Added 'on' option for Period

lib/crono/period.rb

@@ -1,19 +1,20 @@
 module Crono
   # Period describe frequency of performing a task
   class Period
-    def initialize(period, at: nil)
+    DAYS = [:monday, :tuesday, :wednesday, :thursday, :friday, :saturday,
+            :sunday]
+
+    def initialize(period, at: nil, on: nil)
       @period = period
       @at_hour, @at_min = parse_at(at) if at
+      @on = parse_on(on) if on
     end
 
     def next(since: nil)
-      if since.nil?
-        @next = Time.now.change(time_atts)
-        return @next if @next.future?
-        since = Time.now
-      end
-
-      @period.since(since).change(time_atts)
+      return initial_next unless since
+      @next = @period.since(since)
+      @next = @next.beginning_of_week.advance(days: @on) if @on
+      @next.change(time_atts)
     end
 
     def description
@@ -22,6 +23,28 @@ module Crono
       desc
     end
 
+    private
+
+    def initial_next
+      next_time = initial_day.change(time_atts)
+      return next_time if next_time.future?
+      @period.from_now.change(time_atts)
+    end
+
+    def initial_day
+      return Time.now unless @on
+      day = Time.now.beginning_of_week.advance(days: @on)
+      return day if day.future?
+      @period.from_now.beginning_of_week.advance(days: @on)
+    end
+
+    def parse_on(on)
+      day_number = DAYS.index(on)
+      fail "Wrong 'on' day" unless day_number
+      fail "period should be at least 1 week to use 'on'" if @period < 1.week
+      day_number
+    end
+
     def parse_at(at)
       case at
       when String
@@ -34,8 +57,6 @@ module Crono
       end
     end
 
-    private
-
     def time_atts
       { hour: @at_hour, min: @at_min }.compact
     end