Add new practice exercise baffling-birthdays

config.json

@@ -2338,6 +2338,21 @@
         ],
         "difficulty": 5
       },
+      {
+        "slug": "baffling-birthdays",
+        "name": "Baffling Birthdays",
+        "uuid": "e5ebca45-1e54-45b6-89bb-0cddfd23b6e1",
+        "practices": [
+          "randomness"
+        ],
+        "prerequisites": [
+          "randomness",
+          "dates-and-time",
+          "lists",
+          "enum"
+        ],
+        "difficulty": 6
+      },
       {
         "slug": "game-of-life",
         "name": "Game of Life",

exercises/practice/baffling-birthdays/.docs/instructions.md

@@ -0,0 +1,23 @@
+# Instructions
+
+Your task is to estimate the birthday paradox's probabilities.
+
+To do this, you need to:
+
+- Generate random birthdates.
+- Check if a collection of randomly generated birthdates contains at least two with the same birthday.
+- Estimate the probability that at least two people in a group share the same birthday for different group sizes.
+
+~~~~exercism/note
+A birthdate includes the full date of birth (year, month, and day), whereas a birthday refers only to the month and day, which repeat each year.
+Two birthdates with the same month and day correspond to the same birthday.
+~~~~
+
+~~~~exercism/caution
+The birthday paradox assumes that:
+
+- There are 365 possible birthdays (no leap years).
+- Each birthday is equally likely (uniform distribution).
+
+Your implementation must follow these assumptions.
+~~~~

exercises/practice/baffling-birthdays/.docs/introduction.md

@@ -0,0 +1,25 @@
+# Introduction
+
+Fresh out of college, you're throwing a huge party to celebrate with friends and family.
+Over 70 people have shown up, including your mildly eccentric Uncle Ted.
+
+In one of his usual antics, he bets you £100 that at least two people in the room share the same birthday.
+That sounds ridiculous — there are many more possible birthdays than there are guests, so you confidently accept.
+
+To your astonishment, after collecting the birthdays of just 32 guests, you've already found two guests that share the same birthday.
+Accepting your loss, you hand Uncle Ted his £100, but something feels off.
+
+The next day, curiosity gets the better of you.
+A quick web search leads you to the [birthday paradox][birthday-problem], which reveals that with just 23 people, the probability of a shared birthday exceeds 50%.
+
+Ah. So _that's_ why Uncle Ted was so confident.
+
+Determined to turn the tables, you start looking up other paradoxes; next time, _you'll_ be the one making the bets.
+
+~~~~exercism/note
+The birthday paradox is a [veridical paradox][veridical-paradox]: even though it feels wrong, it is actually true.
+
+[veridical-paradox]: https://en.wikipedia.org/wiki/Paradox#Quine's_classification
+~~~~
+
+[birthday-problem]: https://en.wikipedia.org/wiki/Birthday_problem

exercises/practice/baffling-birthdays/.formatter.exs

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

exercises/practice/baffling-birthdays/.gitignore

@@ -0,0 +1,26 @@
+# The directory Mix will write compiled artifacts to.
+/_build/
+
+# If you run "mix test --cover", coverage assets end up here.
+/cover/
+
+# The directory Mix downloads your dependencies sources to.
+/deps/
+
+# Where third-party dependencies like ExDoc output generated docs.
+/doc/
+
+# Ignore .fetch files in case you like to edit your project deps locally.
+/.fetch
+
+# If the VM crashes, it generates a dump, let's ignore it too.
+erl_crash.dump
+
+# Also ignore archive artifacts (built via "mix archive.build").
+*.ez
+
+# Ignore package tarball (built via "mix hex.build").
+baffling_birthdays-*.tar
+
+# Temporary files, for example, from tests.
+/tmp/

exercises/practice/baffling-birthdays/.meta/config.json

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "jiegillet"
+  ],
+  "files": {
+    "solution": [
+      "lib/baffling_birthdays.ex"
+    ],
+    "test": [
+      "test/baffling_birthdays_test.exs"
+    ],
+    "example": [
+      ".meta/example.ex"
+    ]
+  },
+  "blurb": "Estimate the birthday paradox's probabilities.",
+  "source": "Erik Schierboom",
+  "source_url": "https://github.com/exercism/problem-specifications/pull/2539"
+}

