Add months distance methods

This makes the app free from months count assumption thus making it more flexible
for other kinds of calendars.

src/main/java/io/github/persiancalendar/calendar/CivilDate.java

@@ -4,7 +4,7 @@
  * @author Amir
  */
 
-public class CivilDate extends AbstractDate {
+public class CivilDate extends AbstractDate implements YearMonthDate<CivilDate> {
 
     public CivilDate(int year, int month, int dayOfMonth) {
         super(year, month, dayOfMonth);
@@ -73,4 +73,14 @@ protected int[] fromJdn(long jdn) {
         } else
             return julianFromJdn(jdn);
     }
+
+    @Override
+    public CivilDate monthStartOfMonthsDistance(int monthsDistance) {
+        return TwelveMonthsYear.monthStartOfMonthsDistance(this, monthsDistance, CivilDate::new);
+    }
+
+    @Override
+    public int monthsDistanceTo(CivilDate date) {
+        return TwelveMonthsYear.monthsDistanceTo(this, date);
+    }
 }

src/main/java/io/github/persiancalendar/calendar/IslamicDate.java

@@ -8,7 +8,7 @@
  * @author Amir
  */
 
-public class IslamicDate extends AbstractDate {
+public class IslamicDate extends AbstractDate implements YearMonthDate<IslamicDate> {
 
     // Converters
     public static boolean useUmmAlQura = false;
@@ -51,4 +51,14 @@ protected int[] fromJdn(long jdn) {
 
         return result;
     }
+
+    @Override
+    public IslamicDate monthStartOfMonthsDistance(int monthsDistance) {
+        return TwelveMonthsYear.monthStartOfMonthsDistance(this, monthsDistance, IslamicDate::new);
+    }
+
+    @Override
+    public int monthsDistanceTo(IslamicDate date) {
+        return TwelveMonthsYear.monthsDistanceTo(this, date);
+    }
 }

src/main/java/io/github/persiancalendar/calendar/PersianDate.java

@@ -3,7 +3,7 @@
 import io.github.persiancalendar.calendar.persian.AlgorithmicConverter;
 import io.github.persiancalendar.calendar.persian.LookupTableConverter;
 
-public class PersianDate extends AbstractDate {
+public class PersianDate extends AbstractDate implements YearMonthDate<PersianDate> {
 
     public PersianDate(int year, int month, int dayOfMonth) {
         super(year, month, dayOfMonth);
@@ -29,4 +29,14 @@ protected int[] fromJdn(long jdn) {
         int[] result = LookupTableConverter.fromJdn(jdn);
         return result == null ? AlgorithmicConverter.fromJdn(jdn) : result;
     }
+
+    @Override
+    public PersianDate monthStartOfMonthsDistance(int monthsDistance) {
+        return TwelveMonthsYear.monthStartOfMonthsDistance(this, monthsDistance, PersianDate::new);
+    }
+
+    @Override
+    public int monthsDistanceTo(PersianDate date) {
+        return TwelveMonthsYear.monthsDistanceTo(this, date);
+    }
 }

src/main/java/io/github/persiancalendar/calendar/YearMonthDate.java

@@ -0,0 +1,32 @@
+package io.github.persiancalendar.calendar;
+
+interface YearMonthDate<T extends AbstractDate> {
+    // Ideally getYear()/getMonth()/getDay() also should be moved to this interface
+
+    T monthStartOfMonthsDistance(int monthsDistance);
+
+    int monthsDistanceTo(T date);
+
+    interface CreateDate<T extends AbstractDate> {
+        T createDate(int year, int month, int dayOfMonth);
+    }
+
+    class TwelveMonthsYear {
+        static <T extends AbstractDate> T monthStartOfMonthsDistance(
+                T baseDate, int monthsDistance, CreateDate<T> createDate
+        ) {
+            int month = monthsDistance + baseDate.getMonth() - 1; // make it zero based for easier calculations
+            int year = baseDate.getYear() + month / 12;
+            month %= 12;
+            if (month < 0) {
+                year -= 1;
+                month += 12;
+            }
+            return createDate.createDate(year, month + 1, 1);
+        }
+
+        static <T extends AbstractDate> int monthsDistanceTo(T baseDate, T toDate) {
+            return (toDate.getYear() - baseDate.getYear()) * 12 + toDate.getMonth() - baseDate.getMonth();
+        }
+    }
+}

src/test/kotlin/io/github/persiancalendar/calendar/MainTests.kt

@@ -179,4 +179,35 @@ class MainTests {
         assertTrue(dayOfMonth == 1 || dayOfMonth == previousDayOfMonth + 1)
         dayOfMonth
     }.let {}
+
+    @Test
+    fun `Test month distance methods`() {
+        run {
+            val date = CivilDate(2014, 7, 7)
+            assertEquals(0, date.monthsDistanceTo(date))
+            assertEquals(-10, date.monthsDistanceTo(CivilDate(2013, 9, 5)))
+            assertEquals(30, date.monthsDistanceTo(CivilDate(2017, 1, 7)))
+            assertEquals(CivilDate(2013, 12, 1), date.monthStartOfMonthsDistance(-7))
+            assertEquals(CivilDate(2014, 6, 1), date.monthStartOfMonthsDistance(-1))
+            assertEquals(CivilDate(2014, 7, 1), date.monthStartOfMonthsDistance(0))
+            assertEquals(CivilDate(2014, 8, 1), date.monthStartOfMonthsDistance(1))
+            assertEquals(CivilDate(2015, 1, 1), date.monthStartOfMonthsDistance(6))
+        }
+
+        (-40..40).forEach {
+            val date = PersianDate(1318, 2, 5)
+            val dateWithDistance = date.monthStartOfMonthsDistance(it)
+            assertEquals(it, date.monthsDistanceTo(dateWithDistance))
+        }
+        (-40..40).forEach {
+            val date = CivilDate(2016, 7, 26)
+            val dateWithDistance = date.monthStartOfMonthsDistance(it)
+            assertEquals(it, date.monthsDistanceTo(dateWithDistance))
+        }
+        (-40..40).forEach {
+            val date = IslamicDate(1440, 7, 26)
+            val dateWithDistance = date.monthStartOfMonthsDistance(it)
+            assertEquals(it, date.monthsDistanceTo(dateWithDistance))
+        }
+    }
 }