How to create an infinite enumerable of Times? - ruby

Question

I want to be able to have an object extend Enumerable in Ruby to be an infinite list of Mondays (for example).
So it would yield: March 29, April 5, April 12...... etc
How can I implement this in Ruby?

Answer 1

In 1.9 (and probably previous versions using backports), you can easily create enumerator:
require 'date'
def ndays_from(from, step=7)
Enumerator.new {|y|
loop {
y.yield from
from += step
}
}
end
e = ndays_from(Date.today)
p e.take(5)
#=> [#<Date: 2010-03-25 (4910561/2,0,2299161)>, #<Date: 2010-04-01 (4910575/2,0,2299161)>, #<Date: 2010-04-08 (4910589/2,0,2299161)>, #<Date: 2010-04-15 (4910603/2,0,2299161)>, #<Date: 2010-04-22 (4910617/2,0,2299161)>]

Answer 2

Store a Date as instance variable, initialized to a Monday. You would implement an each method which increments the stored date by 7 days using date += 7.

Answer 3

You could do something by extending Date...
#!/usr/bin/ruby
require 'date'
class Date
def current_monday
self - self.wday + 1
end
def next_monday
self.current_monday + 7
end
end
todays_date = Date.today
current_monday = todays_date.current_monday
3.times do |i|
puts current_monday.to_s
current_monday = current_monday.next_monday
end
2010-03-22
2010-03-29
2010-04-05
2010-04-12
...with the usual warnings about extending base classes of course.

Answer 4

You can extend Date class with nw method mondays
class Date
def self.mondays(start_date=Date.today, count=10)
monday = start_date.wday > 1 ? start_date - start_date.wday + 8 : start_date - start_date.wday + 1
mondays = []
count.times { |i| mondays << monday + i*7}
mondays
end
end
Date.mondays will return by default Array of mondays with 10 elements from closest monday to Date.today. You can pass parameters:
Date.mondays(start_date:Date, count:Integer)
start_date - start point to find closest monday
count - number of mondays you are looking
IE:
Date.mondays(Date.parse('11.3.2002'))
Date.mondays(Date.parse('11.3.2002'), 30)

Answer 5

module LazyEnumerable
extend Enumerable
def select(&block)
lazily_enumerate { |enum, value| enum.yield(value) if
block.call(value) }
end
def map(&block)
lazily_enumerate {|enum, value| enum.yield(block.call(value))}
end
def collect(&block)
map(&block)
end
private
def lazily_enumerate(&block)
Enumerator.new do |enum|
self.each do |value|
block.call(enum, value)
end
end
end
end
...........
class LazyInfiniteDays
include LazyEnumerable
attr_reader :day
def self.day_of_week
dow = { :sundays => 0, :mondays => 1, :tuesdays => 2, :wednesdays =>
3, :thursdays => 4, :fridays => 5, :saturdays => 6, :sundays => 7 }
dow.default = -10
dow
end
DAY_OF_WEEK = day_of_week()
def advance_to_midnight_of_next_specified_day(day_sym)
year = DateTime.now.year
month = DateTime.now.month
day_of_month = DateTime.now.day
output_day = DateTime.civil(year, month, day_of_month)
output_day += 1 until output_day.wday == DAY_OF_WEEK[day_sym]
output_day
end
def initialize(day_sym)
#day = advance_to_midnight_of_next_specified_day(day_sym)
end
def each
day = #day.dup
loop {
yield day
day += 7
}
end
def ==(other)
return false unless other.kind_of? LazyInfiniteDays
#day.wday == other.day.wday
end
end

Answer 6

Ruby 2.7 introduced Enumerator#produce for creating an infinite enumerator from any block, which results in a very elegant, very functional way of implementing the original problem:
irb(main):001:0> require 'date'
=> true
irb(main):002:0> puts Date.today
2022-09-23
=> nil
irb(main):003:0> Date.today.friday?
=> true
irb(main):004:0> future_mondays = Enumerator.produce { |date|
date = (date || Date.today).succ
date = date.succ until date.monday?
date
}
=> #<Enumerator: #<Enumerator::Producer:0x00007fa4300b3070>:each>
irb(main):005:0> puts future_mondays.first(5)
2022-09-26
2022-10-03
2022-10-10
2022-10-17
2022-10-24
=> nil
irb(main):006:0> _