addition: reduce, gcd and correct_sign

src/lib.rs

@@ -54,15 +54,27 @@ impl Fraction {
     }
 
     fn gcd(a: i64, b: i64) -> i64 {
-        todo!();
+        if a == 0 {
+            return b;
+        }
+        Fraction::gcd(b % a, a)
     }
 
     fn reduce(&mut self) {
-        todo!();
+        let gdc = Fraction::gcd(self.num, self.den);
+
+        self.num /= gdc;
+        self.den /= gdc;
     }
 
     fn correct_sign(&mut self) {
-        todo!()
+        if self.num < 0 && self.den < 0 {
+            self.num = self.num.abs();
+            self.den = self.den.abs();
+        } else if self.den < 0 {
+            self.num = -self.num;
+            self.den = self.den.abs();
+        }
     }
 }