Add join operation

crates/fj-kernel/src/objects/full/cycle.rs

@@ -50,6 +50,16 @@ impl Cycle {
             .position(|edge| edge.id() == half_edge.id())
     }
 
+    /// Return the number of half-edges in the cycle
+    pub fn len(&self) -> usize {
+        self.half_edges.len()
+    }
+
+    /// Indicate whether the cycle is empty
+    pub fn is_empty(&self) -> bool {
+        self.half_edges.is_empty()
+    }
+
     /// Indicate the cycle's winding, assuming a right-handed coordinate system
     ///
     /// Please note that this is not *the* winding of the cycle, only one of the

crates/fj-kernel/src/operations/build/shell.rs

@@ -2,12 +2,12 @@ use fj_math::Point;
 
 use crate::{
     objects::{Face, Objects, Shell},
-    operations::{Insert, UpdateCycle, UpdateFace, UpdateHalfEdge},
+    operations::{Insert, JoinCycle, UpdateFace},
     services::Service,
     storage::Handle,
 };
 
-use super::{BuildFace, Polygon};
+use super::BuildFace;
 
 /// Build a [`Shell`]
 pub trait BuildShell {
@@ -35,78 +35,34 @@ pub trait BuildShell {
     ) -> Tetrahedron {
         let [a, b, c, d] = points.map(Into::into);
 
-        let [Polygon {
-            face: face_abc,
-            edges: [ab, bc, ca],
-            vertices: [a, b, c],
-        }, Polygon {
-            face: face_bad,
-            edges: [ba, ad, db],
-            vertices: [_, _, d],
-        }, Polygon {
-            face: face_dac,
-            edges: [da, ac, cd],
-            ..
-        }, Polygon {
-            face: face_cbd,
-            edges: [cb, bd, dc],
-            ..
-        }] = [
-            Face::triangle([a, b, c], objects),
-            Face::triangle([b, a, d], objects),
-            Face::triangle([d, a, c], objects),
-            Face::triangle([c, b, d], objects),
-        ];
-
-        let face_bad = face_bad.update_exterior(|cycle| {
-            let ba_joined = ba
-                .replace_start_vertex(b.clone())
-                .replace_global_form(ab.global_form().clone())
-                .insert(objects);
-            let ad_joined = ad.replace_start_vertex(a.clone()).insert(objects);
-
-            cycle
-                .replace_half_edge(&ba, ba_joined)
-                .replace_half_edge(&ad, ad_joined)
-                .insert(objects)
-        });
-        let face_dac = face_dac.update_exterior(|cycle| {
-            let da_joined = da
-                .replace_start_vertex(d.clone())
-                .replace_global_form(ad.global_form().clone())
-                .insert(objects);
-            let ac_joined = ac
-                .replace_start_vertex(a)
-                .replace_global_form(ca.global_form().clone())
-                .insert(objects);
-            let cd_joined = cd.replace_start_vertex(c.clone()).insert(objects);
-
-            cycle
-                .replace_half_edge(&da, da_joined)
-                .replace_half_edge(&ac, ac_joined)
-                .replace_half_edge(&cd, cd_joined)
-                .insert(objects)
-        });
-        let face_cbd = face_cbd.update_exterior(|cycle| {
-            let cb_joined = cb
-                .replace_start_vertex(c)
-                .replace_global_form(bc.global_form().clone())
-                .insert(objects);
-            let bd_joined = bd
-                .replace_start_vertex(b)
-                .replace_global_form(db.global_form().clone())
-                .insert(objects);
-            let dc_joined = dc
-                .replace_start_vertex(d)
-                .replace_global_form(cd.global_form().clone())
-                .insert(objects);
-
-            cycle
-                .replace_half_edge(&cb, cb_joined)
-                .replace_half_edge(&bd, bd_joined)
-                .replace_half_edge(&dc, dc_joined)
-                .insert(objects)
-        });
+        let face_abc = Face::triangle([a, b, c], objects).face;
+        let face_bad =
+            Face::triangle([b, a, d], objects)
+                .face
+                .update_exterior(|cycle| {
+                    cycle
+                        .join_to(face_abc.exterior(), 0..=0, 0..=0, objects)
+                        .insert(objects)
+                });
+        let face_dac =
+            Face::triangle([d, a, c], objects)
+                .face
+                .update_exterior(|cycle| {
+                    cycle
+                        .join_to(face_abc.exterior(), 1..=1, 2..=2, objects)
+                        .join_to(face_bad.exterior(), 0..=0, 1..=1, objects)
+                        .insert(objects)
+                });
+        let face_cbd =
+            Face::triangle([c, b, d], objects)
+                .face
+                .update_exterior(|cycle| {
+                    cycle
+                        .join_to(face_abc.exterior(), 0..=0, 1..=1, objects)
+                        .join_to(face_bad.exterior(), 1..=1, 2..=2, objects)
+                        .join_to(face_dac.exterior(), 2..=2, 2..=2, objects)
+                        .insert(objects)
+                });
 
