Add baffling birthdays practice exercice

config.json

@@ -1211,6 +1211,20 @@
         ],
         "difficulty": 6
       },
+      {
+        "slug": "baffling-birthdays",
+        "name": "Baffling Birthdays",
+        "uuid": "b534049a-5920-4906-9091-0fa6d81a3636",
+        "practices": [],
+        "prerequisites": [
+          "for-loops",
+          "lists",
+          "sets",
+          "randomness",
+          "datetime"
+        ],
+        "difficulty": 6
+      },
       {
         "slug": "bank-account",
         "name": "Bank Account",

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": [
+    "Baboushka"
+  ],
+  "files": {
+    "solution": [
+      "src/main/java/BafflingBirthdays.java"
+    ],
+    "test": [
+      "src/test/java/BafflingBirthdaysTest.java"
+    ],
+    "example": [
+      ".meta/src/reference/java/BafflingBirthdays.java"
+    ],
+    "invalidator": [
+      "build.gradle"
+    ]
+  },
+  "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/src/reference/java/BafflingBirthdays.java

@@ -0,0 +1,50 @@
+import java.time.LocalDate;
+import java.util.ArrayList;
+import java.util.concurrent.ThreadLocalRandom;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Set;
+
+class BafflingBirthdays {
+    private static final int nonLeapYear = 2001;
+    private static final int daysInYear = 365;
+
+    boolean sharedBirthday(List<String> birthdates) {
+        Set<String> seen = new HashSet<>();
+        for (String birthdate : birthdates) {
+            String monthDay = birthdate.substring(5);
+            if (!seen.add(monthDay)) {
+                return true;
+            }
+        }
+        return false;
+    }
+
+    List<String> randomBirthdates(int groupSize) {
+        if (groupSize <= 0) {
+            return List.of();
+        }
+        List<String> birthdates = new ArrayList<>(groupSize);
+        ThreadLocalRandom random = ThreadLocalRandom.current();
+        for (int i = 0; i < groupSize; i++) {
+            int dayOfYear = random.nextInt(1, daysInYear + 1);
+            LocalDate date = LocalDate.ofYearDay(nonLeapYear, dayOfYear);
+            birthdates.add(date.toString());
+        }
+        return birthdates;
+    }
+
+    double estimatedProbabilityOfSharedBirthday(int groupSize) {
+        if (groupSize <= 1) {
+            return 0.0;
+        }
+        if (groupSize > daysInYear) {
+            return 100.0;
+        }
+        double probabilityNoSharedBirthday = 1.0;
+        for (int k = 0; k < groupSize; k++) {
+            probabilityNoSharedBirthday *= (daysInYear - k) / (double) daysInYear;
+        }
+        return (1 - probabilityNoSharedBirthday) * 100.0;
+    }
+}

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/build.gradle

@@ -0,0 +1,25 @@
+plugins {
+    id "java"
+}
+
+repositories {
+    mavenCentral()
+}
+
+dependencies {
+    testImplementation platform("org.junit:junit-bom:5.10.0")
+    testImplementation "org.junit.jupiter:junit-jupiter"
+    testImplementation "org.assertj:assertj-core:3.25.1"
+
+    testRuntimeOnly "org.junit.platform:junit-platform-launcher"
+}
+
+test {
+    useJUnitPlatform()
+
+    testLogging {
+        exceptionFormat = "full"
+        showStandardStreams = true
+        events = ["passed", "failed", "skipped"]
+    }
+}

exercises/practice/baffling-birthdays/gradle/wrapper/gradle-wrapper.properties

@@ -0,0 +1,6 @@
+distributionBase=GRADLE_USER_HOME
+distributionPath=wrapper/dists
+distributionUrl=https\://services.gradle.org/distributions/gradle-9.2.1-bin.zip
+validateDistributionUrl=true
+zipStoreBase=GRADLE_USER_HOME
+zipStorePath=wrapper/dists