bug fix add month algorithm and add a test to check it

src/Traits/AdditionAndSubstraction.php

@@ -11,19 +11,23 @@ public function addYear($count)
  $date = $this->currentCalendar->timestampToDate($this->timestamp + $this->timezoneOffset);
  $timestamp = $this->currentCalendar->dateToTimestamp($date->year + $count, $date->month, $date->day, $date->hour, $date->minute, $date->second);
  $this->timestamp = $timestamp - $this->timezoneOffset;
- 
+
  return $this;
  }
 
  public function addMonth($count)
  {
  $date = $this->currentCalendar->timestampToDate($this->timestamp + $this->timezoneOffset);
  $totalMonth = $date->month + $count;
- $year = $date->year + floor($totalMonth / 12);
- $month = $totalMonth % 12;
- $timestamp = $this->currentCalendar->dateToTimestamp($year, $month, $date->day, $date->hour, $date->minute, $date->second);
+ $year = $date->year + floor($totalMonth / 12) - (($totalMonth % 12) == 0 ? 1 : 0);
+ $month = ($totalMonth % 12) == 0 ? 12 : ($totalMonth % 12);
+
+ $daysInMonth = $this->currentCalendar->daysInMonth($year, $month);
+ $day = $date->day <= $daysInMonth ? $date->day : $daysInMonth;
+
+ $timestamp = $this->currentCalendar->dateToTimestamp($year, $month, $day, $date->hour, $date->minute, $date->second);
  $this->timestamp = $timestamp - $this->timezoneOffset;
- 
+
  return $this;
  }
 
@@ -107,7 +111,7 @@ public function subMinute($count)
  public function subSecond($count)
  {
  $this->timestamp = $this->timestamp - $count;
- 
+
  return $this;
  }
 }

tests/CalendarTest.php

@@ -22,8 +22,9 @@ public function testJalaliDateTime()
  $this->assertTrue($pasoonate->jalali()->setDate(1399,12,11)->daysInMonth() === 30);
 
  $julianDayNumber = $pasoonate->jalali()->setDate(1399,12,11)->getJulianDayNumber();
-
  $this->assertTrue($pasoonate->jalali()->setJulianDayNumber($julianDayNumber)->gregorian()->getJulianDayNumber() == $julianDayNumber);
+
+ $this->assertEquals($pasoonate->jalali()->setDate(1399,1,31)->addMonth(11)->format('yyyy/MM/dd'), '1399/12/30');
  }
 
  public function testGregorianDateTime()