         let faces = [face_abc, face_bad, face_dac, face_cbd]
             .map(|face| face.insert(objects));

crates/fj-kernel/src/operations/join/cycle.rs

@@ -0,0 +1,100 @@
+use std::ops::RangeInclusive;
+
+use crate::{
+    objects::{Cycle, Objects},
+    operations::{Insert, UpdateCycle, UpdateHalfEdge},
+    services::Service,
+};
+
+/// Join a [`Cycle`] to another
+pub trait JoinCycle {
+    /// Join the cycle to another
+    ///
+    /// Joins the cycle to the other at the provided ranges. The ranges specify
+    /// the indices of the half-edges that are joined together.
+    ///
+    /// A modulo operation is applied to all indices before use, so in a cycle
+    /// of 3 half-edges, indices `0` and `3` refer to the same half-edge. This
+    /// allows for specifying a range that crosses the "seam" of the cycle.
+    ///
+    /// # Panics
+    ///
+    /// Panics, if the ranges have different lengths.
+    ///
+    /// # Implementation Note
+    ///
+    /// The use of the `RangeInclusive` type might be a bit limiting, as other
+    /// range types might be more convenient in a given use case. This
+    /// implementation was chosen for now, as it wasn't clear whether the
+    /// additional complexity of using `RangeBounds` would be worth it.
+    ///
+    /// A solution based on `SliceIndex` could theoretically be used, but that
+    /// trait is partially unstable. In addition, it's not clear how it could be
+    /// used generically, as it could yield a range (which can be iterated over)
+    /// or a single item (which can not). This is not a hard problem in
+    /// principle (a single item could just be an iterator of length 1), but I
+    /// don't see it how to address this in Rust in a reasonable way.
+    ///
+    /// Maybe a custom trait that is implemented for `usize` and all range types
+    /// would be the best solution.
+    fn join_to(
+        &self,
+        other: &Cycle,
+        range: RangeInclusive<usize>,
+        other_range: RangeInclusive<usize>,
+        objects: &mut Service<Objects>,
+    ) -> Self;
+}
+
+impl JoinCycle for Cycle {
+    fn join_to(
+        &self,
+        other: &Cycle,
+        range: RangeInclusive<usize>,
+        range_other: RangeInclusive<usize>,
+        objects: &mut Service<Objects>,
+    ) -> Self {
+        assert_eq!(
+            range.end() - range.start(),
+            range_other.end() - range_other.start()
+        );
+
+        let mut cycle = self.clone();
+
+        for (index, index_other) in range.zip(range_other) {
+            let index = index % self.len();
+            let index_other = index_other % self.len();
+
+            let half_edge = self
+                .nth_half_edge(index)
+                .expect("Index must be valid, due to use of `%` above");
+            let half_edge_other = other
+                .nth_half_edge(index_other)
+                .expect("Index must be valid, due to use of `%` above");
+
+            let vertex_a = other
+                .half_edge_after(half_edge_other)
+                .expect("Expected other cycle to contain edge")
+                .start_vertex()
+                .clone();
+            let vertex_b = half_edge_other.start_vertex().clone();
+
+            let next_edge = cycle
+                .half_edge_after(half_edge)
+                .expect("Expected this cycle to contain edge");
+
+            let this_joined = half_edge
+                .replace_start_vertex(vertex_a)
+                .replace_global_form(half_edge_other.global_form().clone())
+                .insert(objects);
+            let next_joined =
+                next_edge.replace_start_vertex(vertex_b).insert(objects);
+
+            cycle = cycle
+                .replace_half_edge(half_edge, this_joined)
+                .replace_half_edge(next_edge, next_joined)
+        }
+
+        cycle
+    }
+}

crates/fj-kernel/src/operations/join/mod.rs

@@ -0,0 +1,3 @@
+mod cycle;
+
+pub use self::cycle::JoinCycle;

crates/fj-kernel/src/operations/mod.rs

@@ -2,6 +2,7 @@
 
 mod build;
 mod insert;
+mod join;
 mod update;
 
 pub use self::{
@@ -10,5 +11,6 @@ pub use self::{
         Polygon, Tetrahedron,
     },
     insert::Insert,
+    join::JoinCycle,
     update::{UpdateCycle, UpdateFace, UpdateHalfEdge, UpdateShell},
 };