface.clone(), core).insert(core);
+        let da = HalfEdge::arc(d, a, angle, surface.clone(), core).insert(core);
+
+        Cycle::empty().add_half_edges([ab, bc, cd, da], core)
     }
 
     /// Build a polygon