Shopify Scripts Buy Set, get X% off script - https://www.shopify.com/partners/blog/109804486-getting-started-with-shopify-scripts-a-practical-walkthrough

shopify-scripts-buy-set-get-x-off-script.rb

################################################################################
# Buy set, get X% off discount script.
#
# This script allows the running of a campaign to give customers X% off a "set"
# of items, if and only if they purchase at least one of each item in the set.
#
# The discount is applied only to all items in the set in the cart.
#
# Written by Gavin Ballard (gavin@discolabs.com)
################################################################################


################################################################################
# START CUSTOMISATION SECTION
#
# You can customise the variables here at the top of the script to adjust the
# products the campaign applies and the message customers will see in their cart
# and at checkout.
################################################################################

# Defines a list of the products that are in the set. Customers will need to
# purchase at least one of each product with this tag to be eligible for the
# discount. Setting this as an empty list (`[]`) will effectively turn off this
# campaign.
PRODUCTS_IN_SET = []

# Set the percentage discount that should be applied if the customer has all
# products in the set in their cart. Will *only* apply to the products in the
# set themselves - other items in the cart won't be affected.
DISCOUNT_PERCENT = 10

# Set the message that will be displayed to customers in their cart and at
# checkout to let them know their discount has been applied.
DISCOUNT_MESSAGE = "#{DISCOUNT_PERCENT}% off for purchasing the set!"


################################################################################
# END CUSTOMISATION SECTION
#
# You shouldn't need to edit anything below this line, unless you're a developer
# and know what you're doing :).
################################################################################


# The campaign class.
class BuySetGetXOffCampaign

  def initialize(selector, discount, partitioner)
    @selector = selector
    @discount = discount
    @partitioner = partitioner
  end

  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

# Select line items by product id.
class ProductsSelector

  def initialize(product_ids)
    @product_ids = Array(product_ids)
  end

  def match?(line_item)
    @product_ids.include?(line_item.variant.product.id)
  end

end

# Apply a percentage discount to a line item.
class PercentageDiscount

  def initialize(percent, message)
    @percent = Decimal.new(percent) / 100.0
    @message = message
  end

  def apply(line_item)
    line_discount = line_item.line_price * @percent
    new_line_price = line_item.line_price - line_discount
    line_item.change_line_price(new_line_price, message: @message)
    puts "Discounted line item with variant #{line_item.variant.id} by #{line_discount}."
  end

end

class ProductSetPartitioner

  def initialize(set_product_ids)
    @set_product_ids = set_product_ids
  end

  def partition(cart, applicable_line_items)
    # If all items in the product set are in the cart, then all applicable line
    # items are eligible for the discount. Otherwise, none of them are.
    cart_product_ids = cart.line_items.map { |line_item| line_item.variant.product.id }.uniq

    if (@set_product_ids - cart_product_ids).empty?
      applicable_line_items
    else
      []
    end
  end

end

# List of campaigns.
CAMPAIGNS = [
  BuySetGetXOffCampaign.new(
    ProductsSelector.new(PRODUCTS_IN_SET),
    PercentageDiscount.new(DISCOUNT_PERCENT, DISCOUNT_MESSAGE),
    ProductSetPartitioner.new(PRODUCTS_IN_SET)
  )
]

# Apply all defined campaiagns to the cart.
CAMPAIGNS.each do |campaign|
  campaign.run(Input.cart)
end

# Export changes.
Output.cart = Input.cart