refactor: Separate Empirical::mean_and_var

src/distribution/empirical.rs

@@ -57,14 +57,15 @@ mod non_nan {
 /// ```
 #[derive(Clone, PartialEq, Debug)]
 pub struct Empirical {
-    mean_and_var: Option<(f64, f64)>,
     // keys are data points, values are number of data points with equal value
     data: BTreeMap<NonNan<f64>, u64>,
 
     // The following fields are only logically valid if !data.is_empty():
     /// Total amount of data points (== sum of all _values_ inside self.data).
     /// Must be 0 iff data.is_empty()
     sum: u64,
+    mean: f64,
+    var: f64,
 }
 
 impl Empirical {
@@ -84,9 +85,10 @@ impl Empirical {
     #[allow(clippy::result_unit_err)]
     pub fn new() -> Result<Empirical, ()> {
         Ok(Empirical {
-            sum: 0,
-            mean_and_var: None,
             data: BTreeMap::new(),
+            sum: 0,
+            mean: 0.0,
+            var: 0.0,
         })
     }
 
@@ -97,17 +99,10 @@ impl Empirical {
         };
 
         self.sum += 1;
-        match self.mean_and_var {
-            Some((mean, var)) => {
-                let sum = self.sum as f64;
-                let var = var + (sum - 1.) * (data_point - mean) * (data_point - mean) / sum;
-                let mean = mean + (data_point - mean) / sum;
-                self.mean_and_var = Some((mean, var));
-            }
-            None => {
-                self.mean_and_var = Some((data_point, 0.));
-            }
-        }
+        let sum = self.sum as f64;
+        self.var += (sum - 1.) * (data_point - self.mean) * (data_point - self.mean) / sum;
+        self.mean += (data_point - self.mean) / sum;
+
         *self.data.entry(map_key).or_insert(0) += 1;
     }
 
@@ -117,21 +112,25 @@ impl Empirical {
             None => return,
         };
 
-        if let (Some(val), Some((mean, var))) = (self.data.remove(&map_key), self.mean_and_var) {
-            if val == 1 && self.data.is_empty() {
-                self.mean_and_var = None;
-                self.sum = 0;
-                return;
-            };
-            // reset mean and var
-            let sum = self.sum as f64;
-            let mean = (sum * mean - data_point) / (sum - 1.);
-            let var = var - (sum - 1.) * (data_point - mean) * (data_point - mean) / sum;
-            self.sum -= 1;
-            if val != 1 {
-                self.data.insert(map_key, val - 1);
-            };
-            self.mean_and_var = Some((mean, var));
+        let val = match self.data.remove(&map_key) {
+            Some(v) => v,
+            None => return,
+        };
+
+        if val == 1 && self.data.is_empty() {
+            self.sum = 0;
+            self.mean = 0.0;
+            self.var = 0.0;
+            return;
+        };
+
+        // reset mean and var
+        let sum = self.sum as f64;
+        self.mean = (sum * self.mean - data_point) / (sum - 1.);
+        self.var -= (sum - 1.) * (data_point - self.mean) * (data_point - self.mean) / sum;
+        self.sum -= 1;
+        if val != 1 {
+            self.data.insert(map_key, val - 1);
         }
     }
 
@@ -232,12 +231,19 @@ impl Min<f64> for Empirical {
 
 impl Distribution<f64> for Empirical {
     fn mean(&self) -> Option<f64> {
-        self.mean_and_var.map(|(mean, _)| mean)
+        if self.data.is_empty() {
+            None
+        } else {
+            Some(self.mean)
+        }
     }
 
     fn variance(&self) -> Option<f64> {
-        self.mean_and_var
-            .map(|(_, var)| var / (self.sum as f64 - 1.))
+        if self.data.is_empty() {
+            None
+        } else {
+            Some(self.var / (self.sum as f64 - 1.))
+        }
     }
 }
 
@@ -293,7 +299,7 @@ mod tests {
     #[test]
     fn test_remove_nonexisting() {
         let mut empirical = Empirical::new().unwrap();
-        
+
         empirical.add(5.2);
         // should not panic
         empirical.remove(10.0);