factor::rho: Refactor to avoid unnecessary allocations

src/uu/factor/src/factor.rs

@@ -36,6 +36,13 @@ impl Factors {
         Factors { f: BTreeMap::new() }
     }
 
+    fn prime(p: u64) -> Factors {
+        debug_assert!(miller_rabin::is_prime(p));
+        let mut f = Factors::one();
+        f.push(p);
+        f
+    }
+
     fn add(&mut self, prime: u64, exp: u8) {
         debug_assert!(exp > 0);
         let n = *self.f.get(&prime).unwrap_or(&0);

src/uu/factor/src/rho.rs

@@ -55,17 +55,15 @@ fn _factor<A: Arithmetic>(num: u64) -> Factors {
         _factor::<A>(n)
     };
 
-    let mut factors = Factors::one();
     if num == 1 {
-        return factors;
+        return Factors::one();
     }
 
     let n = A::new(num);
     let divisor;
     match miller_rabin::test::<A>(n) {
         Prime => {
-            factors.push(num);
-            return factors;
+            return Factors::prime(num);
         }
 
         Composite(d) => {
@@ -77,7 +75,7 @@ fn _factor<A: Arithmetic>(num: u64) -> Factors {
         }
     };
 
-    factors *= _factor(divisor);
+    let mut factors = _factor(divisor);
     factors *= _factor(num / divisor);
     factors
 }