added the book-store exercise

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

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+# Instructions
+
+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 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 (1st, 2nd,3rd, 4th, 5th)
+- 1 group of 3 (1st, 2nd, 3rd)
+
+This would give a total of:
+
+- 5 books at a 25% discount
+- 3 books at a 10% discount
+
+Resulting in:
+
+- 5 × (100% - 25%) × $8 = 5 × $6.00 = $30.00, plus
+- 3 × (100% - 10%) × $8 = 3 × $7.20 = $21.60
+
+Which equals $51.60.
+
+However, a different way to group these 8 books is:
+
+- 1 group of 4 books (1st, 2nd, 3rd, 4th)
+- 1 group of 4 books (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 × (100% - 20%) × $8 = 4 × $6.40 = $25.60, plus
+- 4 × (100% - 20%) × $8 = 4 × $6.40 = $25.60
+
+Which equals $51.20.
+
+And $51.20 is the price with the biggest discount.

exercises/practice/book-store/.gitignore

@@ -0,0 +1,4 @@
+*.beam
+*.ez
+build
+erl_crash.dump

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

@@ -0,0 +1,23 @@
+{
+  "authors": [
+    "massivefermion"
+  ],
+  "files": {
+    "solution": [
+      "src/book_store.gleam"
+    ],
+    "test": [
+      "test/book_store_test.gleam"
+    ],
+    "example": [
+      ".meta/example.gleam"
+    ],
+    "invalidator": [
+      "gleam.toml",
+      "manifest.toml"
+    ]
+  },
+  "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": "https://cyber-dojo.org"
+}

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

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+import gleam/int
+import gleam/list
+import gleam/float
+
+pub fn lowest_price(books: List(Int)) -> Float {
+  books
+  |> list.fold(
+    [#(1, 0), #(2, 0), #(3, 0), #(4, 0), #(5, 0)],
+    fn(acc, book) {
+      let assert Ok(current) = list.key_find(acc, book)
+      list.key_set(acc, book, current + 1)
+    },
+  )
+  |> list.map(fn(item) { item.1 })
+  |> list.sort(int.compare)
+  |> calculate(0.0)
+}
+
+fn calculate(counts: List(Int), acc: Float) -> Float {
+  case counts {
+    [0, 0, 0, 0, a] -> acc +. int.to_float(a) *. 800.0
+
+    [0, 0, 0, a, b] -> calculate([0, 0, 0, a - 1, b - 1], acc +. 2.0 *. 760.0)
+
+    [0, 0, a, b, c] ->
+      calculate([0, 0, a - 1, b - 1, c - 1], acc +. 3.0 *. 720.0)
+
+    [0, a, b, c, d] ->
+      calculate([0, a - 1, b - 1, c - 1, d - 1], acc +. 4.0 *. 640.0)
+
+    [a, b, c, d, e] ->
+      float.min(
+        calculate([a - 1, b - 1, c - 1, d - 1, e - 1], acc +. 5.0 *. 600.0),
+        calculate(
+          list.sort([a, b - 1, c - 1, d - 1, e - 1], int.compare),
+          acc +. 4.0 *. 640.0,
+        ),
+      )
+  }
+}

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

@@ -0,0 +1,64 @@
+# 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 four books and one 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 two each of remaining"
+
+[0b19a24d-e4cf-4ec8-9db2-8899a41af0da]
+description = "Three each of first two books and two each of remaining books"
+
+[bb376344-4fb2-49ab-ab85-e38d8354a58d]
+description = "Four groups of four are cheaper than two groups each of five and three"
+
+[5260ddde-2703-4915-b45a-e54dbbac4303]
+description = "Check that groups of four are created properly even when there are more groups of three than groups of five"
+
+[b0478278-c551-4747-b0fc-7e0be3158b1f]
+description = "One group of one and four is cheaper than one group of two and three"
+
+[cf868453-6484-4ae1-9dfc-f8ee85bbde01]
+description = "One group of one and two plus three groups of four is cheaper than one group of each size"

exercises/practice/book-store/gleam.toml

@@ -0,0 +1,11 @@
+name = "book_store"
+version = "0.1.0"
+
+[dependencies]
+gleam_bitwise = "~> 1.3"
+gleam_otp = "~> 0.7"
+gleam_stdlib = "~> 0.31"
+simplifile = "~> 0.1"
+
+[dev-dependencies]
+exercism_test_runner = "~> 1.4"

