From 4c63ba147001f09c4dd9b3f8f9576b798480dade Mon Sep 17 00:00:00 2001
From: william-wwh <60587573+william-wwh@users.noreply.github.com>
Date: Tue, 13 Oct 2026 16:36:35 +0800
Subject: [PATCH] LINE: resolve deferred TAN first point with typed
 @distance<angle> (#788)

* LINE: resolve deferred TAN first point with typed @distance<angle>

* LINE: collapse the nested if-let (clippy collapsible_if)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Brandon Thomas <echelon@gmail.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
---
 crates/engine/src/cmd/draw.rs | 16 +++++++++++++++-
 crates/engine/src/lib.rs      |  8 ++++++++
 crates/engine/src/prompt.rs   | 10 ++++++++++
 crates/engine/src/snap.rs     | 15 +++++++++++++++
 crates/engine/src/tests.rs    | 21 +++++++++++++++++++++
 5 files changed, 69 insertions(+), 1 deletion(-)

diff --git a/crates/engine/src/cmd/draw.rs b/crates/engine/src/cmd/draw.rs
index 96043de..b073cf1 100644
--- a/crates/engine/src/cmd/draw.rs
+++ b/crates/engine/src/cmd/draw.rs
@@ -428,7 +428,21 @@ impl Interactive for LineM {
             && let Some(first) = self.first
         {
             match i {
-                Input::Point(p) => return self.resolve_first(s, first, snap::LineEnd::Point(p)),
+                Input::Point(p) => {
+                    // A deferred TAN first point with a typed `@d<a` next point: resolve
+                    // exactly from the direction and distance the user gave.
+                    if let Some((dist, ang)) = s.last_polar.take()
+                        && let Some(t) = snap::tangent_point_in_direction(&first.curve, ang, first.at)
+                    {
+                        let e = t + Vec2::new(ang.cos(), ang.sin()) * dist;
+                        self.handles.push(s.add_entity(line(t, e))?);
+                        self.pts = vec![t, e];
+                        s.last_point = e;
+                        self.first = None;
+                        return Ok(Step::Continue);
+                    }
+                    return self.resolve_first(s, first, snap::LineEnd::Point(p));
+                }
                 Input::Deferred(d) => return self.resolve_first(s, first, snap::LineEnd::Deferred(d)),
                 _ => {}
             }
diff --git a/crates/engine/src/lib.rs b/crates/engine/src/lib.rs
index 14802b2..e3f7f83 100644
--- a/crates/engine/src/lib.rs
+++ b/crates/engine/src/lib.rs
@@ -574,6 +574,11 @@ pub struct Session {
     /// The last segment given at a point prompt (its base point and the point): polar angles
     /// relative to the last segment (POLARMODE bit 1) are measured from it.
     pub last_segment: Option<(Vec2, Vec2)>,
+    /// The last typed `@distance<angle` input, as `(distance, angle in radians)`.
+    /// Set by [`Session::typed`] and consumed by commands that resolve a deferred
+    /// tangent first point (e.g. [`cmd::draw::LineM`]) when the next point is typed
+    /// rather than clicked. Cleared at [`Session::start`].
+    pub last_polar: Option<(f64, f64)>,
     /// Point modifiers pending at the current point prompt (`FROM`, `M2P`, `.x`, `<a`…).
     pub point_mods: Vec<pointmod::Frame>,
     /// Cursor position in world coordinates (from the UI; used for direct distance entry).
@@ -635,6 +640,7 @@ impl Session {
             last_command: None,
             last_point: Vec2::ZERO,
             last_segment: None,
+            last_polar: None,
             point_mods: Vec::new(),
             cursor: Vec2::ZERO,
             cursor_deferred: None,
@@ -830,6 +836,7 @@ impl Session {
 
