Implement per-month employee working hours

This is not yet finished, as the new/changed methods in CustomOrder are
quite a mess.

src/main/java/catering/order/CustomOrder.java

@@ -11,6 +11,7 @@ import org.springframework.data.annotation.Id;
 import javax.money.MonetaryAmount;
 import java.time.Duration;
 import java.time.LocalDateTime;
+import java.time.YearMonth;
 import java.time.format.DateTimeFormatter;
 import java.util.HashSet;
 import java.util.Set;
@@ -38,14 +39,39 @@ public class CustomOrder extends Order {
 		this.formatterPattern = cart.getFormatterPattern();
 	}
 
-	public CustomOrder() {}
+	public CustomOrder() { }
+
+	private long getDurationInSeconds(LocalDateTime start, LocalDateTime finish) {
+		if (start.compareTo(finish) > 0) {
+			return 0;
+		}
+		return Duration.between(start, finish).getSeconds();
+	}
 
 	/**
 	 * Helper function to get the amount of hours in between start and finish (analogous to CustomCart)
 	 * @return hours between start and finish
 	 */
 	private long getDurationInHours(LocalDateTime start, LocalDateTime finish) {
-		return Duration.between(start, finish).getSeconds() / 3600;
+		return getDurationInSeconds(start, finish) / 3600;
+	}
+
+	public long getDurationInHours() {
+		return getDurationInHours(start, finish);
+	}
+
+	private static LocalDateTime min(LocalDateTime a, LocalDateTime b) {
+		return (a.compareTo(b)) < 0 ? a : b;
+	}
+
+	private static LocalDateTime max(LocalDateTime a, LocalDateTime b) {
+		return (a.compareTo(b)) > 0 ? a : b;
+	}
+
+	public long getDurationInSecondsDuringMonth(YearMonth month) {
+		LocalDateTime startInMonth = max(this.start, month.atDay(1).atStartOfDay());
+		LocalDateTime finishInMonth = min(this.finish, month.plusMonths(1).atDay(1).atStartOfDay());
+		return getDurationInSeconds(startInMonth, finishInMonth);
 	}
 
 	/**
@@ -53,7 +79,7 @@ public class CustomOrder extends Order {
 	 */
 	public boolean addEmployee(Employee employee) {
 		MonetaryAmount cost = Money.of(12.0, "EUR")
-			.multiply(getDurationInHours(this.start, this.finish)); // TODO: Get salary from employee
+				.multiply(getDurationInHours()); // TODO: Get salary from employee
 
 		if (this.staff.add(employee)) {
 			super.addChargeLine(cost, employee.getId().toString());

src/main/java/catering/staff/StaffManagement.java

@@ -1,6 +1,7 @@
 package catering.staff;
 
 import java.time.LocalDateTime;
+import java.time.YearMonth;
 import java.util.Optional;
 import java.util.Set;
 import java.util.function.Predicate;
@@ -104,4 +105,10 @@ public class StaffManagement {
 				.collect(Collectors.toSet());
 	}
 
+	public double getWorkingHoursByEmployee(Employee e, YearMonth month) {
+		return orderManagement.findAll(Pageable.unpaged()).stream()
+				.filter(order -> order.getStaff().contains(e))
+				.map(order -> order.getDurationInSecondsDuringMonth(month))
+				.collect(Collectors.summingDouble(seconds -> (double) seconds / 3600));
+	}
 }