Added "baffling-birthdays" exercise

.github/workflows/ci.yml

@@ -69,6 +69,7 @@ jobs:
       shell: powershell
       # Delete the exercises that require Boost to avoid issues with Windows setup.
       run: |
+        rm exercises/practice/baffling-birthdays -r
         rm exercises/practice/gigasecond -r
         rm exercises/practice/meetup -r
         cmake .

config.json

@@ -1278,6 +1278,21 @@
           "comparisons"
         ],
         "difficulty": 6
+      },
+      {
+        "slug": "baffling-birthdays",
+        "name": "Baffling Birthdays",
+        "uuid": "1886bed2-3985-4323-a689-22b1a5c75e38",
+        "practices": [
+          "randomness",
+          "datetimes"
+        ],
+        "prerequisites": [
+          "datetimes",
+          "randomness",
+          "numbers"
+        ],
+        "difficulty": 6
       }
     ],
     "foregone": [

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 +1,21 @@
+{
+  "authors": [
+    "marcelweikum"
+  ],
+  "files": {
+    "solution": [
+      "baffling_birthdays.cpp",
+      "baffling_birthdays.h"
+    ],
+    "test": [
+      "baffling_birthdays_test.cpp"
+    ],
+    "example": [
+      ".meta/example.cpp",
+      ".meta/example.h"
+    ]
+  },
+  "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.cpp

@@ -0,0 +1,55 @@
+#include <random>
+#include <unordered_set>
+
+#include "baffling_birthdays.h"
+
+namespace baffling_birthdays {
+
+Date parse_date(const std::string& iso) {
+    try {
+        return boost::gregorian::from_simple_string(iso);
+    } catch (...) {
+        return Date(boost::gregorian::not_a_date_time);
+    }
+}
+
+bool shared_birthday(const std::vector<Date>& dates) {
+    std::unordered_set<unsigned> seen_md;
+    seen_md.reserve(dates.size());
+    for (auto const& d : dates) {
+        if (d.is_not_a_date()) continue;
+        unsigned key = d.month().as_number() * 100 + d.day();
+        if (seen_md.count(key)) return true;
+        seen_md.insert(key);
+    }
+    return false;
+}
+
+std::vector<Date> random_birthdates(std::size_t group_size) {
+    static std::mt19937_64 gen(
+        static_cast<unsigned long>(std::random_device{}()));
+    std::uniform_int_distribution<int> month_dist(1, 12);
+    const int days_in_month[12] = {31, 28, 31, 30, 31, 30,
+                                   31, 31, 30, 31, 30, 31};
+    std::vector<Date> dates;
+    dates.reserve(group_size);
+    for (std::size_t i = 0; i < group_size; ++i) {
+        int m = month_dist(gen);
+        std::uniform_int_distribution<int> day_dist(1, days_in_month[m - 1]);
+        int d = day_dist(gen);
+        dates.push_back(Date(2001, m, d));
+    }
+    return dates;
+}
+
+double estimated_probability_of_shared_birthday(std::size_t group_size) {
+    if (group_size <= 1) return 0.0;
+    if (group_size > 365) return 100.0;
+    double unique_prob = 1.0;
+    for (std::size_t i = 0; i < group_size; ++i) {
+        unique_prob *= (365.0 - static_cast<double>(i)) / 365.0;
+    }
+    return (1.0 - unique_prob) * 100.0;
+}
+
+}  // namespace baffling_birthdays

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

@@ -0,0 +1,36 @@
+#pragma once
+
+#include <boost/date_time/gregorian/gregorian.hpp>
+#include <string>
+#include <vector>
+
+namespace baffling_birthdays {
+
+using Date = boost::gregorian::date;
+
+Date parse_date(const std::string& iso);
+
+bool shared_birthday(const std::vector<Date>& dates);
+std::vector<Date> random_birthdates(std::size_t group_size);
+
+double estimated_probability_of_shared_birthday(std::size_t group_size);
+
+inline bool shared_birthday(const std::vector<std::string>& birthdates) {
+    std::vector<Date> dates;
+    dates.reserve(birthdates.size());
+    for (auto const& s : birthdates) {
+        dates.push_back(parse_date(s));
+    }
+    return shared_birthday(dates);
+}
+
+inline std::vector<std::string> random_birthdates_str(std::size_t group_size) {
+    auto dates = random_birthdates(group_size);
+    std::vector<std::string> out;
+    out.reserve(group_size);
+    for (auto const& d : dates)
+        out.push_back(boost::gregorian::to_iso_extended_string(d));
+    return out;
+}
+
+}  // namespace baffling_birthdays

