Piecewise default interpolator

README.md

@@ -247,10 +247,12 @@ Returns a uniform nonrational B-spline interpolator through the specified array
 
 ### Piecewise
 
-<a name="piecewise" href="#piecewise">#</a> d3.<b>piecewise</b>(<i>interpolate</i>, <i>values</i>) · [Source](https://github.com/d3/d3-interpolate/blob/master/src/piecewise.js), [Examples](https://observablehq.com/@d3/d3-piecewise)
+<a name="piecewise" href="#piecewise">#</a> d3.<b>piecewise</b>([<i>interpolate</i>, ]<i>values</i>) · [Source](https://github.com/d3/d3-interpolate/blob/master/src/piecewise.js), [Examples](https://observablehq.com/@d3/d3-piecewise)
 
 Returns a piecewise interpolator, composing interpolators for each adjacent pair of *values*. The returned interpolator maps *t* in [0, 1 / (*n* - 1)] to *interpolate*(*values*[0], *values*[1]), *t* in [1 / (*n* - 1), 2 / (*n* - 1)] to *interpolate*(*values*[1], *values*[2]), and so on, where *n* = *values*.length. In effect, this is a lightweight [linear scale](https://github.com/d3/d3-scale/blob/master/README.md#linear-scales). For example, to blend through red, green and blue:
 
 ```js
 var interpolate = d3.piecewise(d3.interpolateRgb.gamma(2.2), ["red", "green", "blue"]);
 ```
+
+If  *interpolate* is not specified, defaults to [d3.interpolate](#interpolate).

src/piecewise.js

@@ -1,4 +1,7 @@
+import {default as value} from "./value.js";
+
 export default function piecewise(interpolate, values) {
+  if (values === undefined) values = interpolate, interpolate = value;
   var i = 0, n = values.length - 1, v = values[0], I = new Array(n < 0 ? 0 : n);
   while (i < n) I[i] = interpolate(v, v = values[++i]);
   return function(t) {

test/piecewise-test.js

@@ -0,0 +1,38 @@
+var tape = require("tape"),
+    interpolate = require("../");
+
+tape("piecewise(interpolate, values)(t) returns the expected values", function(test) {
+  var i = interpolate.piecewise(interpolate.interpolate, [0,2,10]);
+  test.strictEqual(i(-1), -4);
+  test.strictEqual(i(0), 0);
+  test.strictEqual(i(0.19), 0.76);
+  test.strictEqual(i(0.21), 0.84);
+  test.strictEqual(i(0.5), 2);
+  test.strictEqual(i(0.75), 6);
+  test.strictEqual(i(1), 10);
+  test.end();
+});
+
+tape("piecewise(values) uses the default interpolator", function(test) {
+  var i = interpolate.piecewise([0,2,10]);
+  test.strictEqual(i(-1), -4);
+  test.strictEqual(i(0), 0);
+  test.strictEqual(i(0.19), 0.76);
+  test.strictEqual(i(0.21), 0.84);
+  test.strictEqual(i(0.5), 2);
+  test.strictEqual(i(0.75), 6);
+  test.strictEqual(i(1), 10);
+  test.end();
+});
+
+tape("piecewise(values) uses the default interpolator/2", function(test) {
+  var i = interpolate.piecewise(["a0","a2","a10"]);
+  test.strictEqual(i(-1), "a-4");
+  test.strictEqual(i(0), "a0");
+  test.strictEqual(i(0.19), "a0.76");
+  test.strictEqual(i(0.21), "a0.84");
+  test.strictEqual(i(0.5), "a2");
+  test.strictEqual(i(0.75), "a6");
+  test.strictEqual(i(1), "a10");
+  test.end();
+});