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Add birthdays exercise #743

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a998879
Fixes to concept tree
meatball133 Dec 6, 2024
ca5f72c
Merge branch 'exercism:main' into main
meatball133 Jan 1, 2026
89ffdb7
Add Baffling Birthdays exercise with instructions, example, and tests
meatball133 Jan 2, 2026
6d949ff
Format files
meatball133 Jan 2, 2026
f77c54f
Fix when suppose to generate for 23 people
meatball133 Jan 2, 2026
dff34d2
2nd attempt
meatball133 Jan 2, 2026
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8 changes: 8 additions & 0 deletions config.json
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Expand Up @@ -1036,6 +1036,14 @@
"prerequisites": [],
"difficulty": 3
},
{
"slug": "baffling-birthdays",
"name": "Baffling Birthdays",
"uuid": "232e58b6-06ca-4401-b4ba-2e16e59902c3",
"practices": [],
"prerequisites": [],
"difficulty": 3
},
{
"slug": "food-chain",
"name": "Food Chain",
Expand Down
23 changes: 23 additions & 0 deletions exercises/practice/baffling-birthdays/.docs/instructions.md
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# 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.
~~~~
25 changes: 25 additions & 0 deletions exercises/practice/baffling-birthdays/.docs/introduction.md
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# 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
19 changes: 19 additions & 0 deletions exercises/practice/baffling-birthdays/.meta/config.json
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{
"authors": [
"meatball133"
],
"files": {
"solution": [
"src/baffling_birthdays.cr"
],
"test": [
"spec/baffling_birthdays_spec.cr"
],
"example": [
".meta/src/example.cr"
]
},
"blurb": "Estimate the birthday paradox's probabilities.",
"source": "Erik Schierboom",
"source_url": "https://github.com/exercism/problem-specifications/pull/2539"
}
29 changes: 29 additions & 0 deletions exercises/practice/baffling-birthdays/.meta/src/example.cr
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class BafflingBirthdays
def self.shared_birthday(birthdates : Array(Time)) : Bool
birthdates.each_with_index do |date, idx|
birthdates.each_with_index do |date_2, idx_2|
if idx == idx_2
next
end
if date.month == date_2.month && date.day == date_2.day
return true
end
end
end
return false
end

def self.random_birthdates(size : Int) : Array(Time)
result = [] of Time
size.times do |_|
time = Time.utc(2025, 1, 1).shift days: Random.rand(365)
result << time
end

return result
end

def self.estimated_probability_of_shared_birthday(size : Int) : Float
return (1..10000).sum { |_| shared_birthday(random_birthdates(size)) ? 1 : 0 } / 100.0
end
end
31 changes: 31 additions & 0 deletions exercises/practice/baffling-birthdays/.meta/test_template.ecr
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require "spec"
require "../src/*"

describe "<%-= to_capitalized(@json["exercise"].to_s) %>" do
<%- @json["cases"].as_a.each do |cases| %>
<%- cases["cases"].as_a.each do |sub_case| %>
<%= status()%> "<%-= sub_case["description"] %>" do
<%- if sub_case["property"] == "sharedBirthday" -%>
birthdays = [<%= sub_case["input"]["birthdates"].as_a.map {|date| "Time.parse_utc(\"#{date}\", \"%Y-%m-%d\")" }.join(", ") %>]
<%= to_capitalized(@json["exercise"].to_s) %>.<%= sub_case["property"].to_s.underscore %>(birthdays).should be_<%= sub_case["expected"] %>
<%- elsif sub_case["property"] == "estimatedProbabilityOfSharedBirthday" -%>
<%= to_capitalized(@json["exercise"].to_s) %>.<%= sub_case["property"].to_s.underscore %>(<%= sub_case["input"]["groupSize"] %>).should be_close <%= (sub_case["expected"]) %>, <%= sub_case["input"]["groupSize"].as_i == 1 ? 0.01 : 1 %>
<%- elsif sub_case["expected"] == "length == groupsize" -%>
(1..10).each do |size|
result = <%= to_capitalized(@json["exercise"].to_s) %>.<%= sub_case["property"].to_s.underscore %>(size)
result.size.should eq(size)
end
<%- elsif (sub_case["expected"]["years"]? || {} of String => Bool)["leapYear"]? -%>
result = <%= to_capitalized(@json["exercise"].to_s) %>.<%= sub_case["property"].to_s.underscore %>(50)
result.all? { |date| Time.leap_year?(date.year) }.should be_false
<%- elsif (sub_case["expected"]["months"]? || {} of String => Bool)["random"]? -%>
result = <%= to_capitalized(@json["exercise"].to_s) %>.<%= sub_case["property"].to_s.underscore %>(100)
result.to_set { |date| date.month }.size.should eq(12)
<%- elsif (sub_case["expected"]["days"]? || {} of String => Bool)["random"]? -%>
