Fix #530. Implement #511.

Author: jmrozanec

src/main/java/com/cronutils/model/time/ExecutionTime.java

@@ -178,20 +178,15 @@ default int countExecutions(ZonedDateTime startDate, ZonedDateTime endDate) {
      */
     default List<ZonedDateTime> getExecutionDates(ZonedDateTime startDate, ZonedDateTime endDate) {
         if (endDate.equals(startDate) || endDate.isBefore(startDate)) {
-            throw new IllegalArgumentException("End date should be after start date");
+            throw new IllegalArgumentException("endDate should take place later in time than startDate");
         }
-
         List<ZonedDateTime> executions = new ArrayList<>();
         ZonedDateTime nextExecutionDate = nextExecution(startDate).orElse(null);
 
         if (nextExecutionDate == null) return Collections.emptyList();
-        executions.add(nextExecutionDate);
-
-        while (nextExecutionDate.isBefore(endDate)) {
-            nextExecutionDate = nextExecution(nextExecutionDate).orElse(null);
-
-            if (nextExecutionDate == null) break;
+        while(nextExecutionDate != null && nextExecutionDate.isBefore(endDate)){
             executions.add(nextExecutionDate);
+            nextExecutionDate = nextExecution(nextExecutionDate).orElse(null);
         }
         return executions;
     }

src/main/java/com/cronutils/utils/CronFrequencyComparator.java

@@ -0,0 +1,24 @@
+package com.cronutils.utils;
+
+import com.cronutils.model.Cron;
+import com.cronutils.model.time.ExecutionTime;
+
+import java.time.ZonedDateTime;
+import java.util.Comparator;
+
+public class CronFrequencyComparator implements Comparator<Cron> {
+    private final ZonedDateTime startDate;
+    private final ZonedDateTime endDate;
+
+    public CronFrequencyComparator(ZonedDateTime startDate, ZonedDateTime endDate){
+        this.startDate = startDate;
+        this.endDate = endDate;
+    }
+
+    @Override
+    public int compare(Cron o1, Cron o2) {
+        int executions1 = ExecutionTime.forCron(o1).countExecutions(startDate, endDate);
+        int executions2 = ExecutionTime.forCron(o2).countExecutions(startDate, endDate);
+        return executions1-executions2;
+    }
+}

src/test/java/com/cronutils/utils/CronFrequencyComparatorTest.java

@@ -0,0 +1,50 @@
+package com.cronutils.utils;
+
+import com.cronutils.model.Cron;
+import com.cronutils.model.CronType;
+import com.cronutils.model.definition.CronDefinition;
+import com.cronutils.model.definition.CronDefinitionBuilder;
+import com.cronutils.parser.CronParser;
+import org.junit.Before;
+import org.junit.Test;
+
+import java.time.LocalDateTime;
+import java.time.ZoneId;
+import java.time.ZonedDateTime;
+
+import static org.junit.Assert.*;
+
+public class CronFrequencyComparatorTest {
+    private CronFrequencyComparator comparator;
+    private CronParser parser;
+    private Cron cron1;
+    private Cron cron2;
+
+    @Before
+    public void setUp() {
+        ZonedDateTime date1 = LocalDateTime.of(2018, 11, 5, 0, 0, 0).atZone(ZoneId.of("UTC"));
+        ZonedDateTime date2 = LocalDateTime.of(2018, 11, 11, 0, 0, 0).atZone(ZoneId.of("UTC"));
+        comparator = new CronFrequencyComparator(date1, date2);
+        final CronDefinition cronDefinition = CronDefinitionBuilder.instanceDefinitionFor(CronType.SPRING53);
+        parser = new CronParser(cronDefinition);
+
+        cron1 = parser.parse("0 0 9-17 * * MON-FRI");//on the hour nine-to-five weekdays -> 9 executions per day, five times a week -> 45 executions per week
+        cron2 = parser.parse("0 0/30 8-10 * * *");//8:00, 8:30, 9:00, 9:30, 10:00 and 10:30 every day -> six executions per day, seven times a week -> 42 executions per week
+    }
+
+    @Test
+    public void compareMoreFrequent() {
+        assertTrue(comparator.compare(cron1, cron2)>0);
+    }
+
+    @Test
+    public void compareLessFrequent() {
+        assertTrue(comparator.compare(cron2, cron1)<0);
+    }
+
+    @Test
+    public void compareEqualFrequent() {
+        Cron cronx = parser.parse("0 0 9-17 * * MON-FRI");//on the hour nine-to-five weekdays -> 9 executions per day, five times a week -> 45 executions per week
+        assertEquals(0, comparator.compare(cron1, cronx));
+    }
+}