shapes: Add arc-through

Thanks to Matthew Brown for the implementation.

src/draw.typ

@@ -1,4 +1,4 @@
 #import "draw/grouping.typ": intersections, group, anchor, copy-anchors, place-anchors, set-ctx, get-ctx, for-each-anchor, on-layer, place-marks
 #import "draw/transformations.typ": set-transform, rotate, translate, scale, set-origin, move-to, set-viewport
 #import "draw/styling.typ": set-style, fill, stroke
-#import "draw/shapes.typ": circle, circle-through, arc, mark, line, grid, content, rect, bezier, bezier-through, catmull, hobby, merge-path, shadow
+#import "draw/shapes.typ": circle, circle-through, arc, arc-through, mark, line, grid, content, rect, bezier, bezier-through, catmull, hobby, merge-path, shadow

src/draw/shapes.typ

@@ -303,6 +303,50 @@
   },)
 }
 
+#let arc-through(
+  a,
+  b,
+  c,
+  name: none,
+  ..style,
+) = get-ctx(ctx => {
+  let (ctx, a) = coordinate.resolve(ctx, a)
+  let (ctx, b) = coordinate.resolve(ctx, b)
+  let (ctx, c) = coordinate.resolve(ctx, c)
+
+  let center = util.calculate-circle-center-3pt(a, b, c)
+  let radius = vector.dist(center, a)
+  let start = {
+    let (x, y, ..) = vector.sub(a, center)
+    calc.atan2(x, y) // Typst's atan2 is (x,y) order!
+  }
+  let delta = vector.angle(a, center, c)
+
+  let side-on-line(a, b, pt) = {
+    let (x1, y1, ..) = a
+    let (x2, y2, ..) = b
+    let (x,  y, ..)  = pt
+    return (x - x1) * (y2 - y1) - (y - y1) * (x2 - x1)
+  }
+
+  let center-is-left = side-on-line(a, c, center) < 0
+  let b-is-left = side-on-line(a, c, b) < 0
+
+  // If the center and point b are on the same side of a-c,
+  // the arcs delta must be > 180deg
+  if center-is-left == b-is-left {
+    delta = 360deg - delta
+  }
+
+  // If b is left of a-c, swap a-c to c-a by using a negative delta
+  if b-is-left {
+    delta *= -1
+  }
+
+  return arc(a, start: start, delta: delta, radius: radius,
+    anchor: "arc-start", name: name, ..style)
+})
+
 #let mark(from, to, ..style) = {
   assert.eq(
     style.pos(),

tests/arc/arc-through/test.typ

@@ -0,0 +1,58 @@
+#set page(width: auto, height: auto)
+#import "/src/lib.typ": *
+
+#let show-points(..pts) = {
+  import draw: *
+  for pt in pts.pos() {
+    circle(pt, radius: .1)
+  }
+}
+
+#let test(a, b, c) = {
+  import draw: *
+  group({
+    anchor("center", (0,0,0))
+    show-points(a, b, c)
+    arc-through(a, b, c)
+  }, name: "g", anchor: "west", padding: .1)
+  set-origin("g.east")
+}
+
+#box(stroke: 2pt + red, canvas(length: 1cm, {
+  import draw: *
+
+  test((0,0), (1, 1), (2, 0))
+  test((0,0), (1,-1), (2, 0))
+  test((0,1), (1, 0), (0,-1))
+  test((0,1), (-1,0), (0,-1))
+}))
+
+#box(stroke: 2pt + red, canvas(length: 1cm, {
+  import draw: *
+
+  for a in range(36, 360 + 36, step: 36) {
+    let a = a * 1deg
+    test((1,0),
+         (calc.cos(a / 2), calc.sin(a / 2)),
+         (calc.cos(a), calc.sin(a)))
+  }
+}))
+
+#box(stroke: 2pt + red, canvas(length: 1cm, {
+  import draw: *
+
+  for d in range(0, 8 + 1) {
+    let d = (d - 2) / 5
+    test((0,0), (1,d), (2,.5))
+  }
+}))
+
+#box(stroke: 2pt + red, canvas(length: 1cm, {
+  import draw: *
+
+  // The style radius must not influence the
+  // arc radius!
+  set-style(radius: 5)
+  set-style(arc: (radius: 5))
+  test((0,0), (1, 1), (2, 0))
+}))