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/CMakeLists.txt

@@ -0,0 +1,78 @@
+# Get the exercise name from the current directory
+get_filename_component(exercise ${CMAKE_CURRENT_SOURCE_DIR} NAME)
+
+# Basic CMake project
+cmake_minimum_required(VERSION 3.5.1)
+
+# Name the project after the exercise
+project(${exercise} CXX)
+
+# Locate Boost date_time library
+set(Boost_USE_MULTITHREADED ON)
+set(Boost_USE_STATIC_RUNTIME OFF)
+find_package(Boost 1.58 REQUIRED COMPONENTS date_time)
+
+# Get a source filename from the exercise name by replacing -'s with _'s
+string(REPLACE "-" "_" file ${exercise})
+
+# Implementation could be only a header
+if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/${file}.cpp)
+    set(exercise_cpp ${file}.cpp)
+else()
+    set(exercise_cpp "")
+endif()
+
+# Use the common Catch library?
+if(EXERCISM_COMMON_CATCH)
+    # For Exercism track development only
+    add_executable(${exercise} ${file}_test.cpp ${exercise_cpp} ${file}.h $<TARGET_OBJECTS:catchlib>)
+elseif(EXERCISM_TEST_SUITE)
+    # The Exercism test suite is being run, the Docker image already
+    # includes a pre-built version of Catch.
+    find_package(Catch2 REQUIRED)
+    add_executable(${exercise} ${file}_test.cpp ${exercise_cpp} ${file}.h)
+    target_link_libraries(${exercise} PRIVATE Catch2::Catch2WithMain)
+    # When Catch is installed system wide we need to include a different
+    # header, we need this define to use the correct one.
+    target_compile_definitions(${exercise} PRIVATE EXERCISM_TEST_SUITE)
+else()
+    # Build executable from sources and headers
+    add_executable(${exercise} ${file}_test.cpp ${exercise_cpp} ${file}.h test/tests-main.cpp)
+endif()
+
+set_target_properties(${exercise} PROPERTIES
+    CXX_STANDARD 17
+    CXX_STANDARD_REQUIRED OFF
+    CXX_EXTENSIONS OFF
+)
+
+set(CMAKE_BUILD_TYPE Debug)
+
+if("${CMAKE_CXX_COMPILER_ID}" MATCHES "(GNU|Clang)")
+    set_target_properties(${exercise} PROPERTIES
+        COMPILE_FLAGS "-Wall -Wextra -Wpedantic -Werror"
+    )
+endif()
+
+# We need boost libraries
+target_link_libraries(${exercise}
+    PRIVATE
+    Boost::date_time
+)
+
+# Configure to run all the tests?
+if(${EXERCISM_RUN_ALL_TESTS})
+    target_compile_definitions(${exercise} PRIVATE EXERCISM_RUN_ALL_TESTS)
+endif()
+
+# Tell MSVC not to warn us about unchecked iterators in debug builds
+# Treat warnings as errors
+# Treat type conversion warnings C4244 and C4267 as level 4 warnings, i.e. ignore them in level 3
+if(${MSVC})
+    set_target_properties(${exercise} PROPERTIES
+        COMPILE_DEFINITIONS_DEBUG _SCL_SECURE_NO_WARNINGS
+        COMPILE_FLAGS "/WX /w44244 /w44267")
+endif()
+
+# Run the tests on every build
+add_custom_target(test_${exercise} ALL DEPENDS ${exercise} COMMAND ${exercise})

exercises/practice/baffling-birthdays/baffling_birthdays.cpp

@@ -0,0 +1,7 @@
+#include "baffling_birthdays.h"
+
+namespace baffling_birthdays {
+
+// TODO: add your solution here
+
+}  // namespace baffling_birthdays

exercises/practice/baffling-birthdays/baffling_birthdays.h

@@ -0,0 +1,7 @@
+#pragma once
+
+namespace baffling_birthdays {
+
+// TODO: add your solution here
+
+}  // namespace baffling_birthdays