Fixed compound unit conversion

src/quantity/unit/unit.rs

@@ -97,12 +97,60 @@ impl Unit {
 		return n;
 	}
 
+	// True if base units are the same
 	pub fn compatible_with(&self, other: &Unit) -> bool {
 		let s = self.clone() * self.to_base_factor().unit;
 		let o = other.clone() * other.to_base_factor().unit;
+
 		return o == s;
 	}
 
+	// True if these two units have a common factor
+	pub fn common_factor(&self, other: &Unit) -> Option<Quantity> {
+
+		if self.unitless() || other.unitless() { return None; }
+
+
+		let mut failed = false;
+
+		// What to convert `other` to before multiplying
+		let mut factor = Quantity::new_rational_from_string("1").unwrap();
+		let mut flag;
+		for (us, _) in self.get_val() {
+			flag = false;
+			for (uo, po) in other.get_val() {
+				if {
+					us.get_base().unit.compatible_with(&uo.get_base().unit)
+				} {
+					factor.insert_unit(us.clone(), po.clone());
+					flag = true;
+					break;
+				}
+			}
+			if !flag { failed = true }
+		}
+
+		if !failed { return Some(factor);}
+
+
+		let mut factor = Quantity::new_rational_from_string("1").unwrap();
+		for (uo, po) in other.get_val() {
+			flag = false;
+			for (us, _) in self.get_val() {
+				if {
+					us.get_base().unit.compatible_with(&uo.get_base().unit)
+				} {
+					factor.insert_unit(us.clone(), po.clone());
+					flag = true;
+					break;
+				}
+			}
+			if !flag { return None; }
+		}
+
+		return Some(factor);
+	}
+
 	pub fn insert(&mut self, u: FreeUnit, p: Scalar) {
 		let v = self.get_val_mut();
 		match v.get_mut(&u) {