WIP use border anchors

src/anchor.typ

@@ -18,6 +18,8 @@
   south: 270deg,
   south-east: 315deg,
 )
+#let compass-angle-names = compass-angle.keys()
+#let compass-angle-names-with-center = compass-angle-names + ("center",)
 
 /// Setup an anchor calculation and handling function for an element. Unifies anchor error checking and calculation of the offset transform.
 ///
@@ -59,13 +61,14 @@
       assert.ne(default, none, message: strfmt("Element '{}' does not have a default anchor!", name))
       anchor = default
     }
-    assert(
-      anchor in anchor-names,
-      message: strfmt("Anchor '{}' not in anchors {} for element '{}'", anchor, repr(anchor-names), name)
-      // message: strfmt("Anchor '{}' not in anchors {}", anchor, repr(anchor-names)) + if name != none { strfmt(" for element '{}'", name) }
-    )
+
+    assert(type(anchor) != str or anchor in anchor-names,
+      message: strfmt("Anchor '{}' not in anchors {} for element '{}'", anchor, repr(anchor-names), name))
 
     let out = callback(anchor)
+    assert(out != none,
+      message: "Anchor '" + repr(anchor) + "' does not exist for element '" + name + "'")
+
     return if transform != none {
       util.apply-transform(
         transform,
@@ -89,7 +92,10 @@
 /// - drawables (drawables): Drawables to test for an intersection against. Ideally should be of type path but all others are ignored.
 /// - angle (angle): The angle to check for a border anchor at.
 /// -> vector
-#let border(center, x-dist, y-dist, drawables, angle) = {
+#let border-intersection(center, x-dist, y-dist, drawables, angle) = {
+  x-dist += util.float-epsilon
+  y-dist += util.float-epsilon
+
   if type(drawables) == dictionary {
     drawables = (drawables,)
   }
@@ -116,5 +122,47 @@
   }
 
   // Find the furthest intersection point from center
-  return util.sort-points-by-distance(center, pts).last()
+  return util.sort-points-by-distance(center, pts).at(-1, default: none)
+}
+
+/// Path anchor function providing:
+/// - named "start", "mid" and "end" anchors
+/// - distance border anchors
+#let path(anchor, segments) = {
+  if anchor == "start" { anchor = 0% }
+  else if anchor == "mid" { anchor = 50% }
+  else if anchor == "end" { anchor = 100% }
+
+  if type(anchor) in (int, float, ratio) {
+    return path-util.point-on-path(segments, anchor)
+  }
+}
+
+/// Border anchor function providing:
+/// - named compass anchors
+/// - angle border anchors
+/// - distance border anchors
+/// Returns none if the anchor could not be handled
+#let border(anchor, center, dx, dy, drawables, with-center: false) = {
+  assert(drawables != none,
+    message: "Shape must be set.")
+
+  // Compass name => angle
+  if type(anchor) == str {
+    if str in compass-angle-names {
+      anchor = compass-angle.at(anchor)
+    } else if with-center and anchor == "center" {
+      return center
+    } else {
+      return none
+    }
+  }
+
+  if type(anchor) == angle {
+    return border-intersection(center, dx, dy, drawables, anchor)
+  } else if type(anchor) in (int, float, ratio) {
+    let p = drawables.filter(d => d.type == "path").first()
+    return path(anchor, p.segments)
+  }
+  return none
 }