Fix rounding error.

src/exponent.js

@@ -0,0 +1,3 @@
+export default function(value) {
+  return Math.floor(Math.log(value) / Math.LN10 + 1e-12);
+};

src/formatAutoPrefix.js

@@ -1,10 +1,6 @@
 import btod from "./btod";
 
-export var exponent;
-
-export function exponentOf(value) { // Fast approximation.
-  return Math.max(-8, Math.min(8, Math.floor((Math.log(value) / (Math.LN10 * 3) + 1e-12)))) * 3;
-};
+export var prefixExponent;
 
 export default function(x, p) {
   var d = btod(x, p);
@@ -15,7 +11,7 @@ export default function(x, p) {
   else i %= 3;
   if (!i) i = 3;
   else if (i < 0) i += 3;
-  exponent = Math.max(-24, Math.min(24, Math.floor((d.exponent - 1) / 3) * 3));
+  prefixExponent = Math.max(-24, Math.min(24, Math.floor((d.exponent - 1) / 3) * 3));
   return d.coefficient.slice(0, i)
       + (d.coefficient.length > i
       ? "." + d.coefficient.slice(i)

src/localeFormat.js

@@ -1,7 +1,8 @@
+import exponent from "./exponent";
 import formatGroup from "./formatGroup";
 import formatSpecifier from "./formatSpecifier";
 import formatTypes from "./formatTypes";
-import {exponent, exponentOf} from "./formatAutoPrefix";
+import {prefixExponent} from "./formatAutoPrefix";
 
 var prefixes = ["y","z","a","f","p","n","µ","m","","k","M","G","T","P","E","Z","Y"];
 
@@ -62,7 +63,7 @@ export default function(locale) {
       value = formatType(value, precision);
 
       // Compute the suffix.
-      var valueSuffix = suffix + (type === "s" ? prefixes[8 + exponent / 3] : "");
+      var valueSuffix = suffix + (type === "s" ? prefixes[8 + prefixExponent / 3] : "");
 
       // Break the formatted value into the integer “value” part that can be
       // grouped, and fractional or exponential “suffix” part that is not.
@@ -102,7 +103,7 @@ export default function(locale) {
 
   function formatPrefix(specifier, value) {
     var f = format((specifier = formatSpecifier(specifier), specifier.type = "f", specifier)),
-        e = exponentOf(value),
+        e = Math.max(-8, Math.min(8, Math.floor(exponent(value) / 3))) * 3,
         k = Math.pow(10, -e),
         prefix = prefixes[8 + e / 3];
     return function(value) {

src/precisionFixed.js

@@ -1,3 +1,5 @@
+import exponent from "./exponent";
+
 export default function(step) {
-  return Math.max(0, -Math.floor(Math.log(Math.abs(step)) / Math.LN10));
+  return Math.max(0, -exponent(Math.abs(step)));
 };

src/precisionPrefix.js

@@ -1,6 +1,6 @@
+import exponent from "./exponent";
 import precisionFixed from "./precisionFixed";
-import {exponentOf} from "./formatAutoPrefix";
 
 export default function(step, value) {
-  return Math.max(0, exponentOf(value) - Math.floor(Math.log(Math.abs(step)) / Math.LN10));
+  return Math.max(0, Math.floor(exponent(value) / 3) * 3 - exponent(Math.abs(step)));
 };

src/precisionRound.js

@@ -1,3 +1,5 @@
+import exponent from "./exponent";
+
 export default function(step, max) {
-  return Math.max(0, Math.floor(Math.log(Math.abs(max)) / Math.LN10) - Math.floor(Math.log(Math.abs(step)) / Math.LN10)) + 1;
+  return Math.max(0, exponent(Math.abs(max)) - exponent(Math.abs(step))) + 1;
 };