Add baffling-birthdays exercise

exercises/baffling-birthdays/canonical-data.json

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+{
+  "exercise": "baffling-birthdays",
+  "comments": [
+    "Dates are formatted as 'YYYY-MM-DD'.",
+    "",
+    "To increase the likelihood of consistent result, the tests should run the code returning random results many times.",
+    "",
+    "The expected probability values should be compared using some tolerance to allow for small deviations."
+  ],
+  "cases": [
+    {
+      "description": "shared birthday",
+      "cases": [
+        {
+          "uuid": "716dcc2b-8fe4-4fc9-8c48-cbe70d8e6b67",
+          "description": "one birthdate",
+          "property": "sharedBirthday",
+          "input": {
+            "birthdates": ["2000-01-01"]
+          },
+          "expected": false
+        },
+        ,
+        {
+          "uuid": "f7b3eb26-bcfc-4c1e-a2de-af07afc33f45",
+          "description": "two birthdates with same year, month, and day",
+          "property": "sharedBirthday",
+          "input": {
+            "birthdates": ["2000-01-01", "2000-01-01"]
+          },
+          "expected": true
+        },
+        {
+          "uuid": "7193409a-6e16-4bcb-b4cc-9ffe55f79b25",
+          "description": "two birthdates with same year and month, but different day",
+          "property": "sharedBirthday",
+          "input": {
+            "birthdates": ["2012-05-09", "2012-05-17"]
+          },
+          "expected": false
+        },
+        {
+          "uuid": "d04db648-121b-4b72-93e8-d7d2dced4495",
+          "description": "two birthdates with same month and day, but different year",
+          "property": "sharedBirthday",
+          "input": {
+            "birthdates": ["1999-10-23", "1988-10-23"]
+          },
+          "expected": true
+        },
+        {
+          "uuid": "3c8bd0f0-14c6-4d4c-975a-4c636bfdc233",
+          "description": "two birthdates with same year, but different month and day",
+          "property": "sharedBirthday",
+          "input": {
+            "birthdates": ["2007-12-19", "2007-04-27"]
+          },
+          "expected": false
+        },
+        {
+          "uuid": "df5daba6-0879-4480-883c-e855c99cdaa3",
+          "description": "two birthdates with different year, month, and day",
+          "property": "sharedBirthday",
+          "input": {
+            "birthdates": ["1997-08-04", "1963-11-23"]
+          },
+          "expected": false
+        },
+        {
+          "uuid": "0c17b220-cbb9-4bd7-872f-373044c7b406",
+          "description": "multiple birthdates without shared birthday",
+          "property": "sharedBirthday",
+          "input": {
+            "birthdates": [
+              "1966-07-29",
+              "1977-02-12",
+              "2001-12-25",
+              "1980-11-10"
+            ]
+          },
+          "expected": false
+        },
+        {
+          "uuid": "966d6b0b-5c0a-4b8c-bc2d-64939ada49f8",
+          "description": "multiple birthdates with one shared birthday",
+          "property": "sharedBirthday",
+          "input": {
+            "birthdates": [
+              "1966-07-29",
+              "1977-02-12",
+              "2001-07-29",
+              "1980-11-10"
+            ]
+          },
+          "expected": true
+        },
+        {
+          "uuid": "b7937d28-403b-4500-acce-4d9fe3a9620d",
+          "description": "multiple birthdates with more than one shared birthday",
+          "property": "sharedBirthday",
+          "input": {
+            "birthdates": [
+              "1966-07-29",
+              "1977-02-12",
+              "2001-12-25",
+              "1980-07-29",
+              "2019-02-12"
+            ]
+          },
+          "expected": true
+        }
+      ]
+    },
+    {
+      "description": "random birthdates",
+      "cases": [
+        {
+          "uuid": "70b38cea-d234-4697-b146-7d130cd4ee12",
+          "description": "generate requested number of birthdates",
+          "scenarios": ["random"],
+          "property": "randomBirthdates",
+          "input": {},
+          "expected": "length == groupsize"
+        },
+        {
+          "uuid": "d9d5b7d3-5fea-4752-b9c1-3fcd176d1b03",
+          "description": "years are not leap years",
+          "scenarios": ["random"],
+          "property": "randomBirthdates",
+          "input": {},
+          "expected": {
+            "years": {
+              "leapYear": false
+            }
+          }
+        },
+        {
+          "uuid": "d1074327-f68c-4c8a-b0ff-e3730d0f0521",
+          "description": "months are random",
+          "scenarios": ["random"],
+          "property": "randomBirthdates",
+          "input": {},
+          "expected": {
+            "months": {
+              "random": true
+            }
+          }
+        },
+        {
+          "uuid": "7df706b3-c3f5-471d-9563-23a4d0577940",
+          "description": "days are random",
+          "scenarios": ["random"],
+          "property": "randomBirthdates",
+          "input": {},
+          "expected": {
+            "days": {
+              "random": true
+            }
+          }
+        }
+      ]
+    },
+    {
+      "description": "estimated probability of at least one shared birthday",
+      "cases": [
+        {
+          "uuid": "89a462a4-4265-4912-9506-fb027913f221",
+          "description": "for one person",
+          "scenarios": ["random"],
+          "property": "estimatedProbabilityOfSharedBirthday",
+          "input": {
+            "groupSize": 1
+          },
+          "expected": 0.0
+        },
+        {
+          "uuid": "ec31c787-0ebb-4548-970c-5dcb4eadfb5f",
+          "description": "among ten people",
+          "scenarios": ["random"],
+          "property": "estimatedProbabilityOfSharedBirthday",
+          "input": {
+            "groupSize": 10
+          },
+          "expected": 11.694818
+        },
+        {
+          "uuid": "b548afac-a451-46a3-9bb0-cb1f60c48e2f",
+          "description": "among twenty-three people",
+          "scenarios": ["random"],
+          "property": "estimatedProbabilityOfSharedBirthday",
+          "input": {
+            "groupSize": 23
+          },
+          "expected": 50.729723
+        },
+        {
+          "uuid": "e43e6b9d-d77b-4f6c-a960-0fc0129a0bc5",
+          "description": "among seventy people",
+          "scenarios": ["random"],
+          "property": "estimatedProbabilityOfSharedBirthday",
+          "input": {
+            "groupSize": 70
+          },
+          "expected": 99.915958
+        }
+      ]
+    }
+  ]
+}

exercises/baffling-birthdays/instructions.md

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+# Instructions
+
+Your task is to estimate the birthday paradox's probabilities.
+
+To do this, you need to:
+
+- Determine if two birthdates have the same month and day.
+- 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/baffling-birthdays/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 their birthdays.
+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/baffling-birthdays/metadata.toml

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+title = "Baffling Birthdays"
+blurb = "Estimate the birthday paradox's probabilities."
+source = "Erik Schierboom"
+source_url = "https://github.com/exercism/problem-specifications/pull/2539"