Fix logic when two different exponents are involved

Author: pitrou

cpp/src/arrow/testing/gtest_util_test.cc

@@ -233,6 +233,10 @@ TEST(TestWithinUlp, Double) {
   CheckWithinUlp(1.0, 1.0000000000000007, 3);
   CheckNotWithinUlp(1.0, 1.0000000000000007, 2);
   CheckNotWithinUlp(1.0, 1.0000000000000007, 1);
+  // left and right have a different exponent but are still very close
+  CheckWithinUlp(1.0, 0.9999999999999999, 1);
+  CheckWithinUlp(1.0, 0.9999999999999988, 11);
+  CheckNotWithinUlp(1.0, 0.9999999999999988, 10);
 
   CheckWithinUlp(123.4567, 123.45670000000015, 11);
   CheckNotWithinUlp(123.4567, 123.45670000000015, 10);
@@ -253,6 +257,10 @@ TEST(TestWithinUlp, Float) {
   CheckWithinUlp(1.0f, 1.0000001f, 1);
   CheckWithinUlp(1.0f, 1.0000013f, 11);
   CheckNotWithinUlp(1.0f, 1.0000013f, 10);
+  // left and right have a different exponent but are still very close
+  CheckWithinUlp(1.0f, 0.99999994f, 1);
+  CheckWithinUlp(1.0f, 0.99999934f, 11);
+  CheckNotWithinUlp(1.0f, 0.99999934f, 10);
 
   CheckWithinUlp(123.456f, 123.456085f, 11);
   CheckNotWithinUlp(123.456f, 123.456085f, 10);

cpp/src/arrow/testing/math.cc

@@ -63,7 +63,8 @@ bool WithinUlpGeneric(Float left, Float right, int n_ulp) {
   if (!std::isfinite(left) || !std::isfinite(right)) {
     return left == right;
   }
-  return WithinUlpOneWay(left, right, n_ulp) || WithinUlpOneWay(right, left, n_ulp);
+  return (std::abs(left) <= std::abs(right)) ? WithinUlpOneWay(left, right, n_ulp)
+                                             : WithinUlpOneWay(right, left, n_ulp);
 }
 
 template <typename Float>