Merge pull request #76 from d3/exact-ends

Exact ends

src/date.js

@@ -1,6 +1,6 @@
 export default function(a, b) {
   var d = new Date;
-  return a = +a, b -= a, function(t) {
-    return d.setTime(a + b * t), d;
+  return a = +a, b = +b, function(t) {
+    return d.setTime(a * (1 - t) + b * t), d;
   };
 }

src/number.js

@@ -1,5 +1,5 @@
 export default function(a, b) {
-  return a = +a, b -= a, function(t) {
-    return a + b * t;
+  return a = +a, b = +b, function(t) {
+    return a * (1 - t) + b * t;
   };
 }

src/round.js

@@ -1,5 +1,5 @@
 export default function(a, b) {
-  return a = +a, b -= a, function(t) {
-    return Math.round(a + b * t);
+  return a = +a, b = +b, function(t) {
+    return Math.round(a * (1 - t) + b * t);
   };
 }

test/array-test.js

@@ -38,3 +38,10 @@ tape("interpolateArray(a, b) interpolates array-like objects", function(test) {
   test.deepEqual(interpolate.interpolateArray(args, [4, 24])(0.5), [3, 18]);
   test.end();
 });
+
+tape("interpolateArray(a, b) gives exact ends for t=0 and t=1", function(test) {
+  var a = [2e+42], b = [335];
+  test.deepEqual(interpolate.interpolateArray(a, b)(1), b);
+  test.deepEqual(interpolate.interpolateArray(a, b)(0), a);
+  test.end();
+});

test/date-test.js

@@ -16,3 +16,10 @@ tape("interpolateDate(a, b) reuses the output datea", function(test) {
   test.strictEqual(i(0.2), i(0.4));
   test.end();
 });
+
+tape("interpolateDate(a, b) gives exact ends for t=0 and t=1", function(test) {
+  var a = new Date(1e8 * 24 * 60 * 60 * 1000), b = new Date(-1e8 * 24 * 60 * 60 * 1000 +1);
+  test.equal(+interpolate.interpolateDate(a, b)(1), +b);
+  test.equal(+interpolate.interpolateDate(a, b)(0), +a);
+  test.end();
+});

test/number-test.js

@@ -18,3 +18,11 @@ tape("interpolateNumber(a, b) interpolates between two numbers a and b", functio
   test.inDelta(i(1.0), 42.0);
   test.end();
 });
+
+
+tape("interpolateNumber(a, b) gives exact ends for t=0 and t=1", function(test) {
+  var a = 2e+42, b = 335;
+  test.equal(interpolate.interpolateNumber(a, b)(1), b);
+  test.equal(interpolate.interpolateNumber(a, b)(0), a);
+  test.end();
+});

test/round-test.js

@@ -22,3 +22,10 @@ tape("interpolateRound(a, b) does not pre-round a and b", function(test) {
   test.equal(i(0.6), 3);
   test.end();
 });
+
+tape("interpolateRound(a, b) gives exact ends for t=0 and t=1", function(test) {
+  var a = 2e+42, b = 335;
+  test.equal(interpolate.interpolateRound(a, b)(1), b);
+  test.equal(interpolate.interpolateRound(a, b)(0), a);
+  test.end();
+});