Extend any surface: natural, tangent and curvature, up to a target (#760) (#1166)

Extend works on any surface: natural (continuing the face's own surface), tangent (G1 run-off) or curvature (G2 run-off), by a distance, up to a plane or face, or back to the surface's bounds, with neighbouring edges lengthened to meet. Untrim and Extend share face parameter-loop helpers. The new SurfaceExtend fields are noted under format 21. Closes #760.

crates/doc/src/document.rs

@@ -1137,11 +1137,20 @@ pub enum FeatureKind {
         #[serde(default, skip_serializing_if = "Option::is_none")]
         face: Option<Vec3>,
     },
-    /// A planar surface body extended outward by `distance` past the edges through `edges`.
+    /// A surface body's boundary edges through `edges` extended outward: by `distance`, or up
+    /// to `up_to`; the surface continued (`mode` natural) or a run-off face along each edge
+    /// (tangent, curvature). `neighbours`: the edges next to an extended edge are lengthened to
+    /// meet it (else the extension's sides run square to the edge).
     SurfaceExtend {
         body: String,
         edges: Vec<Vec3>,
         distance: String,
+        #[serde(default, skip_serializing_if = "ExtendMode::is_natural")]
+        mode: ExtendMode,
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        up_to: Option<ExtendTo>,
+        #[serde(default = "yes", skip_serializing_if = "is_yes")]
+        neighbours: bool,
     },
     /// A surface body trimmed by a plane, or by a face of a body (`tool`), keeping the side of
     /// `keep`.
@@ -1189,6 +1198,37 @@ pub enum FeatureKind {
     },
 }
 
+/// How Extend continues a surface past its edges.
+#[derive(Clone, Copy, Debug, Default, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "snake_case")]
+pub enum ExtendMode {
+    /// The surface itself, continued.
+    #[default]
+    Natural,
+    /// A ruled run-off in the surface's tangent plane (G1).
+    Tangent,
+    /// A run-off that keeps the surface's curvature across the edge (G2).
+    Curvature,
+}
+
+impl ExtendMode {
+    pub fn is_natural(&self) -> bool {
+        *self == ExtendMode::Natural
+    }
+}
+
+/// What Extend extends up to instead of a distance.
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
+#[serde(rename_all = "snake_case")]
+pub enum ExtendTo {
+    /// A plane.
+    Plane(PlaneRef),
+    /// The surface of a body's face (extended).
+    Face(FaceAt),
+    /// The surface's own bounds (natural only): a trim undone.
+    Bounds,
+}
+
 /// How a fillet's size is given.
 #[derive(Clone, Debug, Default, PartialEq, Serialize, Deserialize)]
 #[serde(tag = "type", rename_all = "snake_case")]