[#764] New practice exercise book-store (#778)

Co-authored-by: Angelika Tyborska <angelikatyborska@fastmail.com>

Author: jiegillet

config.json

@@ -2490,6 +2490,27 @@
           "strings"
         ],
         "difficulty": 5
+      },
+      {
+        "slug": "book-store",
+        "name": "Book Store",
+        "uuid": "d6d9687c-8bf0-482e-8309-4f3d706ba753",
+        "prerequisites": [
+          "lists",
+          "pattern-matching",
+          "guards",
+          "multiple-clause-functions",
+          "enum",
+          "list-comprehensions",
+          "maps",
+          "pipe-operator",
+          "ranges",
+          "recursion"
+        ],
+        "practices": [
+          "list-comprehensions"
+        ],
+        "difficulty": 8
       }
     ],
     "foregone": [

exercises/practice/book-store/.docs/instructions.md

@@ -0,0 +1,68 @@
+# Description
+
+To try and encourage more sales of different books from a popular 5 book
+series, a bookshop has decided to offer discounts on multiple book purchases.
+
+One copy of any of the five books costs $8.
+
+If, however, you buy two different books, you get a 5%
+discount on those two books.
+
+If you buy 3 different books, you get a 10% discount.
+
+If you buy 4 different books, you get a 20% discount.
+
+If you buy all 5, you get a 25% discount.
+
+Note: that if you buy four books, of which 3 are
+different titles, you get a 10% discount on the 3 that
+form part of a set, but the fourth book still costs $8.
+
+Your mission is to write a piece of code to calculate the
+price of any conceivable shopping basket (containing only
+books of the same series), giving as big a discount as
+possible.
+
+For example, how much does this basket of books cost?
+
+- 2 copies of the first book
+- 2 copies of the second book
+- 2 copies of the third book
+- 1 copy of the fourth book
+- 1 copy of the fifth book
+
+One way of grouping these 8 books is:
+
+- 1 group of 5 --> 25% discount (1st,2nd,3rd,4th,5th)
+- +1 group of 3 --> 10% discount (1st,2nd,3rd)
+
+This would give a total of:
+
+- 5 books at a 25% discount
+- +3 books at a 10% discount
+
+Resulting in:
+
+- 5 x (8 - 2.00) == 5 x 6.00 == $30.00
+- +3 x (8 - 0.80) == 3 x 7.20 == $21.60
+
+For a total of $51.60
+
+However, a different way to group these 8 books is:
+
+- 1 group of 4 books --> 20% discount  (1st,2nd,3rd,4th)
+- +1 group of 4 books --> 20% discount  (1st,2nd,3rd,5th)
+
+This would give a total of:
+
+- 4 books at a 20% discount
+- +4 books at a 20% discount
+
+Resulting in:
+
+- 4 x (8 - 1.60) == 4 x 6.40 == $25.60
+- +4 x (8 - 1.60) == 4 x 6.40 == $25.60
+
+For a total of $51.20
+
+And $51.20 is the price with the biggest discount.

exercises/practice/book-store/.formatter.exs

@@ -0,0 +1,4 @@
+# Used by "mix format"
+[
+  inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"]
+]

exercises/practice/book-store/.meta/config.json

@@ -0,0 +1,18 @@
+{
+  "authors": ["jiegillet"],
+  "contributors": [],
+  "files": {
+    "example": [
+      ".meta/example.ex"
+    ],
+    "solution": [
+      "lib/book_store.ex"
+    ],
+    "test": [
+      "test/book_store_test.exs"
+    ]
+  },
+  "blurb": "To try and encourage more sales of different books from a popular 5 book series, a bookshop has decided to offer discounts of multiple-book purchases.",
+  "source": "Inspired by the harry potter kata from Cyber-Dojo.",
+  "source_url": "http://cyber-dojo.org"
+}

