Add baffling-birthdays exercise

config.json

@@ -2713,6 +2713,14 @@
           "for-loops"
         ],
         "difficulty": 7
+      },
+      {
+        "slug": "baffling-birthdays",
+        "name": "baffling-birthdays",
+        "uuid": "f75d1dd2-63ee-4258-a4e9-f208fb8e8bd4",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 5
       }
     ],
     "foregone": [

exercises/Exercises.sln

@@ -357,6 +357,8 @@ Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "StateOfTicTacToe", "practic
 EndProject
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "BottleSong", "practice\bottle-song\BottleSong.csproj", "{6BD384E6-225E-4F8A-856C-3079957C6E36}"
 EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "BafflingBirthdays", "practice\baffling-birthdays\BafflingBirthdays.csproj", "{9D596842-830C-417D-807A-6B910EE467B4}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU
@@ -1063,6 +1065,10 @@ Global
 		{6BD384E6-225E-4F8A-856C-3079957C6E36}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{6BD384E6-225E-4F8A-856C-3079957C6E36}.Release|Any CPU.ActiveCfg = Release|Any CPU
 		{6BD384E6-225E-4F8A-856C-3079957C6E36}.Release|Any CPU.Build.0 = Release|Any CPU
+		{9D596842-830C-417D-807A-6B910EE467B4}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{9D596842-830C-417D-807A-6B910EE467B4}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{9D596842-830C-417D-807A-6B910EE467B4}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{9D596842-830C-417D-807A-6B910EE467B4}.Release|Any CPU.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE
@@ -1243,6 +1249,7 @@ Global
 		{8E276065-1371-4CFA-BA20-95225EC6AEBC} = {E276EF69-669A-43E0-88AC-8ABB17A9C026}
 		{67E9BAB3-9805-42F1-9298-E9BBB795140E} = {E276EF69-669A-43E0-88AC-8ABB17A9C026}
 		{6BD384E6-225E-4F8A-856C-3079957C6E36} = {E276EF69-669A-43E0-88AC-8ABB17A9C026}
+		{9D596842-830C-417D-807A-6B910EE467B4} = {E276EF69-669A-43E0-88AC-8ABB17A9C026}
 	EndGlobalSection
 	GlobalSection(ExtensibilityGlobals) = postSolution
 		SolutionGuid = {AB4EA6C9-5461-4024-BDC7-2AE0C3A85CD1}

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

@@ -0,0 +1,15 @@
+# Instructions
+
+Your task is to implement a solution that verifies the Birthday Problem's probabilities.
+
+To do this, you need to:
+
+- Determine if two birthdates match (same month and day).
+- Generate random birthdates.
+- Check if a set of randomly generated birthdates contains at least one matching pair.
+- Calculate the probability of at least one match for different group sizes.
+
+```exercism/caution
+The Birthday Problem assumes that birthdays are uniformly distributed within a single year.
+While the year should vary between different random birthdate generation calls, all birthdates within a single call must share the same year.
+```

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

@@ -0,0 +1,25 @@
+# Introduction
+
+Fresh out of graduation, 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 365 possible birthdays, so you confidently accept.
+
+To your astonishment, after collecting just 32 birthdays, you've already found a match.
+Magnanimous, 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 Problem][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: 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/.editorconfig