exercises/practice/baffling-birthdays/.meta/example.ex

@@ -0,0 +1,55 @@
+defmodule BafflingBirthdays do
+  @moduledoc """
+  Estimate the probability of shared birthdays in a group of people.
+  """
+
+  @spec shared_birthday?(birthdates :: [Date.t()]) :: boolean()
+  def shared_birthday?(birthdates) do
+    result =
+      Enum.reduce_while(birthdates, MapSet.new(), fn date, birthdays ->
+        birthday = {date.month, date.day}
+
+        if MapSet.member?(birthdays, birthday) do
+          {:halt, :found_a_shared_birthday}
+        else
+          {:cont, MapSet.put(birthdays, birthday)}
+        end
+      end)
+
+    result == :found_a_shared_birthday
+  end
+
+  @spec random_birthdates(group_size :: integer()) :: [Date.t()]
+  def random_birthdates(group_size) do
+    for _ <- 1..group_size do
+      year = generate_non_leap_year_january_first(0, 3000)
+      days_to_add = Enum.random(0..364)
+      Date.add(year, days_to_add)
+    end
+  end
+
+  defp generate_non_leap_year_january_first(min, max) do
+    year = Enum.random(min..max)
+    january_first = Date.new!(year, 1, 1)
+
+    if Date.leap_year?(january_first) do
+      Date.add(january_first, 366)
+    else
+      january_first
+    end
+  end
+
+  @spec estimated_probability_of_shared_birthday(group_size :: integer()) :: float()
+  def estimated_probability_of_shared_birthday(group_size) do
+    sample_size = 1000
+
+    shared_birthdays_count =
+      for _ <- 1..sample_size,
+          group_size |> random_birthdates() |> shared_birthday?(),
+          reduce: 0 do
+        count -> count + 1
+      end
+
+    100 * shared_birthdays_count / sample_size
+  end
+end

exercises/practice/baffling-birthdays/.meta/tests.toml

@@ -0,0 +1,61 @@
+# 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.
+
+[716dcc2b-8fe4-4fc9-8c48-cbe70d8e6b67]
+description = "shared birthday -> one birthdate"
+
+[f7b3eb26-bcfc-4c1e-a2de-af07afc33f45]
+description = "shared birthday -> two birthdates with same year, month, and day"
+
+[7193409a-6e16-4bcb-b4cc-9ffe55f79b25]
+description = "shared birthday -> two birthdates with same year and month, but different day"
+
+[d04db648-121b-4b72-93e8-d7d2dced4495]
+description = "shared birthday -> two birthdates with same month and day, but different year"
+
+[3c8bd0f0-14c6-4d4c-975a-4c636bfdc233]
+description = "shared birthday -> two birthdates with same year, but different month and day"
+
+[df5daba6-0879-4480-883c-e855c99cdaa3]
+description = "shared birthday -> two birthdates with different year, month, and day"
+
+[0c17b220-cbb9-4bd7-872f-373044c7b406]
+description = "shared birthday -> multiple birthdates without shared birthday"
+
+[966d6b0b-5c0a-4b8c-bc2d-64939ada49f8]
+description = "shared birthday -> multiple birthdates with one shared birthday"
+
+[b7937d28-403b-4500-acce-4d9fe3a9620d]
+description = "shared birthday -> multiple birthdates with more than one shared birthday"
+
+[70b38cea-d234-4697-b146-7d130cd4ee12]
+description = "random birthdates -> generate requested number of birthdates"
+
+[d9d5b7d3-5fea-4752-b9c1-3fcd176d1b03]
+description = "random birthdates -> years are not leap years"
+
+[d1074327-f68c-4c8a-b0ff-e3730d0f0521]
+description = "random birthdates -> months are random"
+
+[7df706b3-c3f5-471d-9563-23a4d0577940]
+description = "random birthdates -> days are random"
+
+[89a462a4-4265-4912-9506-fb027913f221]
+description = "estimated probability of at least one shared birthday -> for one person"
+
+[ec31c787-0ebb-4548-970c-5dcb4eadfb5f]
+description = "estimated probability of at least one shared birthday -> among ten people"
+
+[b548afac-a451-46a3-9bb0-cb1f60c48e2f]
+description = "estimated probability of at least one shared birthday -> among twenty-three people"
+
+[e43e6b9d-d77b-4f6c-a960-0fc0129a0bc5]
+description = "estimated probability of at least one shared birthday -> among seventy people"

exercises/practice/baffling-birthdays/lib/baffling_birthdays.ex

@@ -0,0 +1,17 @@
+defmodule BafflingBirthdays do
+  @moduledoc """
+  Estimate the probability of shared birthdays in a group of people.
+  """
+
+  @spec shared_birthday?(birthdates :: [Date.t()]) :: boolean()
+  def shared_birthday?(birthdates) do
+  end
+
+  @spec random_birthdates(group_size :: integer()) :: [Date.t()]
+  def random_birthdates(group_size) do
+  end
+
+  @spec estimated_probability_of_shared_birthday(group_size :: integer()) :: float()
+  def estimated_probability_of_shared_birthday(group_size) do
+  end
+end

exercises/practice/baffling-birthdays/mix.exs

@@ -0,0 +1,27 @@
+defmodule BafflingBirthdays.MixProject do
+  use Mix.Project
+
+  def project do
+    [
+      app: :baffling_birthdays,
+      version: "0.1.0",
+      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