Fix cognitive metric when there are no functions

src/macros.rs

@@ -297,7 +297,12 @@ macro_rules! check_metrics {
 
             $(
                 $(
-                    assert!((func_space.metrics.$metric.$func_float() - $true_float_value).abs() < f64::EPSILON);
+                    let value: f64 = $true_float_value;
+                    if !value.is_nan() {
+                        assert!((func_space.metrics.$metric.$func_float() - value).abs() < f64::EPSILON);
+                    } else {
+                        assert!(func_space.metrics.$metric.$func_float().is_nan());
+                    }
                 )*
             )?
         }

src/metrics/cognitive.rs

@@ -72,6 +72,8 @@ impl Stats {
     ///
     /// This value is computed dividing the `Cognitive Complexity` value
     /// for the total number of functions/closures in a space.
+    ///
+    /// If there are no functions in a code, its value is `NAN`.
     pub fn cognitive_average(&self) -> f64 {
         self.cognitive() / self.total_space_functions as f64
     }
@@ -411,6 +413,54 @@ mod tests {
 
     use super::*;
 
+    #[test]
+    fn python_no_cognitive() {
+        check_metrics!(
+            "a = 42",
+            "foo.py",
+            PythonParser,
+            cognitive,
+            [(cognitive, 0, usize)],
+            [(cognitive_average, f64::NAN)]
+        );
+    }
+
+    #[test]
+    fn rust_no_cognitive() {
+        check_metrics!(
+            "let a = 42;",
+            "foo.rs",
+            RustParser,
+            cognitive,
+            [(cognitive, 0, usize)],
+            [(cognitive_average, f64::NAN)]
+        );
+    }
+
+    #[test]
+    fn c_no_cognitive() {
+        check_metrics!(
+            "int a = 42;",
+            "foo.c",
+            CppParser,
+            cognitive,
+            [(cognitive, 0, usize)],
+            [(cognitive_average, f64::NAN)]
+        );
+    }
+
+    #[test]
+    fn mozjs_no_cognitive() {
+        check_metrics!(
+            "var a = 42;",
+            "foo.js",
+            MozjsParser,
+            cognitive,
+            [(cognitive, 0, usize)],
+            [(cognitive_average, f64::NAN)]
+        );
+    }
+
     #[test]
     fn python_simple_function() {
         check_metrics!(