Add palindrome-products practice exercise (#521)

* Add palindrome products practice exercise

* Suggestion from keiravillekode

config.json

@@ -1171,6 +1171,14 @@
           "text_parsing"
         ]
       },
+      {
+        "slug": "palindrome-products",
+        "name": "Palindrome Products",
+        "uuid": "928d3505-7780-4d2d-8fd8-7c8f0aac4033",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 5
+      },
       {
         "slug": "anagram",
         "name": "Anagram",

exercises/practice/palindrome-products/.busted

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

exercises/practice/palindrome-products/.docs/instructions.md

@@ -0,0 +1,36 @@
+# Instructions
+
+Detect palindrome products in a given range.
+
+A palindromic number is a number that remains the same when its digits are reversed.
+For example, `121` is a palindromic number but `112` is not.
+
+Given a range of numbers, find the largest and smallest palindromes which
+are products of two numbers within that range.
+
+Your solution should return the largest and smallest palindromes, along with the factors of each within the range.
+If the largest or smallest palindrome has more than one pair of factors within the range, then return all the pairs.
+
+## Example 1
+
+Given the range `[1, 9]` (both inclusive)...
+
+And given the list of all possible products within this range:
+`[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, 15, 21, 24, 27, 20, 28, 32, 36, 25, 30, 35, 40, 45, 42, 48, 54, 49, 56, 63, 64, 72, 81]`
+
+The palindrome products are all single digit numbers (in this case):
+`[1, 2, 3, 4, 5, 6, 7, 8, 9]`
+
+The smallest palindrome product is `1`.
+Its factors are `(1, 1)`.
+The largest palindrome product is `9`.
+Its factors are `(1, 9)` and `(3, 3)`.
+
+## Example 2
+
+Given the range `[10, 99]` (both inclusive)...
+
+The smallest palindrome product is `121`.
+Its factors are `(11, 11)`.
+The largest palindrome product is `9009`.
+Its factors are `(91, 99)`.

exercises/practice/palindrome-products/.meta/config.json

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "ryanplusplus"
+  ],
+  "files": {
+    "solution": [
+      "palindrome-products.lua"
+    ],
+    "test": [
+      "palindrome-products_spec.lua"
+    ],
+    "example": [
+      ".meta/example.lua"
+    ]
+  },
+  "blurb": "Detect palindrome products in a given range.",
+  "source": "Problem 4 at Project Euler",
+  "source_url": "https://projecteuler.net/problem=4"
+}

exercises/practice/palindrome-products/.meta/example.lua

@@ -0,0 +1,72 @@
+local function reversed(s)
+  local reversed = ''
+  for c in s:gmatch('.') do
+    reversed = c .. reversed
+  end
+  return reversed
+end
+
+local function is_palindrome(n)
+  local s = tostring(n)
+  return s == reversed(s)
+end
+
+local function validate_inputs(min, max)
+  assert(min <= max, 'min must be <= max')
+end
+
+local function smallest(min, max)
+  validate_inputs(min, max)
+
+  local value = math.huge
+  local factors = {}
+
+  for i = min, max do
+    for j = i, max do
+      local n = i * j
+
+      if n > value then
+        break
+      end
+
+      if is_palindrome(n) then
+        if n ~= value then
+          value = n
+          factors = {}
+        end
+        table.insert(factors, { i, j })
+      end
+    end
+  end
+
+  return { value = #factors > 0 and value or nil, factors = factors }
+end
+
+local function largest(min, max)
+  validate_inputs(min, max)
+
+  local value = -math.huge
+  local factors = {}
+
+  for i = max, min, -1 do
+    for j = i, min, -1 do
+      local n = i * j
+
+      if n < value then
+        break
+      end
+
+      if is_palindrome(n) then
+        if n ~= value then
+          value = n
+          factors = {}
+        end
+        table.insert(factors, { j, i })
+      end
+    end
+  end
+
+  return { value = #factors > 0 and value or nil, factors = factors }
+end
+
+return { smallest = smallest, largest = largest }

exercises/practice/palindrome-products/.meta/spec_generator.lua

@@ -0,0 +1,32 @@
+local function render_factors(factors)
+  local rendered = {}
+  for _, factor in ipairs(factors) do
+    table.insert(rendered, string.format('{ %d, %d }', factor[1], factor[2]))
+  end
+  return '{ ' .. table.concat(rendered, ', ') .. ' }'
+end
+
+return {
+  module_name = 'palindrome_products',
+
+  generate_test = function(case)
+    if case.expected.error then
+      local template = [[
+        assert.has_error(function()
+          palindrome_products.%s(%d, %d)
+        end, '%s')]]
+
+      return template:format(case.property, case.input.min, case.input.max, case.expected.error)
+    else
+      local template = [[
+        local expected = {
+          value = %s, --
+          factors = %s
+        }
+        assert.are.same(expected, palindrome_products.%s(%d, %d))]]
+
+      return template:format(case.expected.value or 'nil', render_factors(case.expected.factors), case.property,
+                             case.input.min, case.input.max)
+    end
+  end
+}

