Remove fudge factor when computing exponent.

src/exponent.js

@@ -1,3 +1,5 @@
-export default function(value) {
-  return Math.floor(Math.log(value) / Math.LN10 + 1e-12);
+import formatDecimal from "./formatDecimal";
+
+export default function(x) {
+  return x = formatDecimal(Math.abs(x)), x ? x[1] : NaN;
 };

src/formatAutoPrefix.js

@@ -1,19 +1,21 @@
-import btod from "./btod";
+import formatDecimal from "./formatDecimal";
 
 export var prefixExponent;
 
 export default function(x, p) {
-  var d = btod(x, p);
+  var d = formatDecimal(x, p);
   if (!d) return x + "";
-  var i = d.exponent;
+  var coefficient = d[0],
+      exponent = d[1],
+      i = exponent + 1;
   if (i >= 24) i -= 24;
-  else if (i <= -24) d.coefficient = new Array(-22 - i).join("0") + d.coefficient.slice(0, i + 23), i = 1;
+  else if (i <= -24) coefficient = new Array(-22 - i).join("0") + coefficient.slice(0, i + 23), i = 1;
   else i %= 3;
   if (!i) i = 3;
   else if (i < 0) i += 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)
-      : new Array(i - d.coefficient.length + 1).join("0"));
+  prefixExponent = Math.max(-24, Math.min(24, Math.floor(exponent / 3) * 3));
+  return coefficient.slice(0, i)
+      + (coefficient.length > i
+      ? "." + coefficient.slice(i)
+      : new Array(i - coefficient.length + 1).join("0"));
 };

src/formatDecimal.js

@@ -0,0 +1,14 @@
+// Computes the decimal coefficient and exponent of the specified number x with
+// significant digits p, where x is positive and p is in [1, 21] or undefined.
+// For example, formatDecimal(1.23) returns ["123", -1].
+export default function(x, p) {
+  if ((i = (x = x.toExponential(p - 1)).indexOf("e")) < 0) return null; // NaN, ±Infinity
+  var i, coefficient = x.slice(0, i);
+
+  // The string returned by toExponential either has the form \d\.\d+e[-+]\d+
+  // (e.g., 1.2e+3) or the form \de[-+]\d+ (e.g., 1e+3).
+  return [
+    coefficient.length > 1 ? coefficient[0] + coefficient.slice(2) : coefficient,
+    +x.slice(i + 1)
+  ];
+};

src/formatRounded.js

@@ -1,13 +1,15 @@
-import btod from "./btod";
+import formatDecimal from "./formatDecimal";
 
 export default function(x, p) {
-  var d = btod(x, p);
+  var d = formatDecimal(x, p);
   if (!d) return x + "";
-  var i = d.exponent;
+  var coefficient = d[0],
+      exponent = d[1],
+      i = exponent + 1;
   return i <= 0
-      ? "0." + new Array(1 - i).join("0") + d.coefficient
-      : d.coefficient.slice(0, i)
-      + (d.coefficient.length > i
-      ? "." + d.coefficient.slice(i)
-      : new Array(i - d.coefficient.length + 1).join("0"));
+      ? "0." + new Array(1 - i).join("0") + coefficient
+      : coefficient.slice(0, i)
+      + (coefficient.length > i
+      ? "." + coefficient.slice(i)
+      : new Array(i - coefficient.length + 1).join("0"));
 };

src/formatRoundedPercentage.js

@@ -1,13 +1,15 @@
-import btod from "./btod";
+import formatDecimal from "./formatDecimal";
 
 export default function(x, p) {
-  var d = btod(x, p);
+  var d = formatDecimal(x, p);
   if (!d) return x + "";
-  var i = d.exponent + 2;
+  var coefficient = d[0],
+      exponent = d[1],
+      i = exponent + 3;
   return i <= 0
-      ? "0." + new Array(1 - i).join("0") + d.coefficient
-      : d.coefficient.slice(0, i)
-      + (d.coefficient.length > i
-      ? "." + d.coefficient.slice(i)
-      : new Array(i - d.coefficient.length + 1).join("0"));
+      ? "0." + new Array(1 - i).join("0") + coefficient
+      : coefficient.slice(0, i)
+      + (coefficient.length > i
+      ? "." + coefficient.slice(i)
+      : new Array(i - coefficient.length + 1).join("0"));
 };

src/precisionPrefix.js

@@ -2,5 +2,5 @@ import exponent from "./exponent";
 import precisionFixed from "./precisionFixed";
 
 export default function(step, value) {
-  return Math.max(0, Math.floor(exponent(value) / 3) * 3 - exponent(Math.abs(step)));
+  return Math.max(0, Math.max(-8, Math.min(8, Math.floor(exponent(value) / 3))) * 3 - exponent(Math.abs(step)));
 };

test/precisionPrefix-test.js

@@ -0,0 +1,46 @@
+var tape = require("tape"),
+    format = require("../");
+
+// A generalization from µ to all prefixes:
+// test.equal(format.precisionPrefix(1e-6, 1e-6), 0); // 1µ
+// test.equal(format.precisionPrefix(1e-6, 1e-7), 0); // 10µ
+// test.equal(format.precisionPrefix(1e-6, 1e-8), 0); // 100µ
+tape("precisionPrefix(step, value) returns zero if step has the same units as value", function(test) {
+  for (var i = -24; i <= 24; i += 3) {
+    for (var j = i; j < i + 3; ++j) {
+      test.equal(format.precisionPrefix(+("1e" + i), +("1e" + j)), 0);
+    }
+  }
+  test.end();
+});
+
+// A generalization from µ to all prefixes:
+// test.equal(format.precisionPrefix(1e-9, 1e-6), 3); // 0.001µ
+// test.equal(format.precisionPrefix(1e-8, 1e-6), 2); // 0.01µ
+// test.equal(format.precisionPrefix(1e-7, 1e-6), 1); // 0.1µ
+tape("precisionPrefix(step, value) returns greater than zero if fractional digits are needed", function(test) {
+  for (var i = -24; i <= 24; i += 3) {
+    for (var j = i - 4; j < i; ++j) {
+      test.equal(format.precisionPrefix(+("1e" + j), +("1e" + i)), i - j);
+    }
+  }
+  test.end();
+});
+
+tape("precisionPrefix(step, value) returns the expected precision when value is less than one yocto", function(test) {
+  test.equal(format.precisionPrefix(1e-24, 1e-24), 0); // 1y
+  test.equal(format.precisionPrefix(1e-25, 1e-25), 1); // 0.1y
+  test.equal(format.precisionPrefix(1e-26, 1e-26), 2); // 0.01y
+  test.equal(format.precisionPrefix(1e-27, 1e-27), 3); // 0.001y
+  test.equal(format.precisionPrefix(1e-28, 1e-28), 4); // 0.0001y
+  test.end();
+});
+
+tape("precisionPrefix(step, value) returns the expected precision when value is greater than than one yotta", function(test) {
+  test.equal(format.precisionPrefix(1e24, 1e24), 0); // 1Y
+  test.equal(format.precisionPrefix(1e24, 1e25), 0); // 10Y
+  test.equal(format.precisionPrefix(1e24, 1e26), 0); // 100Y
+  test.equal(format.precisionPrefix(1e24, 1e27), 0); // 1000Y
+  test.equal(format.precisionPrefix(1e23, 1e27), 1); // 1000.0Y
+  test.end();
+});