szTheory/streakable

# Represents a streak of calendar days as computed
# by a date column on an association.
#
# So for example if you have a User that has_many :posts, then
# +Streak.new(user, :posts, :created_at).length+ will tell you how many
# consecutive days a given user created posts.
class Streak
  # the base ActiveRecord object instance for this streak calculation
  attr_reader :instance

  # the AR association through which we want to grab a column to caculate a streak
  attr_reader :association

  # an AR column resolving to a date. the column that we want to calculate a calendar date streak against
  attr_reader :column

  # whether to include today in the streak length 
  # calculation or not. If this is true, then you are assuming there 
  # is still time today for the streak to be extended
  attr_reader :except_today

  # Creates a new Streak
  # 
  # @param [ActiveRecord::Base] instance an ActiveRecord object instance
  # @param [Symbol] association a key representing the ActiveRecord association on the instance
  # @param [Symbol] column a key representing the column on the association that you want to calculate the streak against
  # @param [Boolean] except_today whether to include today in the streak length calculation or not. If this is true, then you are assuming there is still time today for the streak to be extended
  def initialize(instance, association, column, except_today: false)
    @instance = instance
    @association = association
    @column = column
    # Don't penalize the current day being absent when determining streaks
    @except_today = except_today
  end

  # Calculate the length of this calendar day streak
  # 
  # @param [Boolean] longest if true, calculate the longest day streak in the sequence, 
  # not just the current one
  def length(longest: false)
    # no streaks
    if streaks.empty?
      0

    # calculate the longest one?
    elsif longest
      streaks.sort do |x, y|
        y.size <=> x.size
      end.first.size

    # default streak calculation
    else
      # pull the first streak
      streak = streaks.first
      
      # either the streak includes today,
      # or we don't care about today and it includes yesterday
      if streak.include?(Date.current) || except_today && streak.include?(Date.current - 1.day)
        streak.size
      else
        0
      end
    end
  end

  # Get a list of all calendar day streaks, sorted descending 
  # (from most recent to farthest away)
  # Includes 1-day streaks. If you want to filter
  # the results further, for example if you want 2 only
  # include 2+ day streaks, you'll have to filter on the result
  def streaks
    return [] if days.empty?

    streaks = []
    streak = []
    days.each.with_index do |day, i|
      # first day
      if i == 0
        # since this is the first one,
        # push to our new streak
        streak << day

      # consecutive day, the previous day was "tomorrow" 
      # relative to day (since we're date descending)
      elsif days[i-1] == (day+1.day)
        # push to existing streak
        streak << day

      # streak was broken
      else
        # push our current streak
        streaks << streak

        # start a new streak
        # and push day to our new streak
        streak = []
        streak << day
      end

      # the jig is up, push the current streak
      if i == (days.size-1) 
        streaks << streak 
      end
    end
   
    streaks
  end

  # TODO: add class methods/scopes to calculate streaks, days
  # scrap code from old method below:
  # 
  # date_strings = instance.send(association).order(column => :desc).pluck(column)
  # dates = date_strings.map(&:to_date)
  # dates.sort.reverse.uniq

  private
    def days
      @days ||= begin
        instance.send(association).map do |x|
          x.send(column).in_time_zone.to_date
        end.sort do |x, y|
          x <=> y
        end.reverse.uniq
      end
    end
end