From a1ab7617053ff68d8d34fa43829b5ab66807ef27 Mon Sep 17 00:00:00 2001
From: Hanno Braun <hanno@braun-odw.eu>
Date: Fri, 1 Apr 2022 18:47:45 +0200
Subject: [PATCH] Add `CycleBuilder`

---
 fj-kernel/src/topology/builder.rs | 41 ++++++++++++++++++++++++++++++-
 fj-kernel/src/topology/edges.rs   |  7 +++++-
 2 files changed, 46 insertions(+), 2 deletions(-)

diff --git a/fj-kernel/src/topology/builder.rs b/fj-kernel/src/topology/builder.rs
index 9e5949974..55ea4e906 100644
--- a/fj-kernel/src/topology/builder.rs
+++ b/fj-kernel/src/topology/builder.rs
@@ -5,7 +5,7 @@ use crate::{
     shape::{Handle, Shape, ValidationResult},
 };
 
-use super::{Edge, Vertex};
+use super::{Cycle, Edge, Vertex};
 
 /// API for building a [`Vertex`]
 pub struct VertexBuilder<'r> {
@@ -93,3 +93,42 @@ impl<'r> EdgeBuilder<'r> {
         Ok(edge)
     }
 }
+
+/// API for building a [`Cycle`]
+pub struct CycleBuilder<'r> {
+    shape: &'r mut Shape,
+}
+
+impl<'r> CycleBuilder<'r> {
+    /// Construct a new instance of `CycleBuilder`
+    pub fn new(shape: &'r mut Shape) -> Self {
+        Self { shape }
+    }
+
+    /// Build a polygon from a list of points
+    pub fn polygon(
+        self,
+        points: impl IntoIterator<Item = impl Into<Point<3>>>,
+    ) -> ValidationResult<Cycle> {
+        // A polygon is closed, so we need to add the first point at the end
+        // again, for the next step.
+        let mut points: Vec<_> = points.into_iter().map(Into::into).collect();
+        if let Some(point) = points.first().cloned() {
+            points.push(point);
+        }
+
+        let mut edges = Vec::new();
+        for ab in points.windows(2) {
+            // Can't panic, as we passed `2` to `windows`.
+            //
+            // Can be cleaned up, once `array_windows` is stable.
+            let points = [ab[0], ab[1]];
+
+            let edge =
+                Edge::build(self.shape).line_segment_from_points(points)?;
+            edges.push(edge);
+        }
+
+        self.shape.insert(Cycle { edges })
+    }
+}
diff --git a/fj-kernel/src/topology/edges.rs b/fj-kernel/src/topology/edges.rs
index 7871481ba..cf5aad58c 100644
--- a/fj-kernel/src/topology/edges.rs
+++ b/fj-kernel/src/topology/edges.rs
@@ -5,7 +5,7 @@ use crate::{
     shape::{Handle, Shape},
 };
 
-use super::{vertices::Vertex, EdgeBuilder};
+use super::{builder::CycleBuilder, vertices::Vertex, EdgeBuilder};
 
 /// A cycle of connected edges
 ///
@@ -29,6 +29,11 @@ pub struct Cycle {
 }
 
 impl Cycle {
+    /// Build a cycle using the [`CycleBuilder`] API
+    pub fn build(shape: &mut Shape) -> CycleBuilder {
+        CycleBuilder::new(shape)
+    }
+
     /// Access the edges that this cycle refers to
     ///
     /// This is a convenience method that saves the caller from dealing with the