ARC: refuse zero-radius arcs instead of adding them (fixes #85) (#123)

The `arc` command checked its three-point form for collinear points but
added the arcs built by its start/center/end and center/radius forms
unchecked, so radius 0 (or a start point on the center) added a
degenerate Arc, dirtied the drawing and recorded an undo step. Every
form now goes through the existing finite_arc check and fails with the
same "these values do not define an arc" error the arc.* variants and
the interactive ARC already use, leaving the drawing unchanged.

Fixes #85.

crates/engine/src/cmd/curves.rs

@@ -43,7 +43,7 @@ pub fn is_locked(s: &Session, h: Handle) -> bool {
 
 // ---------------- arc constructions ----------------
 
-fn finite_arc(a: Arc) -> Option<Arc> {
+pub(crate) fn finite_arc(a: Arc) -> Option<Arc> {
     (a.center.is_finite() && a.radius.is_finite() && a.radius > EPS && a.start.is_finite() && a.end.is_finite()).then_some(a)
 }
 

crates/engine/src/cmd/draw.rs

@@ -172,6 +172,9 @@ fn run_arc(s: &mut Session, p: &Value) -> Result<Value> {
     } else {
         return Err(bad("arc", "give {p1,p2,p3}, {start,center,end} or {center,radius,start,end}"));
     };
+    // Zero or non-finite radius (e.g. start == center): refuse, like the interactive ARC and the
+    // arc.* variants, before anything is added (#85).
+    let a = super::curves::finite_arc(a).ok_or_else(|| bad("arc", "these values do not define an arc"))?;
     added(s.add_entity(arc(&a))?)
 }
 

crates/engine/src/tests.rs

@@ -846,3 +846,24 @@ fn deferred_pick_elsewhere_is_a_plain_point() {
     });
     assert_eq!(c, Some(Vec2::new(0.0, 5.0)));
 }
+
+/// #85: an `arc` request that builds a zero-radius arc is refused and the drawing is unchanged.
+#[test]
+fn arc_refuses_zero_radius() {
+    for params in [json!({"center": [0, 0], "radius": 0, "start": 0, "end": 90}), json!({"start": [0, 0], "center": [0, 0], "end": [1, 0]})] {
+        let mut s = Session::new();
+        let err = s.execute("arc", &params).unwrap_err().to_string();
+        assert!(err.contains("do not define an arc"), "{params}: {err}");
+        let info = s.execute("drawing.inspect", &json!({})).unwrap();
+        assert_eq!(info["entityCount"], 0, "{params}: {info}");
+        assert_eq!(info["dirty"], false, "{params}: {info}");
+        assert_eq!(info["undo"], json!([]), "{params}: {info}");
+    }
+    // The positive-radius control still adds the quarter circle.
+    let mut s = Session::new();
+    s.execute("arc", &json!({"center": [0, 0], "radius": 5, "start": 0, "end": 90})).unwrap();
+    match &s.doc().unwrap().model.iter().next().unwrap().kind {
+        EntityKind::Arc(a) => assert_eq!((a.radius, a.start, a.end), (5.0, 0.0, std::f64::consts::FRAC_PI_2)),
+        other => panic!("{other:?}"),
+    }
+}