Avoid rounding error. (#200)

* fix #199; avoid rounding error

* remove noop ceil

Author: mbostock

src/ticks.js

@@ -15,16 +15,18 @@ export default function(start, stop, count) {
   if ((step = tickIncrement(start, stop, count)) === 0 || !isFinite(step)) return [];
 
   if (step > 0) {
-    start = Math.ceil(start / step);
-    stop = Math.floor(stop / step);
-    ticks = new Array(n = Math.ceil(stop - start + 1));
-    while (++i < n) ticks[i] = (start + i) * step;
+    let r0 = Math.round(start / step), r1 = Math.round(stop / step);
+    if (r0 * step < start) ++r0;
+    if (r1 * step > stop) --r1;
+    ticks = new Array(n = r1 - r0 + 1);
+    while (++i < n) ticks[i] = (r0 + i) * step;
   } else {
     step = -step;
-    start = Math.ceil(start * step);
-    stop = Math.floor(stop * step);
-    ticks = new Array(n = Math.ceil(stop - start + 1));
-    while (++i < n) ticks[i] = (start + i) / step;
+    let r0 = Math.round(start * step), r1 = Math.round(stop * step);
+    if (r0 / step < start) ++r0;
+    if (r1 / step > stop) --r1;
+    ticks = new Array(n = r1 - r0 + 1);
+    while (++i < n) ticks[i] = (r0 + i) / step;
   }
 
   if (reverse) ticks.reverse();

test/ticks-test.js

@@ -105,3 +105,7 @@ tape("ticks(start, stop, count) returns the reverse of ticks(stop, start, count)
   test.deepEqual(array.ticks(10, -10,  2), array.ticks(-10, 10,  2).reverse());
   test.deepEqual(array.ticks(10, -10,  1), array.ticks(-10, 10,  1).reverse());
 });
+
+tape("ticks(start, stop, count) handles precision problems", (test) => {
+  test.deepEqual(array.ticks(0.98, 1.14, 10), [0.98, 1, 1.02, 1.04, 1.06, 1.08, 1.1, 1.12, 1.14]);
+});