As you may recall, before life (real life, not Conway's) got rather busy, I left you hanging off the cliff of Dave Thomas's Code Kata #9. I had finished the "just make it work" part. Now for the "decouple the price rule format" part.
As Dave (the other Dave) says, there are many ways to do it. I chose to use the Strategy Pattern. (If you're not familiar with design patterns, you really need to go read up on them.)
As you can see in the code below, the piece that gets swapped out is actually very small. It does nothing but parse the pricing rules. How they get applied remains the same.
#! /usr/bin/ruby
# Dave Thomas Code Kata #9: Supermarket Checkout
# See http://codekata.pragprog.com/2007/01/kata_nine_back_.html
# Solution by Dave Aronson
# VERSION B: decouple the pricing format.
class CheckOut
def initialize rules, parser
@items = {}
# only really need this in total, but IRL a customer
# might just want a price-check on something....
@pricer = Pricer.new rules, parser
end
def scan sku
@items[sku] = 0 if @items[sku].nil?
@items[sku] += 1
end
def total
tot = 0
@items.each_pair { |sku, qty| tot += @pricer.price sku, qty }
tot
end
end
# separate from CheckOut 'cuz IRL there may be price-check stations
class Pricer
def initialize rules, parser
@prices = parser.parse rules
end
def price sku, quantity
qty = quantity
tot = 0
while qty > 0 do
deal_qty, deal_price = find_best_deal sku, qty
num_deals = qty / deal_qty
tot += num_deals * deal_price
qty -= num_deals * deal_qty
end
tot
end
private
def find_best_deal sku, qty
prices = @prices[sku]
if prices.nil?
puts "ERROR: NO PRICE FOUND for #{sku}!"
return nil
end
deal = nil
prices.each_pair do |rule_qty, rule_tot|
if rule_qty > qty then
break
else
# Inefficient to keep overwriting this,
# but shouldn't happen too much....
deal = [rule_qty, rule_tot]
end
end
puts "ERROR: NO PRICE FOUND for #{qty} of #{sku}!" if deal.nil?
deal
end
end
# now we get to the heart of the matter:
# different ways to parse prices.
# essentially the strategy pattern.
class CSVPriceParser
def parse rules
prices = {}
rules.each do |line|
line.chomp!
cur_sku, qty, tot = line.split ","
prices[cur_sku] = {} if prices[cur_sku].nil?
prices[cur_sku][qty.to_i] = tot.to_i
end
prices
end
end
CSV_RULES = [
"A,1,50",
"A,3,130",
"B,1,30",
"B,2,45",
"C,1,20",
"D,1,15",
]
class YAMLPriceParser
def parse rules
prices = {}
cur_sku = ''
rules.each do |line|
line.chomp!
if line[0..0] == "\t"
qty, tot = line[1..-1].split "\t" # 1 to ignore tab
# really should check for cur_sku not being set yet,
# though that would mean it's an invalid file....
prices[cur_sku] = {} if prices[cur_sku].nil?
prices[cur_sku][qty.to_i] = tot.to_i
else cur_sku = line
end
end
prices
end
end
YAML_RULES = [
"A",
"\t1\t50",
"\t3\t130",
"B",
"\t1\t30",
"\t2\t45",
"C",
"\t1\t20",
"D",
"\t1\t15",
]
# TESTS, initially supplied by Dave Thomas,
# but refactored by Dave Aronson
# to support swapping out strategies
require 'test/unit'
class TestPrice < Test::Unit::TestCase
def helper_test_incremental rules, parser
co = CheckOut.new rules, parser
assert_equal 0, co.total
co.scan "A"; assert_equal( 50, co.total)
co.scan "B"; assert_equal( 80, co.total)
co.scan "A"; assert_equal(130, co.total)
co.scan "A"; assert_equal(160, co.total)
co.scan "B"; assert_equal(175, co.total)
end
def helper_test_totals rules, parser
assert_equal( 0, price("", rules, parser))
assert_equal( 50, price("A", rules, parser))
assert_equal( 80, price("AB", rules, parser))
assert_equal(115, price("CDBA", rules, parser))
assert_equal(100, price("AA", rules, parser))
assert_equal(130, price("AAA", rules, parser))
assert_equal(180, price("AAAA", rules, parser))
assert_equal(230, price("AAAAA", rules, parser))
assert_equal(260, price("AAAAAA", rules, parser))
assert_equal(160, price("AAAB", rules, parser))
assert_equal(175, price("AAABB", rules, parser))
assert_equal(190, price("AAABBD", rules, parser))
assert_equal(190, price("DABABA", rules, parser))
end
def price goods, rules, parser
co = CheckOut.new rules, parser
goods.split(//).each { |item| co.scan(item) }
co.total
end
def helper_test rules, parser
helper_test_incremental rules, parser
helper_test_totals rules, parser
end
def test_rulesets
helper_test CSV_RULES, CSVPriceParser.new
helper_test YAML_RULES, YAMLPriceParser.new
end
endAs always (just had to continue starting each paragraph with "As"!), it's now your turn. How would you have designed and/or implemented this differently? Leave a comment.
