math/cmplx: prevent infinite loop in tanSeries

The condition to determine if any further iterations are needed is
evaluated to false in case it encounters a NaN. Instead, flip the
condition to keep looping until the factor is greater than the machine
roundoff error.

Updates #17577

Change-Id: I058abe73fcd49d3ae4e2f7b33020437cc8f290c3
Reviewed-on: https://go-review.googlesource.com/31952
Reviewed-by: Robert Griesemer <gri@golang.org>

diff --git a/src/math/cmplx/cmath_test.go b/src/math/cmplx/cmath_test.go
index d904be880d829ce0447cf96e6e5e2e7fe2e7a6f9..7a5c485a3103bd565d0cad3a2a6fb80edc9e7178 100644 (file)
--- a/src/math/cmplx/cmath_test.go
+++ b/src/math/cmplx/cmath_test.go
@@ -759,6 +759,14 @@ func TestTanh(t *testing.T) {
        }
 }
 
+// See issue 17577
+func TestInfiniteLoopIntanSeries(t *testing.T) {
+       want := Inf()
+       if got := Cot(0); got != want {
+               t.Errorf("Cot(0): got %g, want %g", got, want)
+       }
+}
+
 func BenchmarkAbs(b *testing.B) {
        for i := 0; i < b.N; i++ {
                Abs(complex(2.5, 3.5))

diff --git a/src/math/cmplx/tan.go b/src/math/cmplx/tan.go
index 03c351ad67b44cbf7ae1824b528da90254717ca8..299055215595c7bac76eec5ae6bfc5487298eed1 100644 (file)
--- a/src/math/cmplx/tan.go
+++ b/src/math/cmplx/tan.go
@@ -139,7 +139,9 @@ func tanSeries(z complex128) float64 {
                t = y2 - x2
                t /= f
                d += t
-               if math.Abs(t/d) <= MACHEP {
+               if !(math.Abs(t/d) > MACHEP) {
+                       // Caution: Use ! and > instead of <= for correct behavior if t/d is NaN.
+                       // See issue 17577.
                        break
                }
        }