@@ -0,0 +1,141 @@
+###############################
+# Core EditorConfig Options   #
+###############################
+
+; This file is for unifying the coding style for different editors and IDEs.
+; More information at:
+; https://docs.microsoft.com/en-us/visualstudio/ide/editorconfig-code-style-settings-reference?view=vs-2017
+; https://docs.microsoft.com/en-us/visualstudio/ide/create-portable-custom-editor-options?view=vs-2017
+
+root = true
+
+[*]
+indent_style = space
+
+[BafflingBirthdays.cs]
+indent_size = 4
+
+###############################
+# .NET Coding Conventions     #
+###############################
+
+# Organize usings
+dotnet_sort_system_directives_first = true
+dotnet_separate_import_directive_groups = true
+
+# this. preferences
+dotnet_style_qualification_for_field = false:suggestion
+dotnet_style_qualification_for_property = false:suggestion
+dotnet_style_qualification_for_method = false:suggestion
+dotnet_style_qualification_for_event = false:suggestion
+
+# Language keywords vs BCL types preferences
+dotnet_style_predefined_type_for_locals_parameters_members = true:suggestion
+dotnet_style_predefined_type_for_member_access = true:suggestion
+
+# Parentheses preferences
+dotnet_style_parentheses_in_arithmetic_binary_operators = never_if_unnecessary:none
+dotnet_style_parentheses_in_relational_binary_operators = never_if_unnecessary:none
+dotnet_style_parentheses_in_other_binary_operators = never_if_unnecessary:none
+dotnet_style_parentheses_in_other_operators = never_if_unnecessary:suggestion
+
+# Modifier preferences
+dotnet_style_require_accessibility_modifiers = always:suggestion
+dotnet_style_readonly_field = true:suggestion
+
+# Expression-level preferences
+dotnet_style_object_initializer = true:suggestion
+dotnet_style_collection_initializer = true:suggestion
+dotnet_style_explicit_tuple_names = true:suggestion
+dotnet_style_prefer_inferred_tuple_names = true:suggestion
+dotnet_style_prefer_inferred_anonymous_type_member_names = true:suggestion
+dotnet_style_prefer_auto_properties = true:suggestion
+dotnet_style_prefer_is_null_check_over_reference_equality_method = true:suggestion
+dotnet_style_prefer_conditional_expression_over_assignment = true:suggestion
+dotnet_style_prefer_conditional_expression_over_return = true:suggestion
+dotnet_style_coalesce_expression = true:suggestion
+dotnet_style_null_propagation = true:suggestion
+
+###############################
+# Naming Conventions          #
+###############################
+
+# Style Definitions
+dotnet_naming_style.pascal_case_style.capitalization = pascal_case
+
+# Use PascalCase for constant fields
+dotnet_naming_rule.constant_fields_should_be_pascal_case.severity = suggestion
+dotnet_naming_rule.constant_fields_should_be_pascal_case.symbols = constant_fields
+dotnet_naming_rule.constant_fields_should_be_pascal_case.style = pascal_case_style
+dotnet_naming_symbols.constant_fields.applicable_kinds = field
+dotnet_naming_symbols.constant_fields.applicable_accessibilities = *
+dotnet_naming_symbols.constant_fields.required_modifiers = const
+
+###############################
+# C# Code Style Rules         #
+###############################
+
+# var preferences
+csharp_style_var_for_built_in_types = true:none
+csharp_style_var_when_type_is_apparent = true:none
+csharp_style_var_elsewhere = true:none
+
+# Expression-bodied members
+csharp_style_expression_bodied_methods = true:suggestion
+csharp_style_expression_bodied_constructors = true:suggestion
+csharp_style_expression_bodied_operators = true:suggestion
+csharp_style_expression_bodied_properties = true:suggestion
+csharp_style_expression_bodied_indexers = true:suggestion
+csharp_style_expression_bodied_accessors = true:suggestion
+
+# Pattern-matching preferences
+csharp_style_pattern_matching_over_is_with_cast_check = true:suggestion
+csharp_style_pattern_matching_over_as_with_null_check = true:suggestion
+
+# Null-checking preferences
+csharp_style_throw_expression = true:suggestion
+csharp_style_conditional_delegate_call = true:suggestion
+
+# Modifier preferences
+csharp_preferred_modifier_order = public,private,protected,internal,static,extern,new,virtual,abstract,sealed,override,readonly,unsafe,volatile,async:suggestion
+
+# Expression-level preferences
+csharp_prefer_braces = true:none
+csharp_prefer_simple_default_expression = true:suggestion
+csharp_style_deconstructed_variable_declaration = true:suggestion
+csharp_style_pattern_local_over_anonymous_function = true:suggestion
+csharp_style_inlined_variable_declaration = true:suggestion
+
+###############################
+# C# Formatting Rules         #
+###############################
+
+# New line preferences
+csharp_new_line_before_open_brace = all
+csharp_new_line_before_else = true
+csharp_new_line_before_catch = true
+csharp_new_line_before_finally = true
+csharp_new_line_before_members_in_object_initializers = false
+csharp_new_line_before_members_in_anonymous_types = false
+csharp_new_line_between_query_expression_clauses = true
+
+# Indentation preferences
+csharp_indent_case_contents = true
+csharp_indent_switch_labels = true
+csharp_indent_labels = flush_left
+
+# Space preferences
+csharp_space_after_cast = false
+csharp_space_after_keywords_in_control_flow_statements = true
+csharp_space_between_method_declaration_parameter_list_parentheses = false
+csharp_space_between_method_call_parameter_list_parentheses = false
+csharp_space_before_colon_in_inheritance_clause = true
+csharp_space_after_colon_in_inheritance_clause = true
+csharp_space_around_binary_operators = before_and_after
+csharp_space_between_method_declaration_empty_parameter_list_parentheses = false
+csharp_space_between_method_call_name_and_opening_parenthesis = false
+csharp_space_between_method_call_empty_parameter_list_parentheses = false
+
+# Wrapping preferences
+csharp_preserve_single_line_blocks = true
+csharp_preserve_single_line_statements = true