exercises/practice/book-store/.meta/example.ex

@@ -0,0 +1,67 @@
+defmodule BookStore do
+  use Agent
+  @type book :: integer
+
+  @base_price 800
+  @discount %{
+    1 => @base_price,
+    2 => div(@base_price * 95, 100),
+    3 => div(@base_price * 90, 100),
+    4 => div(@base_price * 80, 100),
+    5 => div(@base_price * 75, 100)
+  }
+
+  @doc """
+  Calculate lowest price (in cents) for a shopping basket containing books.
+  """
+  @spec total(basket :: [book]) :: integer
+  def total(basket) do
+    # We use an Agent to memoize the discounts
+    {:ok, _pid} = Agent.start_link(fn -> %{} end, name: :discount)
+
+    basket
+    |> frequencies
+    |> Enum.sort()
+    |> discounts()
+    |> Enum.min()
+  end
+
+  def frequencies(list) do
+    list
+    |> Enum.map(&%{&1 => 1})
+    |> Enum.reduce(%{}, &Map.merge(&1, &2, fn _item, a, b -> a + b end))
+    |> Map.values()
+  end
+
+  def discounts([]), do: [0]
+  def discounts([n]), do: [n * @base_price]
+
+  def discounts(frequencies) do
+    cached = Agent.get(:discount, &Map.get(&1, frequencies))
+
+    if cached do
+      cached
+    else
+      discount =
+        for num_books <- 2..5,
+            {picked, left} <- pick_n_distinct_from(num_books, frequencies),
+            remove_picked = picked |> Enum.map(&(&1 - 1)) |> Enum.reject(&(&1 == 0)),
+            costs <- discounts(Enum.sort(remove_picked ++ left)),
+            do: costs + num_books * @discount[num_books]
+
+      Agent.update(:discount, &Map.put(&1, frequencies, discount))
+      discount
+    end
+  end
+
+  def pick_n_distinct_from(n, list) when length(list) < n, do: %{}
+  def pick_n_distinct_from(0, list), do: %{[] => list}
+  def pick_n_distinct_from(n, list) when length(list) == n, do: %{list => []}
+
+  def pick_n_distinct_from(n, list) do
+    for pick <- Enum.uniq(list),
+        {picked, left} <- pick_n_distinct_from(n - 1, list -- [pick]),
+        into: %{},
+        do: {Enum.sort([pick | picked]), left}
+  end
+end

exercises/practice/book-store/.meta/tests.toml

@@ -0,0 +1,54 @@
+# This is an auto-generated file.
+#
+# Regenerating this file via `configlet sync` will:
+# - Recreate every `description` key/value pair
+# - Recreate every `reimplements` key/value pair, where they exist in problem-specifications
+# - Remove any `include = true` key/value pair (an omitted `include` key implies inclusion)
+# - Preserve any other key/value pair
+#
+# As user-added comments (using the # character) will be removed when this file
+# is regenerated, comments can be added via a `comment` key.
+[17146bd5-2e80-4557-ab4c-05632b6b0d01]
+description = "Only a single book"
+
+[cc2de9ac-ff2a-4efd-b7c7-bfe0f43271ce]
+description = "Two of the same book"
+
+[5a86eac0-45d2-46aa-bbf0-266b94393a1a]
+description = "Empty basket"
+
+[158bd19a-3db4-4468-ae85-e0638a688990]
+description = "Two different books"
+
+[f3833f6b-9332-4a1f-ad98-6c3f8e30e163]
+description = "Three different books"
+
+[1951a1db-2fb6-4cd1-a69a-f691b6dd30a2]
+description = "Four different books"
+
+[d70f6682-3019-4c3f-aede-83c6a8c647a3]
+description = "Five different books"
+
+[78cacb57-911a-45f1-be52-2a5bd428c634]
+description = "Two groups of four is cheaper than group of five plus group of three"
+
+[f808b5a4-e01f-4c0d-881f-f7b90d9739da]
+description = "Two groups of four is cheaper than groups of five and three"
+
+[fe96401c-5268-4be2-9d9e-19b76478007c]
+description = "Group of four plus group of two is cheaper than two groups of three"
+
+[68ea9b78-10ad-420e-a766-836a501d3633]
+description = "Two each of first 4 books and 1 copy each of rest"
+
+[c0a779d5-a40c-47ae-9828-a340e936b866]
+description = "Two copies of each book"
+
+[18fd86fe-08f1-4b68-969b-392b8af20513]
+description = "Three copies of first book and 2 each of remaining"
+
+[0b19a24d-e4cf-4ec8-9db2-8899a41af0da]
+description = "Three each of first 2 books and 2 each of remaining books"
+
+[bb376344-4fb2-49ab-ab85-e38d8354a58d]
+description = "Four groups of four are cheaper than two groups each of five and three"

exercises/practice/book-store/lib/book_store.ex

@@ -0,0 +1,11 @@
+defmodule BookStore do
+  @typedoc "A book is represented by its number in the 5-book series"
+  @type book :: 1 | 2 | 3 | 4 | 5
+
+  @doc """
+  Calculate lowest price (in cents) for a shopping basket containing books.
+  """
+  @spec total(basket :: [book]) :: integer
+  def total(basket) do
+  end
+end

exercises/practice/book-store/mix.exs

@@ -0,0 +1,28 @@
+defmodule BookStore.MixProject do
+  use Mix.Project
+
+  def project do
+    [
+      app: :book_store,
+      version: "0.1.0",
+      # elixir: "~> 1.8",
+      start_permanent: Mix.env() == :prod,
+      deps: deps()
+    ]
+  end
+
+  # Run "mix help compile.app" to learn about applications.
+  def application do
+    [
+      extra_applications: [:logger]
+    ]
+  end
+
+  # Run "mix help deps" to learn about dependencies.
+  defp deps do
+    [
+      # {:dep_from_hexpm, "~> 0.3.0"},
+      # {:dep_from_git, git: "https://github.com/elixir-lang/my_dep.git", tag: "0.1.0"}
+    ]
+  end
+end