Add d3.interpolateHue; fixes #36.

README.md

@@ -216,6 +216,10 @@ var interpolator = d3.interpolateRgb.gamma(2.2)("purple", "orange");
 
 See Eric Brasseur’s article, [Gamma error in picture scaling](https://web.archive.org/web/20160112115812/http://www.4p8.com/eric.brasseur/gamma.html), for more on gamma correction.
 
+<a name="interpolateHue" href="#interpolateHue">#</a> d3.<b>interpolateHue</b>(<i>a</i>, <i>b</i>) [<>](https://github.com/d3/d3-interpolate/blob/master/src/hue.js "Source")
+
+Returns an interpolator between the two hue angles *a* and *b*. If either hue is NaN, the opposing value is used. The shortest path between hues is used. The return value of the interpolator is a number in [0, 360).
+
 ### Splines
 
 Whereas standard interpolators blend from a starting value *a* at *t* = 0 to an ending value *b* at *t* = 1, spline interpolators smoothly blend multiple input values for *t* in [0,1] using piecewise polynomial functions. Only cubic uniform nonrational [B-splines](https://en.wikipedia.org/wiki/B-spline) are currently supported, also known as basis splines.

d3-interpolate.sublime-project

@@ -2,12 +2,16 @@
   "folders": [
     {
       "path": ".",
-      "file_exclude_patterns": [
-        "*.sublime-workspace"
-      ],
-      "folder_exclude_patterns": [
-        "build"
-      ]
+      "file_exclude_patterns": ["*.sublime-workspace"],
+      "folder_exclude_patterns": ["build"]
+    }
+  ],
+  "build_systems": [
+    {
+      "name": "yarn test",
+      "cmd": ["yarn", "test"],
+      "file_regex": "\\((...*?):([0-9]*):([0-9]*)\\)",
+      "working_dir": "$project_path"
     }
   ]
 }

index.js

@@ -4,6 +4,7 @@ export {default as interpolateBasis} from "./src/basis";
 export {default as interpolateBasisClosed} from "./src/basisClosed";
 export {default as interpolateDate} from "./src/date";
 export {default as interpolateDiscrete} from "./src/discrete";
+export {default as interpolateHue} from "./src/hue";
 export {default as interpolateNumber} from "./src/number";
 export {default as interpolateObject} from "./src/object";
 export {default as interpolateRound} from "./src/round";

src/hue.js

@@ -0,0 +1,9 @@
+import {hue} from "./color";
+
+export default function(a, b) {
+  var i = hue(+a, +b);
+  return function(t) {
+    var x = i(t);
+    return x - 360 * Math.floor(x / 360);
+  };
+}

test/hue-test.js

@@ -0,0 +1,48 @@
+var tape = require("tape"),
+    interpolate = require("../");
+
+tape("interpolateHue(a, b) interpolate numbers", function(test) {
+  var i = interpolate.interpolateHue("10", "20");
+  test.strictEqual(i(0.0), 10);
+  test.strictEqual(i(0.2), 12);
+  test.strictEqual(i(0.4), 14);
+  test.strictEqual(i(0.6), 16);
+  test.strictEqual(i(0.8), 18);
+  test.strictEqual(i(1.0), 20);
+  test.end();
+});
+
+tape("interpolateHue(a, b) returns a if b is NaN", function(test) {
+  var i = interpolate.interpolateHue(10, NaN);
+  test.equal(i(0.0), 10);
+  test.equal(i(0.5), 10);
+  test.equal(i(1.0), 10);
+  test.end();
+});
+
+tape("interpolateHue(a, b) returns b if a is NaN", function(test) {
+  var i = interpolate.interpolateHue(NaN, 20);
+  test.equal(i(0.0), 20);
+  test.equal(i(0.5), 20);
+  test.equal(i(1.0), 20);
+  test.end();
+});
+
+tape("interpolateHue(a, b) returns NaN if both a and b are NaN", function(test) {
+  var i = interpolate.interpolateHue(NaN, NaN);
+  test.equal(isNaN(i(0.0)), true);
+  test.equal(isNaN(i(0.5)), true);
+  test.equal(isNaN(i(1.0)), true);
+  test.end();
+});
+
+tape("interpolateHue(a, b) uses the shortest path", function(test) {
+  var i = interpolate.interpolateHue(10, 350);
+  test.equal(i(0.0), 10);
+  test.equal(i(0.2), 6);
+  test.equal(i(0.4), 2);
+  test.equal(i(0.6), 358);
+  test.equal(i(0.8), 354);
+  test.equal(i(1.0), 350);
+  test.end();
+});