Shopify Plus - Shopify Script - Buy X products of the same vendor, get 1 free.

buy-X-get-1-free-template.rb

# Buy X products of the same vendor, get 1 free.

class VendorSelector

  def initialize(vendor, tag)
    @vendor = vendor
    @tag = tag
  end

  # Returns whether a or not a line item matches the selector.
  #
  # Example
  # -------
  # Given `VendorSelector.new("Vendor Name Here", "your tag here")` and
  # a line_item with a variant with vendor = "Vendor Name Here" AND tag = "your tag here"
  #
  #   selector.match?(line_item) # returns true
  #
  def match?(line_item)
    (line_item.variant.product.vendor == @vendor) && (line_item.variant.product.tags.include?(@tag))
  end
end


# DiscountPercentage
# ==================
#
# Example
# -------
#   * 100% off the discounted line item
#
class DiscountPercentage

  # Initializes the discount.
  #
  # Arguments
  # ---------
  #
  # * percent
  #   The percentage that the item will be discounted.
  #
  # * message
  #   The console message to show for the discount.
  #
  def initialize(percent, message)
    # Calculate the percentage, making sure decimal values are used for precision.
    #
    @percent = Decimal.new(percent) / 100.0
    @message = message
  end

  # Applies the discount on a line item.
  #
  # Example
  # -------
  # Given `DiscountPercentage.new(10, "Great discount")` and the following line item:
  #
  #   * Quantity = 3, Price = 10
  #
  # The discount will give $1 off per quantity, for a total of $3 off.
  #
  def apply(line_item)
    # Calculate the discount for this line item.
    line_discount = line_item.line_price * @percent

    # Calculate the discounted line price.
    new_line_price = line_item.line_price - line_discount

    # Apply the new line price to this line item.
    # The given message may be displayed in cart pages and confirmation emails to describe the applied discount.
    #
    line_item.change_line_price(new_line_price, message: @message)

    # Print a debugging line to the console.
    puts "Discounted line item with variant #{line_item.variant.id} by #{line_discount}."
  end
end


# LowestPricePartitioner
# ====================
#
# The `LowestPricePartitioner` is used when a specific quantity of items are discounted.
# It discounts the lowest priced items of the specifed quantity of items.
#
# Example
# -------
# Given `LowestPricePartitioner.new(2,1)` and a cart containing the following line items:
#
#   * (A) Quantity = 2, Price = 5
#   * (B) Quantity = 3, Price = 10
#
# The partitioner will:
#
#   * Sort them by ascending price (A, B)
#   * Count the total items to be discounted (1)
#   * Take 1 of A to be discounted
#
# The items to be discounted will be (before discount):
#
#   * (A) Quantity = 1, Price = 5
#
class LowestPricePartitioner

  # Initializes the partitioner.
  #
  # Arguments
  # ---------
  #
  # * paid_item_count
  #   The number of items to skip before selecting items to discount.
  #
  # * discounted_item_count
  #   The number of items to return for discounting.
  #
  # Example
  # -------
  # To create a campaign such as "Buy five, the 6th item is discounted":
  #
  #    LowestPricePartitioner.new(5,1)
  #
  def initialize(paid_item_count, discounted_item_count)
    @paid_item_count = paid_item_count
    @discounted_item_count = discounted_item_count
  end

  # Partitions the items and returns the items that are to be discounted.
  #
  # Arguments
  # ---------
  #
  # * cart
  #   The cart to which split items will be added (typically Input.cart).
  #
  # * line_items
  #   The selected items that are applicable for the campaign.
  #
  # Example
  # -------
  #
  # To create a campaign such that for all items under $5, the 3rd one is discounted:
  #
  #    selected_items = Input.cart.line_items.select{|item| item.variant.price < Money.new(cents: 5_00)}
  #    partitioner = LowestPricePartitioner.new(2,1)
  #    items_to_discount = partitioner.partition(Input.cart, selected_items)
  #
  # After this, the campaign has to apply discounts to `items_to_discount`.
  #
  def partition(cart, applicable_line_items)
    # Sort the items by price from low to high.
    sorted_items = applicable_line_items.sort_by{|line_item| line_item.variant.price}
    # Find the total quantity of items.
    total_applicable_quantity = sorted_items.map(&:quantity).reduce(0, :+)
    # Find the quantity of items that must be discounted.
    discounted_items_remaining = Integer(total_applicable_quantity / (@paid_item_count + @discounted_item_count) * @discounted_item_count)
    # Create an array of items to return.
    discounted_items = []

    # Loop over all the items and find those to be discounted.
    sorted_items.each do |line_item|
      # Exit the loop if all discounted items have been found.
      break if discounted_items_remaining == 0
      # The item will be discounted.
      discounted_item = line_item
      if line_item.quantity > discounted_items_remaining
        # If the item has more quantity than what must be discounted, split it.
        discounted_item = line_item.split(take: discounted_items_remaining)

        # Insert the newly-created item in the cart, right after the original item.
        position = cart.line_items.find_index(line_item)
        cart.line_items.insert(position + 1, discounted_item)
      end

      # Decrement the items left to be discounted.
      discounted_items_remaining -= discounted_item.quantity
      # Add the item to be returned.
      discounted_items.push(discounted_item)
    end

    # Return the items to be discounted.
    discounted_items
  end
end


# BuyGetCampaign
# ==============
#
# Example campaigns
# -----------------
#
#   * Buy five, get one free
#   * Buy one, get one 50% off
#   * Buy two items and get a third for $5 off
#
class BuyGetCampaign

  # Initializes the campaign.
  #
  # Arguments
  # ---------
  #
  # * selector
  #   The selector finds eligible items for this campaign.
  #
  # * discount
  #   The discount changes the prices of the items returned by the partitioner.
  #
  # * partitioner
  #   The partitioner takes all applicable items, and returns only those that are to be discounted.
  #   In a "Buy five items, get the sixth for free" campaign,
  #   the partitioner would skip five items and return the sixth item.
  #
  def initialize(selector, discount, partitioner)
    @selector = selector
    @discount = discount
    @partitioner = partitioner
  end

  # Runs the campaign on the given cart.
  #
  # Example
  # -------
  # To run the campaign on the input cart:
  #
  #    campaign.run(Input.cart)
  #
  def run(cart)
    applicable_items = cart.line_items.select do |line_item|
      @selector.match?(line_item)
    end
    discounted_items = @partitioner.partition(cart, applicable_items)

    discounted_items.each do |line_item|
      @discount.apply(line_item)
    end
  end
end


# Use an array to keep track of the discount campaigns desired.
# The CAMPAIGNS settings here give every 6th item with the same vendor "Vendor Name Here" for free. (Change these settings accordingly.)
CAMPAIGNS = [
  BuyGetCampaign.new(
    VendorSelector.new("Vendor Name Here", "your tag here"),
    DiscountPercentage.new(100, "6TH ITEM IS FREE!"),
    LowestPricePartitioner.new(5,1),
  )
]

# Iterate through each of the discount campaigns.
CAMPAIGNS.each do |campaign|
  # Apply the campaign onto the cart.
  campaign.run(Input.cart)
end


# In order to reflect the changes to the line items, the output of the script needs to be specified.
Output.cart = Input.cart