Add support for multiple hulls

README.md

@@ -99,6 +99,10 @@ These methods are used in d3-geo-voronoi’s [geoContour](https://github.com/Fil
 
 Sets the *triangulate* function. Defaults to d3.Delaunay.from. See [Reusing a  tricontour triangulation](https://observablehq.com/@fil/reusing-a-tricontour-triangulation) and [UK tricontour](https://observablehq.com/@fil/tricontours-with-a-personalized-triangulation) for detailed examples.
 
+The *triangulate* function should create an object which is compatible with d3.Delaunay, i.e. it must have `points`, `halfedges`, `hull` and `triangles`.
+However, in order to support hulls made up of multiple connected components, `hull` does not have to be an `Int32Array` but can be an `Array` of `Int32Array` (one per connected component of the hull).
+This allows to handle the case where the domain is made up of multiple pieces and/or has holes.
+
 [<img src="https://raw.githubusercontent.com/Fil/d3-tricontour/main/img/tricontour-triangulation.jpg" alt="UK tricontour" width="320">](https://observablehq.com/@fil/tricontours-with-a-personalized-triangulation)
 
 
@@ -108,4 +112,4 @@ Sets the *pointInterpolate* function. Arguments: *i*, *j*, *0≤a<1*. Defaults t
 
 <a href="#ringsort" name="ringsort">#</a> _tricontour_.<b>ringsort</b>(_[ringsort]_)
 
-Sets the *ringsort* function.
+Sets the *ringsort* function.

src/tricontour.js

@@ -111,9 +111,11 @@ export default function() {
     // sanity check
     for (const d of values) if (!isFinite(d)) throw ["Invalid value", d];
 
-    const { halfedges, hull, inedges, triangles } = triangulation,
+    const { halfedges, hull: hull_or_hulls, inedges, triangles } = triangulation,
       n = values.length;
 
+    const hulls = Array.isArray(hull_or_hulls[0]) ? hull_or_hulls : [hull_or_hulls];
+
     function edgealpha(i) {
       return alpha(triangles[i], triangles[next(i)]);
     }
@@ -163,7 +165,15 @@ export default function() {
 
         // or follow the hull
         else {
-          let h = (hull.indexOf(triangles[i]) + 1) % hull.length;
+          let idx, hull;
+          for (hull of hulls) {
+            idx = hull.indexOf(triangles[i]);
+            if (idx >= 0) {
+              // `triangles[i]` is on the hull
+              break;
+            }
+          }
+          let h = (idx + 1) % hull.length;
 
           while (values[hull[h]] < v0) {
             // debugger;
@@ -195,14 +205,16 @@ export default function() {
     }
 
     // special case all values on the hull are >=v0, add the hull
-    if (hull.every(d => values[d] >= v0)) {
-      rings.unshift(
-        Array.from(hull)
-          .concat([hull[0]])
-          .map(i => pointInterpolate(i, i, 0))
-      );
+    for (const hull of hulls) {
+      if (hull.every(d => values[d] >= v0)) {
+        rings.unshift(
+          Array.from(hull)
+            .concat([hull[0]])
+            .map(i => pointInterpolate(i, i, 0))
+        );
+      }
     }
 
     return ringsort(rings); // return [rings] if we don't need to sort
   }
-}
+}