Add d3.geo.berghaus.invert.

geo/projection/berghaus.js

@@ -17,6 +17,19 @@ function berghaus(n) {
     return p;
   }
 
+  forward.invert = function(x, y) {
+    var r = Math.sqrt(x * x + y * y);
+    if (r > π / 2) {
+      var θ = Math.atan2(y, x),
+          θ0 = k * Math.round((θ - π / 2) / k) + π / 2,
+          s = θ > θ0 ? -1 : 1,
+          cotα = 1 / Math.tan(s * Math.acos((r * Math.cos(θ0 - θ) - π) / Math.sqrt(π * π + r * r - 2 * π * r * Math.cos(θ0 - θ))));
+      θ = θ0 + 2 * Math.atan((cotα + s * Math.sqrt(cotα * cotα - 3)) / 3);
+      x = r * Math.cos(θ), y = r * Math.sin(θ);
+    }
+    return berghausAzimuthalEquidistant.invert(x, y);
+  };
+
   return forward;
 }
 

geo/projection/test/berghaus-test.js

@@ -0,0 +1,26 @@
+require("./env");
+
+var vows = require("vows"),
+    assert = require("assert");
+
+var suite = vows.describe("d3.geo.berghaus");
+
+suite.addBatch({
+  "berghaus": {
+    topic: d3.geo.berghaus,
+    "projections and inverse projections": function(berghaus) {
+      assert.equalInverse(berghaus, [  0,   0], [480,        250]);
+      assert.equalInverse(berghaus, [  0, -45], [480,        367.809724]);
+      assert.equalInverse(berghaus, [  0,  45], [480,        132.190275]);
+      assert.equalInverse(berghaus, [-90,   0], [244.380550, 250]);
+      assert.equalInverse(berghaus, [ 90,   0], [715.619449, 250]);
+      assert.equalInverse(berghaus, [-80,  15], [277.038148, 194.777583]);
+      assert.equalInverse(berghaus, [  1,   1], [482.617728, 247.381873]);
+      assert.equalInverse(berghaus, [ 15,  45], [510.778518, 131.080938]);
+      assert.equalInverse(berghaus, [120,  30], [750.967904, 114.867516]);
+      assert.equalInverse(berghaus, [110,  10], [759.454234, 183.963114]);
+    }
+  }
+});
+
+suite.export(module);