Add baffling-birthdays exercise (#28)

* Add `baffling-birthdays` exercise

* Fix tests

Author: ErikSchierboom

config.json

@@ -216,6 +216,14 @@
         "practices": [],
         "prerequisites": [],
         "difficulty": 4
+      },
+      {
+        "slug": "baffling-birthdays",
+        "name": "Baffling Birthdays",
+        "uuid": "4bb8b322-f565-4140-b656-58397cbe8e11",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 5
       }
     ]
   },

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 +1,19 @@
+{
+  "authors": [
+    "erikschierboom"
+  ],
+  "files": {
+    "solution": [
+      "solution.jai"
+    ],
+    "test": [
+      "tests.jai"
+    ],
+    "example": [
+      ".meta/example.jai"
+    ]
+  },
+  "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.jai

@@ -0,0 +1,50 @@
+shared_birthday :: (birthdates: [] Calendar_Time) -> bool {
+    unique_birthdays: [..] int;
+
+    for birthdate: birthdates {
+        birthday_key := cast(s16)birthdate.month_starting_at_0 * 31 + birthdate.day_of_month_starting_at_0;
+        is_unique, _ := array_add_if_unique(*unique_birthdays, birthday_key);
+        if !is_unique return true;
+    }
+
+    return false;
+}
+
+random_birthdates :: (count: int) -> [] Calendar_Time {
+     birthdates : [..] Calendar_Time;
+     array_reserve(*birthdates, count);
+     for 1..count array_add(*birthdates, random_birthdate());
+     return birthdates;
+}
+
+random_birthdate :: () -> Calendar_Time {
+    is_leap_year :: (year: int) -> bool {
+        return year % 4 == 0 && (year % 100 != 0 || year % 400 == 0);
+    }
+
+    DAYS_IN_MONTH := u64.[31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
+
+    year  := cast(s32) (random_get() % 100 + 1920);
+    month := cast(s8)  (random_get() % 12);
+    day   := cast(s8)  (random_get() % DAYS_IN_MONTH[month]);
+
+    if is_leap_year(year) year -= 1;
+
+    return .{year = year, month_starting_at_0 = month, day_of_month_starting_at_0 = day};
+}
+
+estimated_probability_of_shared_birthday :: (group_size: int) -> float {
+    ITERATION_COUNT :: 1000;
+    
+    matching_birthday_iterations := 0.0;
+
+    for 1..ITERATION_COUNT {
+        birthdates := random_birthdates(group_size);
+        if shared_birthday(birthdates) matching_birthday_iterations += 1;
+    }
+    
+    return (matching_birthday_iterations / ITERATION_COUNT) * 100;
+}
+
+#import "Basic";
+#import "Random";

exercises/practice/baffling-birthdays/.meta/generator.jai

@@ -0,0 +1,76 @@
+to_test_case :: (canonical_case: CanonicalCase) -> Test {
+    using test: Test;
+    name = to_test_name(canonical_case.description_path);
+    skip = canonical_case.index > 0;
+
+    if canonical_case.property == {
+        case "sharedBirthday";
+            _, birthdates := get_property(canonical_case.input, "birthdates");
+            
+            lines: [..]string;
+            array_add(*lines, "birthdates : [..] Calendar_Time;");
+
+            for birthdate: birthdates.array {
+                parts := split(birthdate.str, "-");
+                year := to_integer(parts[0]);
+                month := to_integer(parts[1]);
+                day := to_integer(parts[2]);
+                array_add(*lines, tprint("array_add(*birthdates, .{year = %, month_starting_at_0 = %, day_of_month_starting_at_0 = %});", to_code(year), to_code(month - 1), to_code(day - 1)));
+            }
+            array_add(*lines, tprint("assert_equal(%, shared_birthday(birthdates));", to_code(canonical_case.expected.boolean)));
+            multiline_body = lines;
+        case "randomBirthdates";
+            lines: [..]string;
+
+            if canonical_case.expected.type == .STRING && canonical_case.expected.str == "length == groupsize" {
+                array_add(*lines, "for 1..10 {");
+                array_add(*lines, tprint("    assert_equal(it, random_birthdates(it).count);"));
+                array_add(*lines, "}");
+            } else if get_property(canonical_case.expected, "years", "leapYear") {
+                array_add(*lines, "is_leap_year :: (year: int) -> bool {");
+                array_add(*lines, "    return year % 4 == 0 && (year % 100 != 0 || year % 400 == 0);");
+                array_add(*lines, "}");
+                array_add(*lines, "");
+                array_add(*lines, "for birthdate: random_birthdates(50) {");
+                array_add(*lines, tprint("    assert_equal(false, is_leap_year(birthdate.year));"));
+                array_add(*lines, "}");
+            } else if get_property(canonical_case.expected, "months", "random") {
+                array_add(*lines, "generated_unique_months: [..]s8;");
+                array_add(*lines, "for birthdate: random_birthdates(100) {");
+                array_add(*lines, tprint("    array_add_if_unique(*generated_unique_months, birthdate.month_starting_at_0);"));
+                array_add(*lines, "}");
+                array_add(*lines, "");
+                array_add(*lines, tprint("assert_equal(12, generated_unique_months.count);"));
+            } else if get_property(canonical_case.expected, "days", "random") {
+                array_add(*lines, "generated_unique_days: [..]s8;");
+                array_add(*lines, "for birthdate: random_birthdates(300) {");
+                array_add(*lines, tprint("    array_add_if_unique(*generated_unique_days, birthdate.day_of_month_starting_at_0);"));
+                array_add(*lines, "}");
+                array_add(*lines, "");
+                array_add(*lines, tprint("assert_equal(31, generated_unique_days.count);"));
+            } else {
+                assert(false, "Unexpected randomBirthdates test case");
+            }
+
+            multiline_body = lines;
+        case "estimatedProbabilityOfSharedBirthday";
+            _, group_size := get_property(canonical_case.input, "groupSize");
+            expected := ifx canonical_case.expected.number then to_code(canonical_case.expected.number) else "0.0";
+
+            lines: [..]string;
+            array_add(*lines, tprint("estimated_probability := estimated_probability_of_shared_birthday(%);", to_code(group_size)));
+            array_add(*lines, tprint("expected_probability := %;", expected));
+            array_add(*lines, tprint("assert_equal(true, estimated_probability > expected_probability - 1.0);"));
+            array_add(*lines, tprint("assert_equal(true, estimated_probability < expected_probability + 1.0);"));
+
+            multiline_body = lines;
+    }
+
+    return test;
+}
+
+#run run_generator("baffling-birthdays", to_test_case);
+
+#import "Basic";
+#import "String";
+#import,dir "../../../shared/generator";

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"