[baffling-birthdays] Add new practice exercise

config.json

@@ -286,6 +286,14 @@
         "prerequisites": [],
         "difficulty": 4
       },
+      {
+        "slug": "baffling-birthdays",
+        "name": "Baffling Birthdays",
+        "uuid": "2b4c4891-a73c-4996-9ce9-7ece0ad2d56d",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 4
+      },
       {
         "slug": "food-chain",
         "name": "Food Chain",

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/.meta/config.json

@@ -0,0 +1,22 @@
+{
+  "authors": [
+    "IsaacG"
+  ],
+  "files": {
+    "solution": [
+      "baffling_birthdays.gd"
+    ],
+    "test": [
+      "baffling_birthdays_test.gd",
+      "run_tests",
+      "lib/test_runner.gd",
+      "project.godot"
+    ],
+    "example": [
+      ".meta/example.gd"
+    ]
+  },
+  "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.gd

@@ -0,0 +1,47 @@
+const LOOPS = 5000
+
+
+func shared_birthday(birthdays):
+	var seen = {}
+	for birthday in birthdays:
+		var d = Time.get_datetime_dict_from_datetime_string(birthday, false)
+		var n = d["month"] * 100 + d["day"]
+		if n in seen:
+			return true
+		seen[n] = true
+	return false
+
+
+func random_birthdates(count):
+	var dates = []
+	var limit = {
+		1: 31, 2: 28, 3: 31, 4: 30, 5: 31, 6: 30, 7: 31, 8: 31, 9: 30, 10: 31, 11: 30, 12: 31
+	}
+	var rng = RandomNumberGenerator.new()
+	for i in range(count):
+		var month = rng.randi_range(1, 12)
+		(
+			dates
+			. append(
+				{
+					"year": 2001,
+					"month": month,
+					"day": rng.randi_range(1, limit[month]),
+				}
+			)
+		)
+	return dates
+
+
+func estimated_probability_of_shared_birthday(count):
+	var rng = RandomNumberGenerator.new()
+	var shared = 0.0
+	for i in range(LOOPS):
+		var seen = {}
+		for j in count:
+			var date = rng.randi_range(1, 365)
+			if date in seen:
+				shared += 1
+				break
+			seen[date] = true
+	return 100 * shared / LOOPS

exercises/practice/baffling-birthdays/.meta/template.j2

@@ -0,0 +1,60 @@
+{% for case in cases %}{% if case.property == "sharedBirthday" %}
+func test_{{ case.func_name }}(solution_script):
+	var got = solution_script.{{ case.property | camel_to_snake }}({{ case.given | quote }})
+	var want = {{ case.expected | quote }}
+	return [got, want]
+
+{% endif %}{% endfor %}{% for count in [0, 1, 2, 5, 10] %}
+func test_random_birthdates_count_{{ count }}(solution_script):
+	var got = len(solution_script.random_birthdates({{ count }}))
+	var want = {{ count }}
+	return [got, want]
+
+{% endfor %}
+func test_random_birthdates_not_leapyear(solution_script):
+	var is_leap = false
+	var dates = solution_script.random_birthdates(300)
+	for date in dates:
+		if date["year"] % 400 == 0 or date["year"] % 4 == 0 and date["year"] % 100 != 0:
+			is_leap = true
+			break
+	return [is_leap, false]
+
+
+func test_random_birthdates_random_months(solution_script):
+	var months = {}
+	var dates = solution_script.random_birthdates(600)
+	for date in dates:
+		if date["month"] not in months:
+			months[date["month"]] = 0
+		months[date["month"]] += 1
+	var is_distributed = len(months) == 12
+	for month in months:
+		if months[month] < 15:
+			is_distributed = false
+
+	return [is_distributed, true]
+
+
+func test_random_birthdates_random_days(solution_script):
+	var days = {}
+	var dates = solution_script.random_birthdates(600)
+	for date in dates:
+		if date["day"] not in days:
+			days[date["day"]] = 0
+		days[date["day"]] += 1
+	var is_distributed = len(days) > 25
+	for day in days:
+		if days[day] < 5:
+			is_distributed = false
+
+	return [is_distributed, true]
+
+{% for case in cases %}{% if case.property == "estimatedProbabilityOfSharedBirthday" %}
+func test_{{ case.func_name }}(solution_script):
+	var got = solution_script.{{ case.property | camel_to_snake }}({{ case.given | quote }})
+	var want = {{ case.expected | quote }}
+	return [got - 1 < want and want < got + 1, true]
+
+{% endif %}{% endfor %}
+

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/baffling_birthdays.gd

@@ -0,0 +1,11 @@
+func shared_birthday(birthdays):
+	pass
+
+
+func random_birthdates(count):
+	# return a list of `count` date dicts, each with a year, month and day.
+	pass
+
+
+func estimated_probability_of_shared_birthday(count):
+	pass