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

@@ -0,0 +1,17 @@
+{
+  "authors": [
+    "erikschierboom"
+  ],
+  "files": {
+    "solution": [
+      "BafflingBirthdays.cs"
+    ],
+    "test": [
+      "BafflingBirthdaysTests.cs"
+    ],
+    "example": [
+      ".meta/Example.cs"
+    ]
+  },
+  "blurb": ""
+}

exercises/practice/baffling-birthdays/BafflingBirthdays.cs

@@ -0,0 +1,34 @@
+public static class BafflingBirthdays
+{
+    public static bool MatchingBirthday(DateOnly birthday1, DateOnly birthday2) =>
+        birthday1.Month == birthday2.Month && birthday1.Day == birthday2.Day;
+
+    private static DateOnly RandomBirthdate(int year)
+    {
+        var month = Random.Shared.Next(1, 12 + 1);
+        var day = Random.Shared.Next(1, DateTime.DaysInMonth(year, month) + 1);
+        return new DateOnly(year, month, day);
+    }
+
+    public static DateOnly[] RandomBirthdates(int numberOfBirthdays)
+    {
+        var year = Random.Shared.Next(1900, DateTime.Now.Year + 1);
+        return Enumerable.Range(0, numberOfBirthdays).Select(_ => RandomBirthdate(year)).ToArray();
+    }
+
+    public static bool HasMatchingBirthdays(DateOnly[] birthdays) =>
+        birthdays
+            .SelectMany((birthday, i) => birthdays.Skip(i + 1), MatchingBirthday)
+            .Any(matching => matching);
+
+    public static double LikelihoodHasMatchingBirthday(int numberOfBirthdays)
+    {
+        var matchingBirthdayIterationCount = 0;
+        for (var i = 0; i < 10000; i++)
+        {
+            if (HasMatchingBirthdays(RandomBirthdates(numberOfBirthdays)))
+                matchingBirthdayIterationCount++;
+        }
+        return matchingBirthdayIterationCount / 100.0;
+    }
+}

exercises/practice/baffling-birthdays/BafflingBirthdays.csproj

@@ -0,0 +1,16 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net8.0</TargetFramework>
+  </PropertyGroup>
+  <ItemGroup>
+    <Using Include="Xunit" />
+  </ItemGroup>
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.12.0" />
+    <PackageReference Include="xunit.v3" Version="1.1.0" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="3.0.2" />
+  </ItemGroup>
+</Project>