refactor: add explicit NaN check

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---

Author: kgryte

lib/node_modules/@stdlib/math/base/special/factorial2/manifest.json

@@ -40,7 +40,8 @@
         "@stdlib/math/base/assert/is-even",
         "@stdlib/constants/float64/pinf",
         "@stdlib/constants/float64/max-nth-double-factorial",
-        "@stdlib/math/base/assert/is-nonnegative-integer"
+        "@stdlib/math/base/assert/is-nonnegative-integer",
+        "@stdlib/math/base/assert/is-nan"
       ]
     },
     {
@@ -57,7 +58,8 @@
         "@stdlib/math/base/assert/is-even",
         "@stdlib/constants/float64/pinf",
         "@stdlib/constants/float64/max-nth-double-factorial",
-        "@stdlib/math/base/assert/is-nonnegative-integer"
+        "@stdlib/math/base/assert/is-nonnegative-integer",
+        "@stdlib/math/base/assert/is-nan"
       ]
     },
     {
@@ -74,7 +76,8 @@
         "@stdlib/math/base/assert/is-even",
         "@stdlib/constants/float64/pinf",
         "@stdlib/constants/float64/max-nth-double-factorial",
-        "@stdlib/math/base/assert/is-nonnegative-integer"
+        "@stdlib/math/base/assert/is-nonnegative-integer",
+        "@stdlib/math/base/assert/is-nan"
       ]
     }
   ]

lib/node_modules/@stdlib/math/base/special/factorial2/src/main.c

@@ -18,6 +18,7 @@
 
 #include "stdlib/math/base/special/factorial2.h"
 #include "stdlib/math/base/assert/is_even.h"
+#include "stdlib/math/base/assert/is_nan.h"
 #include "stdlib/math/base/assert/is_nonnegative_integer.h"
 #include "stdlib/constants/float64/pinf.h"
 #include "stdlib/constants/float64/max_nth_double_factorial.h"
@@ -37,7 +38,8 @@ double stdlib_base_factorial2( const double n ) {
 	double out;
 	double v;
 	double i;
-	if ( !stdlib_base_is_nonnegative_integer( n ) ) {
+
+	if ( stdlib_base_is_nan( n ) || !stdlib_base_is_nonnegative_integer( n ) ) {
 		return 0.0 / 0.0; // NaN
 	}
 	if ( n > STDLIB_CONSTANT_FLOAT64_MAX_NTH_DOUBLE_FACTORIAL ) {

lib/node_modules/@stdlib/math/base/special/factorial2/test/test.native.js

@@ -25,6 +25,7 @@ var tape = require( 'tape' );
 var isnan = require( '@stdlib/math/base/assert/is-nan' );
 var incrspace = require( '@stdlib/array/base/incrspace' );
 var PINF = require( '@stdlib/constants/float64/pinf' );
+var NINF = require( '@stdlib/constants/float64/ninf' );
 var tryRequire = require( '@stdlib/utils/try-require' );
 
 
@@ -62,6 +63,24 @@ tape( 'if provided a negative integer, the function returns `NaN`', opts, functi
 	t.end();
 });
 
+tape( 'if provided negative infinity, the function returns `NaN`', opts, function test( t ) {
+	var v = factorial2( NINF );
+	t.strictEqual( isnan( v ), true, 'returns expected value' );
+	t.end();
+});
+
+tape( 'if provided positive infinity, the function returns `+infinity`', opts, function test( t ) {
+	var v = factorial2( PINF );
+	t.strictEqual( v, PINF, 'returns expected value' );
+	t.end();
+});
+
+tape( 'if provided `NaN`, the function returns `NaN`', opts, function test( t ) {
+	var v = factorial2( NaN );
+	t.strictEqual( isnan( v ), true, 'returns expected value' );
+	t.end();
+});
+
 tape( 'the function evaluates the double factorial', opts, function test( t ) {
 	var expected;
 	var x;