exercises/practice/palindrome-products/.meta/tests.toml

@@ -0,0 +1,49 @@
+# 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.
+
+[5cff78fe-cf02-459d-85c2-ce584679f887]
+description = "find the smallest palindrome from single digit factors"
+
+[0853f82c-5fc4-44ae-be38-fadb2cced92d]
+description = "find the largest palindrome from single digit factors"
+
+[66c3b496-bdec-4103-9129-3fcb5a9063e1]
+description = "find the smallest palindrome from double digit factors"
+
+[a10682ae-530a-4e56-b89d-69664feafe53]
+description = "find the largest palindrome from double digit factors"
+
+[cecb5a35-46d1-4666-9719-fa2c3af7499d]
+description = "find the smallest palindrome from triple digit factors"
+
+[edab43e1-c35f-4ea3-8c55-2f31dddd92e5]
+description = "find the largest palindrome from triple digit factors"
+
+[4f802b5a-9d74-4026-a70f-b53ff9234e4e]
+description = "find the smallest palindrome from four digit factors"
+
+[787525e0-a5f9-40f3-8cb2-23b52cf5d0be]
+description = "find the largest palindrome from four digit factors"
+
+[58fb1d63-fddb-4409-ab84-a7a8e58d9ea0]
+description = "empty result for smallest if no palindrome in the range"
+
+[9de9e9da-f1d9-49a5-8bfc-3d322efbdd02]
+description = "empty result for largest if no palindrome in the range"
+
+[12e73aac-d7ee-4877-b8aa-2aa3dcdb9f8a]
+description = "error result for smallest if min is more than max"
+
+[eeeb5bff-3f47-4b1e-892f-05829277bd74]
+description = "error result for largest if min is more than max"
+
+[16481711-26c4-42e0-9180-e2e4e8b29c23]
+description = "smallest product does not use the smallest factor"

exercises/practice/palindrome-products/palindrome-products.lua

@@ -0,0 +1,9 @@
+local function smallest(min, max)
+
+end
+
+local function largest(min, max)
+
+end
+
+return { smallest = smallest, largest = largest }

exercises/practice/palindrome-products/palindrome-products_spec.lua

@@ -0,0 +1,103 @@
+local palindrome_products = require('palindrome-products')
+
+describe('palindrome-products', function()
+  it('find the smallest palindrome from single digit factors', function()
+    local expected = {
+      value = 1, --
+      factors = { { 1, 1 } }
+    }
+    assert.are.same(expected, palindrome_products.smallest(1, 9))
+  end)
+
+  it('find the largest palindrome from single digit factors', function()
+    local expected = {
+      value = 9, --
+      factors = { { 1, 9 }, { 3, 3 } }
+    }
+    assert.are.same(expected, palindrome_products.largest(1, 9))
+  end)
+
+  it('find the smallest palindrome from double digit factors', function()
+    local expected = {
+      value = 121, --
+      factors = { { 11, 11 } }
+    }
+    assert.are.same(expected, palindrome_products.smallest(10, 99))
+  end)
+
+  it('find the largest palindrome from double digit factors', function()
+    local expected = {
+      value = 9009, --
+      factors = { { 91, 99 } }
+    }
+    assert.are.same(expected, palindrome_products.largest(10, 99))
+  end)
+
+  it('find the smallest palindrome from triple digit factors', function()
+    local expected = {
+      value = 10201, --
+      factors = { { 101, 101 } }
+    }
+    assert.are.same(expected, palindrome_products.smallest(100, 999))
+  end)
+
+  it('find the largest palindrome from triple digit factors', function()
+    local expected = {
+      value = 906609, --
+      factors = { { 913, 993 } }
+    }
+    assert.are.same(expected, palindrome_products.largest(100, 999))
+  end)
+
+  it('find the smallest palindrome from four digit factors', function()
+    local expected = {
+      value = 1002001, --
+      factors = { { 1001, 1001 } }
+    }
+    assert.are.same(expected, palindrome_products.smallest(1000, 9999))
+  end)
+
+  it('find the largest palindrome from four digit factors', function()
+    local expected = {
+      value = 99000099, --
+      factors = { { 9901, 9999 } }
+    }
+    assert.are.same(expected, palindrome_products.largest(1000, 9999))
+  end)
+
+  it('empty result for smallest if no palindrome in the range', function()
+    local expected = {
+      value = nil, --
+      factors = {}
+    }
+    assert.are.same(expected, palindrome_products.smallest(1002, 1003))
+  end)
+
+  it('empty result for largest if no palindrome in the range', function()
+    local expected = {
+      value = nil, --
+      factors = {}
+    }
+    assert.are.same(expected, palindrome_products.largest(15, 15))
+  end)
+
+  it('error result for smallest if min is more than max', function()
+    assert.has_error(function()
+      palindrome_products.smallest(10000, 1)
+    end, 'min must be <= max')
+  end)
+
+  it('error result for largest if min is more than max', function()
+    assert.has_error(function()
+      palindrome_products.largest(2, 1)
+    end, 'min must be <= max')
+  end)
+
+  it('smallest product does not use the smallest factor', function()
+    local expected = {
+      value = 10988901, --
+      factors = { { 3297, 3333 } }
+    }
+    assert.are.same(expected, palindrome_products.smallest(3215, 4000))
+  end)
+end)