     /// Start a command by name or alias as if typed at the command line.
     pub fn start(&mut self, name: &str) -> Result<()> {
+        self.last_polar = None;
         let name = name.trim();
         let (transparent, name) = match name.strip_prefix('\'') {
             Some(n) => (true, n),
@@ -1320,6 +1327,7 @@ impl Session {
         }
         if prompt.accept.point {
             if let Some(p) = prompt::parse_point_with(tt, self.last_point, &self.angle_settings()) {
+                self.last_polar = prompt::parse_polar_text(tt, &self.angle_settings());
                 return self.input(Input::Point(p));
             }
             // Direct distance entry along the tracking path the cursor is on, from the point it
diff --git a/crates/engine/src/prompt.rs b/crates/engine/src/prompt.rs
index e531e3c..1976614 100644
--- a/crates/engine/src/prompt.rs
+++ b/crates/engine/src/prompt.rs
@@ -51,6 +51,16 @@ pub struct Prompt {
     pub picker: Option<Picker>,
 }
 
+/// Parse `@distance<angle` (relative polar), returning `(distance, angle in radians)`.
+/// Returns `None` for any other form.
+pub fn parse_polar_text(text: &str, angles: &crate::units::AngleSettings) -> Option<(f64, f64)> {
+    let body = text.trim().strip_prefix('@')?;   // relative form only
+    let (d, a) = body.split_once('<')?;
+    let dist = d.trim().parse::<f64>().ok()?;
+    let angle = angles.amount(a.trim())?;
+    (dist.is_finite() && angle.is_finite()).then_some((dist, angle))
+}
+
 /// A visual chooser for a prompt ([`Prompt::picker`]).
 #[derive(Clone, Debug, PartialEq, Serialize)]
 #[serde(rename_all = "camelCase")]
diff --git a/crates/engine/src/snap.rs b/crates/engine/src/snap.rs
index e2a8488..22ef5c4 100644
--- a/crates/engine/src/snap.rs
+++ b/crates/engine/src/snap.rs
@@ -96,6 +96,21 @@ pub struct Deferred {
     pub at: Vec2,
 }
 
+/// The point on `curve` whose tangent has direction `dir`, picking the solution
+/// nearest `hint`. Used to resolve a deferred TAN first point when the next point
+/// is typed as `@distance<angle`.
+pub fn tangent_point_in_direction(curve: &SnapCurve, dir: f64, hint: Vec2) -> Option<Vec2> {
+    match curve {
+        SnapCurve::Circle { center, radius } => {
+            let n = dir + std::f64::consts::FRAC_PI_2;
+            let c1 = *center + Vec2::new(n.cos(), n.sin()) * *radius;
+            let c2 = *center - Vec2::new(n.cos(), n.sin()) * *radius;
+            Some(if c1.dist(hint) < c2.dist(hint) { c1 } else { c2 })
+        }
+        _ => None,
+    }
+}
+
 /// One end of a line being solved: a known point or a deferred snap.
 #[derive(Clone, Copy, Debug, PartialEq)]
 pub enum LineEnd {
diff --git a/crates/engine/src/tests.rs b/crates/engine/src/tests.rs
index 03d9f24..1b59178 100644
--- a/crates/engine/src/tests.rs
+++ b/crates/engine/src/tests.rs
@@ -983,6 +983,27 @@ fn line_first_point_deferred_tangent() {
     assert!(line_ends(&s)[1].0.near(b, 1e-12));
 }
 
+#[test]
+fn line_deferred_tangent_with_typed_polar() {
+    let mut s = Session::new();
+    s.cmdline("circle 0,0 5").unwrap();
+    s.execute("osnap", &json!({"modes": ["tan"]})).unwrap();
+    s.cmdline("line").unwrap();
+    snap_click(&mut s, Vec2::new(0.1, 5.2)); // deferred TAN, near the top of the circle
+    s.cmdline("@150<218").unwrap();
+    s.cmdline("").unwrap();
+    let ends = line_ends(&s);
+    assert_eq!(ends.len(), 1);
+    let (a, b) = ends[0];
+    // The start is the tangent point on the circle.
+    assert!((a.len() - 5.0).abs() < 1e-9, "start not on circle: {a:?}");
+    // The typed direction and distance are exact.
+    let v = b - a;
+    assert!((v.len() - 150.0).abs() < 1e-9, "length was {}", v.len());
+    let angle = v.y.atan2(v.x).to_degrees().rem_euclid(360.0);
+    assert!((angle - 218.0).abs() < 1e-9, "angle was {angle}");
+}
+
 #[test]
 fn line_belt_tangent_to_tangent() {
     let mut s = Session::new();