Add baffling-birthdays

bin/generate-spec

@@ -2,6 +2,37 @@
 
 local json = require 'dkjson'
 
+local utils = {}
+
+utils.map = function(t, f)
+  local mapped = {}
+  for i, v in ipairs(t) do
+    mapped[i] = f(v)
+  end
+  return mapped
+end
+
+utils.stringify = function(s)
+  s = s:gsub('\n', '\\n')
+  if s:match("'") and not s:match('"') then
+    return '"' .. s .. '"'
+  else
+    return "'" .. s:gsub("'", "\\'") .. "'"
+  end
+end
+
+utils.snake_case = function(str)
+  local s = str:gsub('%u', function(c)
+    return '_' .. c:lower()
+  end)
+  if s:sub(1, 1) == '_' then
+    s = s:sub(2)
+  end
+  return s
+end
+
+package.loaded['utils'] = utils
+
 local function read_file(path)
   local f = assert(io.open(path, 'r'))
   local contents = f:read('*a')
@@ -81,12 +112,12 @@ local function process(node)
     return table.concat(output, '\n')
   else
     local template = [[
-      it('%s', function()
+      it(%s, function()
         %s
       end)]]
 
     return template:format(
-      node.description:lower():gsub('\'', '\\\''),
+      utils.stringify(node.description:lower()),
       spec_generator.generate_test(node)
     )
   end

config.json

@@ -1481,6 +1481,14 @@
           "exception_handling",
           "graphs"
         ]
+      },
+      {
+        "slug": "baffling-birthdays",
+        "name": "Baffling Birthdays",
+        "uuid": "54dd8d8f-b58c-47d0-8c77-22dd5fcdf14f",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 3
       }
     ]
   },

exercises/practice/anagram/.meta/spec_generator.lua

@@ -1,13 +1,7 @@
-local function map(t, f)
-  local mapped = {}
-  for i, v in ipairs(t) do
-    mapped[i] = f(v)
-  end
-  return mapped
-end
+local utils = require 'utils'
 
 local function render_list(list)
-  return '{ ' .. table.concat(map(list, function(v)
+  return '{ ' .. table.concat(utils.map(list, function(v)
     return "'" .. v .. "'"
   end), ', ') .. ' }'
 end

exercises/practice/baffling-birthdays/.busted

@@ -0,0 +1,5 @@
+return {
+  default = {
+    ROOT = { '.' }
+  }
+}

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": [
+    "ryanplusplus"
+  ],
+  "files": {
+    "solution": [
+      "baffling-birthdays.lua"
+    ],
+    "test": [
+      "baffling-birthdays_spec.lua"
+    ],
+    "example": [
+      ".meta/example.lua"
+    ]
+  },
+  "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.lua

@@ -0,0 +1,41 @@
+local baffling_birthdays = {}
+
+baffling_birthdays.shared_birthday = function(birthdates)
+  local seen = {}
+  for _, birthdate in ipairs(birthdates) do
+    local without_year = birthdate:sub(6)
+    if seen[without_year] then
+      return true
+    end
+    seen[without_year] = true
+  end
+  return false
+end
+
+baffling_birthdays.random_birthdates = function(count)
+  local birthdates = {}
+
+  local non_leap_year = 2026
+  local seconds_per_day = 24 * 60 * 60
+  local start = os.time({ year = non_leap_year, month = 1, day = 1, hour = 12 })
+  for i = 1, count do
+    local offset = math.random(0, 364)
+    local t = start + offset * seconds_per_day
+    table.insert(birthdates, os.date('%Y-%m-%d', t))
+  end
+
+  return birthdates
+end
+
+baffling_birthdays.estimated_probability_of_shared_birthday = function(group_size)
+  local shared_count = 0
+  for i = 1, 10000 do
+    local birthdates = baffling_birthdays.random_birthdates(group_size)
+    if baffling_birthdays.shared_birthday(birthdates) then
+      shared_count = shared_count + 1
+    end
+  end
+  return shared_count / 100
+end
+
+return baffling_birthdays

exercises/practice/baffling-birthdays/.meta/spec_generator.lua

@@ -0,0 +1,64 @@
+local utils = require 'utils'
+
+return {
+  module_name = 'baffling_birthdays',
+
+  generate_test = function(case)
+    if type(case.expected) == 'table' then
+      if case.expected.years and case.expected.years.leapYear ~= nil then
+        return [[
+          local birthdates = baffling_birthdays.random_birthdates(1000)
+          for _, birthdate in ipairs(birthdates) do
+            local year = tonumber(birthdate:sub(1, 4))
+            assert.is_false(year % 4 == 0 and (year % 100 ~= 0 or year % 400 == 0))
+          end]]
+      elseif case.expected.months and case.expected.months.random ~= nil then
+        return [[
+          local birthdates = baffling_birthdays.random_birthdates(1000)
+          local seen = {}
+          local unique_count = 0
+          for _, birthdate in ipairs(birthdates) do
+            local month = tonumber(birthdate:sub(6, 7))
+            if not seen[month] then
+              seen[month] = true
+              unique_count = unique_count + 1
+            end
+          end
+          assert.are.equal(12, unique_count)]]
+      elseif case.expected.days and case.expected.days.random ~= nil then
+        return [[
+          local birthdates = baffling_birthdays.random_birthdates(1000)
+          local seen = {}
+          local unique_count = 0
+          for _, birthdate in ipairs(birthdates) do
+            local day = tonumber(birthdate:sub(9, 10))
+            if not seen[day] then
+              seen[day] = true
+              unique_count = unique_count + 1
+            end
+          end
+          assert.are.equal(31, unique_count)]]
+      else
+        error('Unhandled case')
+      end
+    elseif case.expected == 'length == groupsize' then
+      return [[
+        for i = 1, 10 do
+          assert.are.equal(i, #baffling_birthdays.random_birthdates(i))
+        end]]
+    elseif case.property == 'estimatedProbabilityOfSharedBirthday' then
+      local template = [[
+        local actual = baffling_birthdays.%s(%s)
+        assert.are.near(%s, actual, 1)]]
+
+      return template:format(utils.snake_case(case.property), case.input.groupSize, case.expected)
+    else
+      local template = [[
+        local actual = baffling_birthdays.%s(%s)
+        assert.are.equal(%s, actual)]]
+
+      local input = '{ ' .. table.concat(utils.map(case.input.birthdates, utils.stringify), ', ') .. ' }'
+      return template:format(utils.snake_case(case.property), input, case.expected)
+    end
+  end
+}

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/baffling-birthdays.lua

@@ -0,0 +1,12 @@
+local baffling_birthdays = {}
+
+baffling_birthdays.shared_birthday = function(birthdates)
+end
+
+baffling_birthdays.random_birthdates = function(count)
+end
+
+baffling_birthdays.estimated_probability_of_shared_birthday = function(group_size)
+end
+
+return baffling_birthdays