From c2a5b1237635875b20c46d47577355ced733db14 Mon Sep 17 00:00:00 2001
From: jcopperfield <33193571+jcopperfield@users.noreply.github.com>
Date: Wed, 21 Feb 2018 00:44:01 +0100
Subject: [PATCH] Bugfix: Improve polyfill function of log10 to return whole
 powers of 10 (#5275)

* Bugfix: Improve polyfill function of log10 to return whole powers of 10
        as integer values, as it caused endless loop in IE11 in the tick
        creation loop.

* Compare floating-point numbers directly instead of using unnecessary division.
---
 src/core/core.helpers.js         | 8 +++++++-
 test/specs/core.helpers.tests.js | 8 ++++++--
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/src/core/core.helpers.js b/src/core/core.helpers.js
index bdce895cf..347fd958e 100644
--- a/src/core/core.helpers.js
+++ b/src/core/core.helpers.js
@@ -159,7 +159,13 @@ module.exports = function(Chart) {
 			return Math.log10(x);
 		} :
 		function(x) {
-			return Math.log(x) / Math.LN10;
+			var exponent = Math.log(x) * Math.LOG10E; // Math.LOG10E = 1 / Math.LN10.
+			// Check for whole powers of 10,
+			// which due to floating point rounding error should be corrected.
+			var powerOf10 = Math.round(exponent);
+			var isPowerOf10 = x === Math.pow(10, powerOf10);
+
+			return isPowerOf10 ? powerOf10 : exponent;
 		};
 	helpers.toRadians = function(degrees) {
 		return degrees * (Math.PI / 180);
diff --git a/test/specs/core.helpers.tests.js b/test/specs/core.helpers.tests.js
index 44446d38a..eb96ef8c7 100644
--- a/test/specs/core.helpers.tests.js
+++ b/test/specs/core.helpers.tests.js
@@ -194,8 +194,12 @@ describe('Core helper tests', function() {
 
 	it('should do a log10 operation', function() {
 		expect(helpers.log10(0)).toBe(-Infinity);
-		expect(helpers.log10(1)).toBe(0);
-		expect(helpers.log10(1000)).toBeCloseTo(3, 1e-9);
+
+		// Check all allowed powers of 10, which should return integer values
+		var maxPowerOf10 = Math.floor(helpers.log10(Number.MAX_VALUE));
+		for (var i = 0; i < maxPowerOf10; i += 1) {
+			expect(helpers.log10(Math.pow(10, i))).toBe(i);
+		}
 	});
 
 	it('should correctly determine if two numbers are essentially equal', function() {