Early exit from tanh for values >=20 <=-20 as JS loses precision for …

…true value at this integer
paulmillr · Mar 1, 2016 · 7b2414b · 7b2414b · Yaffle · Mar 9, 2016
1 parent d1ef165
commit 7b2414b
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Showing 2 changed files with 7 additions and 3 deletions.
diff --git a/es6-shim.js b/es6-shim.js
@@ -1957,8 +1957,9 @@
     tanh: function tanh(value) {
       var x = Number(value);
       if (Number.isNaN(x) || x === 0) { return x; }
-      if (x === Infinity) { return 1; }
-      if (x === -Infinity) { return -1; }
+      // can exit early at +-20 as JS loses precision for true value at this integer
+      if (x >= 20) { return 1; }
+      if (x <= -20) { return -1; }
       var a = Math.expm1(x);
       var b = Math.expm1(-x);
       if (a === Infinity) { return 1; }

diff --git a/test/math.js b/test/math.js
@@ -605,7 +605,10 @@ describe('Math', function () {
       expect(isNegativeZero(Math.tanh(-0))).to.equal(true);
       expect(Math.tanh(Infinity)).to.equal(1);
       expect(Math.tanh(-Infinity)).to.equal(-1);
-      expect(Math.tanh(90)).to.almostEqual(1);
+      expect(Math.tanh(19)).to.almostEqual(1);
+      expect(Math.tanh(-19)).to.almostEqual(-1);
+      expect(Math.tanh(20)).to.equal(1); // JS loses precision for true value at this integer
+      expect(Math.tanh(-20)).to.equal(-1); // JS loses precision for true value at this integer
       expect(Math.tanh(10)).to.almostEqual(0.9999999958776927);
       expect(Math.tanh(-2e-17)).to.equal(-2e-17);
     });