Allow users to inject custom interpolators

index.js

@@ -1,4 +1,4 @@
-export {default as interpolate} from "./src/value";
+export {default as interpolate, customInterpolate} from "./src/value";
 export {default as interpolateArray} from "./src/array";
 export {default as interpolateBasis} from "./src/basis";
 export {default as interpolateBasisClosed} from "./src/basisClosed";

src/array.js

@@ -1,6 +1,6 @@
-import value from "./value";
+import interpolate from "./value";
 
-export default function(a, b) {
+export default function(a, b, value = interpolate) {
   var nb = b ? b.length : 0,
       na = a ? Math.min(nb, a.length) : 0,
       x = new Array(na),
@@ -9,7 +9,6 @@ export default function(a, b) {
 
   for (i = 0; i < na; ++i) x[i] = value(a[i], b[i]);
   for (; i < nb; ++i) c[i] = b[i];
-
   return function(t) {
     for (i = 0; i < na; ++i) c[i] = x[i](t);
     return c;

src/object.js

@@ -1,6 +1,6 @@
-import value from "./value";
+import interpolate from "./value";
 
-export default function(a, b) {
+export default function(a, b, value = interpolate) {
   var i = {},
       c = {},
       k;

src/value.js

@@ -7,14 +7,36 @@ import object from "./object";
 import string from "./string";
 import constant from "./constant";
 
-export default function(a, b) {
-  var t = typeof b, c;
-  return b == null || t === "boolean" ? constant(b)
-      : (t === "number" ? number
-      : t === "string" ? ((c = color(b)) ? (b = c, rgb) : string)
-      : b instanceof color ? rgb
-      : b instanceof Date ? date
-      : Array.isArray(b) ? array
-      : typeof b.valueOf !== "function" && typeof b.toString !== "function" || isNaN(b) ? object
-      : number)(a, b);
+const defaultInterpolators = {
+  rgb,
+  array,
+  date,
+  number,
+  object,
+  string,
+  constant
+};
+
+export function customInterpolate(interpolators) {
+  var rgb = interpolators.rgb || defaultInterpolators.rgb;
+  var array = interpolators.array || defaultInterpolators.array;
+  var date = interpolators.date || defaultInterpolators.date;
+  var number = interpolators.number || defaultInterpolators.number;
+  var object = interpolators.object || defaultInterpolators.object;
+  var string = interpolators.string || defaultInterpolators.string;
+  var constant = interpolators.constant || defaultInterpolators.constant;
+
+  return function interpolate(a, b) {
+    var t = typeof b, c;
+    return b == null || t === "boolean" ? constant(b)
+        : (t === "number" ? number
+        : t === "string" ? ((c = color(b)) ? (b = c, rgb) : string)
+        : b instanceof color ? rgb
+        : b instanceof Date ? date
+        : Array.isArray(b) ? array
+        : typeof b.valueOf !== "function" && typeof b.toString !== "function" || isNaN(b) ? object
+        : number)(a, b, interpolate);
+  }
 }
+
+export default customInterpolate({});

test/custom-interpolators-test.js

@@ -0,0 +1,47 @@
+var tape = require("tape"),
+    hcl = require('../').interpolateHcl,
+    color = require("d3-color"),
+    customInterpolate = require("../").customInterpolate;
+
+tape("customInterpolate(interpolators) uses a custom interpolator to ignore some object keys", function(test) {
+  var ignore = {color: true};
+  var ignoreKeyObjectInterpolator = function(a, b, value) {
+    var i = {},
+        c = {},
+        k;
+
+    if (a === null || typeof a !== "object") a = {};
+    if (b === null || typeof b !== "object") b = {};
+
+    for (k in b) {
+      if (k in a && !(k in ignore)) {
+        i[k] = value(a[k], b[k]);
+      } else {
+        c[k] = b[k];
+      }
+    }
+
+    return function(t) {
+      for (k in i) c[k] = i[k](t);
+      return c;
+    };
+  }
+
+  var interpolate = customInterpolate({object: ignoreKeyObjectInterpolator});
+  var i = interpolate({color: "red", x: 1, y: 2}, {color: "red", x: 3, y: 2});
+  test.deepEqual(i(0.5), {color: "red", x: 2, y: 2});
+
+  test.end();
+});
+
+tape("customInterpolate(interpolators) swaps color interpolators", function(test) {
+  var interpolate = customInterpolate({rgb: hcl});
+  var i = interpolate(color.hcl(10, 50, 50), color.hcl(350, 50, 50));
+  test.equal(i(0.0), "rgb(196, 79, 106)");
+  test.equal(i(0.2), "rgb(196, 79, 112)");
+  test.equal(i(0.4), "rgb(195, 79, 118)");
+  test.equal(i(0.6), "rgb(193, 79, 124)");
+  test.equal(i(0.8), "rgb(191, 80, 129)");
+  test.equal(i(1.0), "rgb(189, 81, 135)");
+  test.end();
+});