exercises/practice/book-store/manifest.toml

@@ -0,0 +1,25 @@
+# This file was generated by Gleam
+# You typically do not need to edit this file
+
+packages = [
+  { name = "exercism_test_runner", version = "1.4.0", build_tools = ["gleam"], requirements = ["gleam_community_ansi", "simplifile", "gleam_erlang", "gleam_json", "gap", "glance", "gleam_stdlib"], otp_app = "exercism_test_runner", source = "hex", outer_checksum = "336FBF790841C2DC25EB77B35E76A09EFDB9771D7D813E0FDBC71A50CB79711D" },
+  { name = "gap", version = "0.7.0", build_tools = ["gleam"], requirements = ["gleam_stdlib", "gleam_community_ansi"], otp_app = "gap", source = "hex", outer_checksum = "AF290C27B3FAE5FE64E1B7E9C70A9E29AA0F42429C0592D375770C1C51B79D36" },
+  { name = "glance", version = "0.7.1", build_tools = ["gleam"], requirements = ["gleam_stdlib", "glexer"], otp_app = "glance", source = "hex", outer_checksum = "B646A08970990D9D7A103443C5CD46F9D4297BF05F188767777FCC14ADF395EA" },
+  { name = "gleam_bitwise", version = "1.3.0", build_tools = ["gleam"], requirements = [], otp_app = "gleam_bitwise", source = "hex", outer_checksum = "E2A46EE42E5E9110DAD67E0F71E7358CBE54D5EC22C526DD48CBBA3223025792" },
+  { name = "gleam_community_ansi", version = "1.1.0", build_tools = ["gleam"], requirements = ["gleam_bitwise", "gleam_community_colour", "gleam_stdlib"], otp_app = "gleam_community_ansi", source = "hex", outer_checksum = "6E4E0CF2B207C1A7FCD3C21AA43514D67BC7004F21F82045CDCCE6C727A14862" },
+  { name = "gleam_community_colour", version = "1.1.0", build_tools = ["gleam"], requirements = ["gleam_stdlib", "gleam_bitwise"], otp_app = "gleam_community_colour", source = "hex", outer_checksum = "D27CE357ECB343929A8CEC3FBA0B499943A47F0EE1F589EE16AFC2DC21C61E5B" },
+  { name = "gleam_erlang", version = "0.22.0", build_tools = ["gleam"], requirements = ["gleam_stdlib"], otp_app = "gleam_erlang", source = "hex", outer_checksum = "367D8B41A7A86809928ED1E7E55BFD0D46D7C4CF473440190F324AFA347109B4" },
+  { name = "gleam_json", version = "0.6.0", build_tools = ["gleam"], requirements = ["thoas", "gleam_stdlib"], otp_app = "gleam_json", source = "hex", outer_checksum = "C6CC5BEECA525117E97D0905013AB3F8836537455645DDDD10FE31A511B195EF" },
+  { name = "gleam_otp", version = "0.7.0", build_tools = ["gleam"], requirements = ["gleam_stdlib", "gleam_erlang"], otp_app = "gleam_otp", source = "hex", outer_checksum = "ED7381E90636E18F5697FD7956EECCA635A3B65538DC2BE2D91A38E61DCE8903" },
+  { name = "gleam_stdlib", version = "0.31.0", build_tools = ["gleam"], requirements = [], otp_app = "gleam_stdlib", source = "hex", outer_checksum = "6D1BC5B4D4179B9FEE866B1E69FE180AC2CE485AD90047C0B32B2CA984052736" },
+  { name = "glexer", version = "0.6.2", build_tools = ["gleam"], requirements = ["gleam_stdlib"], otp_app = "glexer", source = "hex", outer_checksum = "703D2347F5180B2BCEA4D258549B0D91DACD0905010892BAC46D04D913B84D1F" },
+  { name = "simplifile", version = "0.1.14", build_tools = ["gleam"], requirements = ["gleam_stdlib"], otp_app = "simplifile", source = "hex", outer_checksum = "10EA0207796F20488A3A166C50A189C9385333F3C9FAC187729DE7B9CE4ADDBC" },
+  { name = "thoas", version = "0.4.1", build_tools = ["rebar3"], requirements = [], otp_app = "thoas", source = "hex", outer_checksum = "4918D50026C073C4AB1388437132C77A6F6F7C8AC43C60C13758CC0ADCE2134E" },
+]
+
+[requirements]
+exercism_test_runner = { version = "~> 1.4" }
+gleam_bitwise = { version = "~> 1.3" }
+gleam_otp = { version = "~> 0.7" }
+gleam_stdlib = { version = "~> 0.31" }
+simplifile = { version = "~> 0.1" }

exercises/practice/book-store/src/book_store.gleam

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+pub fn lowest_price(basket: List(Int)) {
+  todo
+}

