From 7d60c72de0500c912b5c358b8f19383a9dbf3794 Mon Sep 17 00:00:00 2001
From: Orual <orual@nonbinary.computer>
Date: Sun, 2 Aug 2026 20:39:40 -0400
Subject: [PATCH] PM-78: distinguish conditional-line endpoints and controls

Add a typed accessor for type-5 geometry while preserving the existing serde shape, with regression coverage for endpoint/control roles.

Epic: PM-86
Task: PM-78
---
 crates/polymodel-ldraw-core/src/geom.rs | 15 ++++++++++++++
 crates/polymodel-ldraw-core/src/lib.rs  | 26 +++++++++++++++++++++++++
 2 files changed, 41 insertions(+)

diff --git a/crates/polymodel-ldraw-core/src/geom.rs b/crates/polymodel-ldraw-core/src/geom.rs
index 0a0525b..52cd9db 100644
--- a/crates/polymodel-ldraw-core/src/geom.rs
+++ b/crates/polymodel-ldraw-core/src/geom.rs
@@ -255,6 +255,21 @@ pub struct GeometryRecord {
     pub vertices: Vec<Point3>,
 }
 
+impl GeometryRecord {
+    /// Return the endpoint and control-point pairs for a conditional line.
+    ///
+    /// The fixed-size array references make the type-5 geometry contract
+    /// explicit to callers without changing the serialized representation.
+    pub fn conditional_endpoints(&self) -> Option<(&[Point3; 2], &[Point3; 2])> {
+        if self.line_type != LineType::Five || self.vertices.len() != 4 {
+            return None;
+        }
+        let endpoints = self.vertices.get(..2)?.try_into().ok()?;
+        let controls = self.vertices.get(2..)?.try_into().ok()?;
+        Some((endpoints, controls))
+    }
+}
+
 pub(crate) fn parse_primitive(line: &ScannedLine) -> Option<ParsedPrimitive> {
     match line.line_type {
         Some(LineType::One) => {
diff --git a/crates/polymodel-ldraw-core/src/lib.rs b/crates/polymodel-ldraw-core/src/lib.rs
index 84f84c1..26c6eee 100644
--- a/crates/polymodel-ldraw-core/src/lib.rs
+++ b/crates/polymodel-ldraw-core/src/lib.rs
@@ -72,6 +72,32 @@ mod tests {
         assert_eq!(ledger.snapshot().reservations, 0);
     }
     #[test]
+    fn conditional_geometry_exposes_endpoints_and_controls() {
+        let ledger = ReservationLedger::new();
+        let result = LdrawParser
+            .parse_bytes(
+                b"5 16 0 0 0 1 0 0 0 1 0 2 0 0\n",
+                options(&ledger),
+                None,
+            )
+            .unwrap();
+        let geometry = &result.models[0].geometry[0];
+        let (endpoints, controls) = geometry
+            .conditional_endpoints()
+            .expect("type-5 geometry should expose its point roles");
+        assert_eq!(endpoints[0], Point3::new(0.0, 0.0, 0.0));
+        assert_eq!(endpoints[1], Point3::new(1.0, 0.0, 0.0));
+        assert_eq!(controls[0], Point3::new(0.0, 1.0, 0.0));
+        assert_eq!(controls[1], Point3::new(2.0, 0.0, 0.0));
+        assert!(GeometryRecord {
+            line_type: LineType::Two,
+            colour: ColourCode::from(16_u16),
+            vertices: Vec::new(),
+        }
+        .conditional_endpoints()
+        .is_none());
+    }
+    #[test]
     fn bfc_state_machine_certify_then_nocertify() {
         let fixture = corpus_fixture("bfc-directives");
         let bytes = fixture.bytes;
-- 
2.51.2