result = <%= to_capitalized(@json["exercise"].to_s) %>.<%= sub_case["property"].to_s.underscore %>(300)
result.to_set { |date| date.day }.size.should eq(31)
<% end %>
end
<% end %>
<% end %>
end
61 changes: 61 additions & 0 deletions exercises/practice/baffling-birthdays/.meta/tests.toml
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# 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"
96 changes: 96 additions & 0 deletions exercises/practice/baffling-birthdays/spec/baffling_birthdays_spec.cr
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require "spec"
require "../src/*"

describe "BafflingBirthdays" do
it "one birthdate" do
birthdays = [Time.parse_utc("2000-01-01", "%Y-%m-%d")]
BafflingBirthdays.shared_birthday(birthdays).should be_false
end

pending "two birthdates with same year, month, and day" do
birthdays = [Time.parse_utc("2000-01-01", "%Y-%m-%d"), Time.parse_utc("2000-01-01", "%Y-%m-%d")]
BafflingBirthdays.shared_birthday(birthdays).should be_true
end

pending "two birthdates with same year and month, but different day" do
birthdays = [Time.parse_utc("2012-05-09", "%Y-%m-%d"), Time.parse_utc("2012-05-17", "%Y-%m-%d")]
BafflingBirthdays.shared_birthday(birthdays).should be_false
end

pending "two birthdates with same month and day, but different year" do
birthdays = [Time.parse_utc("1999-10-23", "%Y-%m-%d"), Time.parse_utc("1988-10-23", "%Y-%m-%d")]
BafflingBirthdays.shared_birthday(birthdays).should be_true
end

pending "two birthdates with same year, but different month and day" do
birthdays = [Time.parse_utc("2007-12-19", "%Y-%m-%d"), Time.parse_utc("2007-04-27", "%Y-%m-%d")]
BafflingBirthdays.shared_birthday(birthdays).should be_false
end

pending "two birthdates with different year, month, and day" do
birthdays = [Time.parse_utc("1997-08-04", "%Y-%m-%d"), Time.parse_utc("1963-11-23", "%Y-%m-%d")]
BafflingBirthdays.shared_birthday(birthdays).should be_false
end

pending "multiple birthdates without shared birthday" do
birthdays = [
Time.parse_utc("1966-07-29", "%Y-%m-%d"), Time.parse_utc("1977-02-12", "%Y-%m-%d"), Time.parse_utc("2001-12-25", "%Y-%m-%d"),
Time.parse_utc("1980-11-10", "%Y-%m-%d"),
]
BafflingBirthdays.shared_birthday(birthdays).should be_false
end

pending "multiple birthdates with one shared birthday" do
birthdays = [
Time.parse_utc("1966-07-29", "%Y-%m-%d"), Time.parse_utc("1977-02-12", "%Y-%m-%d"), Time.parse_utc("2001-07-29", "%Y-%m-%d"),
Time.parse_utc("1980-11-10", "%Y-%m-%d"),
]
BafflingBirthdays.shared_birthday(birthdays).should be_true
end

pending "multiple birthdates with more than one shared birthday" do
birthdays = [
Time.parse_utc("1966-07-29", "%Y-%m-%d"), Time.parse_utc("1977-02-12", "%Y-%m-%d"), Time.parse_utc("2001-12-25", "%Y-%m-%d"),
Time.parse_utc("1980-07-29", "%Y-%m-%d"), Time.parse_utc("2019-02-12", "%Y-%m-%d"),
]
BafflingBirthdays.shared_birthday(birthdays).should be_true
end

pending "generate requested number of birthdates" do
(1..10).each do |size|
result = BafflingBirthdays.random_birthdates(size)
result.size.should eq(size)
end
end

pending "years are not leap years" do
result = BafflingBirthdays.random_birthdates(50)
result.all? { |date| Time.leap_year?(date.year) }.should be_false
end

pending "months are random" do
result = BafflingBirthdays.random_birthdates(100)
result.to_set { |date| date.month }.size.should eq(12)
end

pending "days are random" do
result = BafflingBirthdays.random_birthdates(300)
result.to_set { |date| date.day }.size.should eq(31)
end

pending "for one person" do
BafflingBirthdays.estimated_probability_of_shared_birthday(1).should be_close 0.0, 0.01
end

pending "among ten people" do
BafflingBirthdays.estimated_probability_of_shared_birthday(10).should be_close 11.694818, 1
end

pending "among twenty-three people" do
BafflingBirthdays.estimated_probability_of_shared_birthday(23).should be_close 50.729723, 1
end

pending "among seventy people" do
BafflingBirthdays.estimated_probability_of_shared_birthday(70).should be_close 99.915958, 1
end
end
12 changes: 12 additions & 0 deletions exercises/practice/baffling-birthdays/src/baffling_birthdays.cr
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class BafflingBirthdays
# Write your code for the 'BafflingBirthdays' exercise in this file.

def self.shared_birthday(birthdates : Array(Time)) : Bool
end

def self.random_birthdates(size : Int) : Array(Time)
end

def self.estimated_probability_of_shared_birthday(size : Int) : Float
end
end
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