feat(semester-service): add service that delivers semester date ranges and lists for forms

app/services/semester.rb

+class Semester
+  # params:
+  #   context - default is now, but optionally any other point in time
+  def initialize(year = nil, semester_number = nil)
+    @context = if year && semester_number == 2
+                 Time.zone.local(year, 6, 1)
+               elsif year && semester_number == 1
+                 Time.zone.local(year - 1, 12, 1)
+               else
+                 Time.zone.now
+               end
+  end
+
+  def number(date_time = nil)
+    if (6..11).cover?(date_time&.month || @context.month)
+      2
+    else
+      1
+    end
+  end
+
+  def year_number(count: 1, direction: :previous, set_semester: nil)
+    string_semester = set_semester || public_send(direction, count)
+    "#{string_semester.end.year},#{number(string_semester.end)}"
+  end
+
+  # running semester where in now
+  def current
+    @current ||= semester_range_from_start(semester_start_time(@context))
+  end
+
+  # params:
+  #   count - shift of semesters - only positive integers
+  def previous(count = 1)
+    return current if count == 0
+
+    current.begin.advance(months: -(6 * count))..current.end.advance(months: -(6 * count))
+  end
+
+  # params:
+  #   count - shift of semesters - only positive integers
+  def next(count = 1)
+    return current if count == 0
+
+    current.begin.advance(months: 6 * count)..current.end.advance(months: 6 * count)
+  end
+
+  # list of semester datetime ranges
+  #
+  # params:
+  #   count - amount of semesters
+  #   direction - [:next || :previous]
+  #   with_current - include current semester as first in array
+  def list(count = 3, direction = :previous, with_current: true)
+    list = (1..count).to_a.map do |step|
+      public_send(direction, step)
+    end
+    if with_current
+      [current] + list
+    else
+      list
+    end
+  end
+
+  def collection(count = 3, direction = :previous, with_current: true)
+    list(count, direction, with_current: with_current).map do |semester|
+      semester_number = number(semester.end)
+      semester_year = semester.end.year
+      [
+        "#{semester_number}. Semester #{semester_year} (#{I18n.l(semester.begin.to_date)} - #{I18n.l(semester.end.to_date)})",
+        year_number(set_semester: semester)
+      ]
+    end
+  end
+
+  private
+
+  def semester_start_time(date_time)
+    if (6..11).cover?(date_time.month)
+      Time.zone.local(date_time.year, 6, 1)
+    elsif date_time.month == 12
+      Time.zone.local(date_time.year, 12, 1)
+    else
+      Time.zone.local(date_time.year - 1, 12, 1)
+    end
+  end
+
+  def semester_range_from_start(date_time)
+    date_time..date_time.advance(months: 5).end_of_month
+  end
+end