exercises/practice/book-store/test/book_store_test.gleam

@@ -0,0 +1,151 @@
+import exercism/test_runner
+import exercism/should
+import book_store
+
+pub fn main() {
+  test_runner.main()
+}
+
+pub fn only_a_single_book_test() {
+  let input = [1]
+  let output = book_store.lowest_price(input)
+  let expected = 800.0
+  output
+  |> should.equal(expected)
+}
+
+pub fn two_of_the_same_book_test() {
+  let input = [2, 2]
+  let output = book_store.lowest_price(input)
+  let expected = 1600.0
+  output
+  |> should.equal(expected)
+}
+
+pub fn empty_basket_test() {
+  let input = []
+  let output = book_store.lowest_price(input)
+  let expected = 0.0
+  output
+  |> should.equal(expected)
+}
+
+pub fn two_different_books_test() {
+  let input = [1, 2]
+  let output = book_store.lowest_price(input)
+  let expected = 1520.0
+  output
+  |> should.equal(expected)
+}
+
+pub fn three_different_books_test() {
+  let input = [1, 2, 3]
+  let output = book_store.lowest_price(input)
+  let expected = 2160.0
+  output
+  |> should.equal(expected)
+}
+
+pub fn four_different_books_test() {
+  let input = [1, 2, 3, 4]
+  let output = book_store.lowest_price(input)
+  let expected = 2560.0
+  output
+  |> should.equal(expected)
+}
+
+pub fn five_different_books_test() {
+  let input = [1, 2, 3, 4, 5]
+  let output = book_store.lowest_price(input)
+  let expected = 3000.0
+  output
+  |> should.equal(expected)
+}
+
+pub fn two_groups_of_four_is_cheaper_than_group_of_five_plus_group_of_three_test() {
+  let input = [1, 1, 2, 2, 3, 3, 4, 5]
+  let output = book_store.lowest_price(input)
+  let expected = 5120.0
+  output
+  |> should.equal(expected)
+}
+
+pub fn two_groups_of_four_is_cheaper_than_groups_of_five_and_three_test() {
+  let input = [1, 1, 2, 3, 4, 4, 5, 5]
+  let output = book_store.lowest_price(input)
+  let expected = 5120.0
+  output
+  |> should.equal(expected)
+}
+
+pub fn group_of_four_plus_group_of_two_is_cheaper_than_two_groups_of_three_test() {
+  let input = [1, 1, 2, 2, 3, 4]
+  let output = book_store.lowest_price(input)
+  let expected = 4080.0
+  output
+  |> should.equal(expected)
+}
+
+pub fn two_each_of_first_four_books_and_one_copy_each_of_rest_test() {
+  let input = [1, 1, 2, 2, 3, 3, 4, 4, 5]
+  let output = book_store.lowest_price(input)
+  let expected = 5560.0
+  output
+  |> should.equal(expected)
+}
+
+pub fn two_copies_of_each_book_test() {
+  let input = [1, 1, 2, 2, 3, 3, 4, 4, 5, 5]
+  let output = book_store.lowest_price(input)
+  let expected = 6000.0
+  output
+  |> should.equal(expected)
+}
+
+pub fn three_copies_of_first_book_and_two_each_of_remaining_test() {
+  let input = [1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 1]
+  let output = book_store.lowest_price(input)
+  let expected = 6800.0
+  output
+  |> should.equal(expected)
+}
+
+pub fn three_each_of_first_two_books_and_two_each_of_remaining_books_test() {
+  let input = [1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 1, 2]
+  let output = book_store.lowest_price(input)
+  let expected = 7520.0
+  output
+  |> should.equal(expected)
+}
+
+pub fn four_groups_of_four_are_cheaper_than_two_groups_each_of_five_and_three_test() {
+  let input = [1, 1, 2, 2, 3, 3, 4, 5, 1, 1, 2, 2, 3, 3, 4, 5]
+  let output = book_store.lowest_price(input)
+  let expected = 10_240.0
+  output
+  |> should.equal(expected)
+}
+
+pub fn check_that_groups_of_four_are_created_properly_even_when_there_are_more_groups_of_three_than_groups_of_five_test() {
+  let input = [1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 4, 4, 5, 5]
+  let output = book_store.lowest_price(input)
+  let expected = 14_560.0
+  output
+  |> should.equal(expected)
+}
+
+pub fn one_group_of_one_and_four_is_cheaper_than_one_group_of_two_and_three_test() {
+  let input = [1, 1, 2, 3, 4]
+  let output = book_store.lowest_price(input)
+  let expected = 3360.0
+  output
+  |> should.equal(expected)
+}
+
+pub fn one_group_of_one_and_two_plus_three_groups_of_four_is_cheaper_than_one_group_of_each_size_test() {
+  let input = [1, 2, 2, 3, 3, 3, 4, 4, 4, 4, 5, 5, 5, 5, 5]
+  let output = book_store.lowest_price(input)
+  let expected = 10_000.0
+  output
+  |> should.equal(expected)
+}