Sync palindrome products exercise with problem specification

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

@@ -12,7 +12,8 @@
     "Insti",
     "kotp",
     "markijbema",
-    "tryantwit"
+    "tryantwit",
+    "themetar"
   ],
   "files": {
     "solution": [

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

@@ -1,43 +1,42 @@
 Palindrome = Struct.new(:value, :factors)
 
 class Palindromes
-  attr_reader :range
-  def initialize(options)
-    max = options.fetch(:max_factor)
-    min = options.fetch(:min_factor) { 1 }
-    @range = (min..max)
-  end
+  private
 
-  def generate
-    @palindromes = {}
-    range.each do |i|
-      range.each do |j|
-        product = i * j
-        if palindrome?(product)
-          palindrome = @palindromes[product] || Palindrome.new(product, [])
-          palindrome.factors << [i, j].sort
-          palindrome.factors.uniq!
-          @palindromes[product] = palindrome
-        end
-      end
-    end
+  attr_reader :min, :max
+
+  def initialize(max_factor:, min_factor: 1)
+    raise ArgumentError, "min must be <= max" unless min_factor <= max_factor
+
+    @min = min_factor
+    @max = max_factor
   end
 
   def palindrome?(number)
     number.to_s == number.to_s.reverse
   end
 
-  def sort
-    @palindromes.sort_by do |key, _palindrome|
-      key
+  def factors(palindrome)
+    (min..Math.sqrt(palindrome)).each_with_object([]) do |number, factors|
+      div, mod = palindrome.divmod(number)
+      factors << [number, div] if div.between?(min, max) && mod.zero?
     end
   end
 
-  def largest
-    sort.last[1]
+  def find_palindrome(enum)
+    enum.lazy.
+      filter { |number| palindrome? number }.
+      map { |number| Palindrome.new number, factors(number) }.
+      find { |palindrome| !palindrome.factors.empty? }
   end
 
-  def smallest
-    sort.first[1]
+  public
+
+  attr_reader :smallest, :largest
+
+  def generate
+    @smallest = find_palindrome(min**2..max**2) || Palindrome.new(nil, [])
+
+    @largest = find_palindrome((max**2..min**2).step(-1)) || Palindrome.new(nil, [])
   end
 end

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

@@ -9,6 +9,9 @@
 # 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"
 
@@ -23,3 +26,24 @@ 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_test.rb

@@ -2,7 +2,16 @@
 require_relative 'palindrome_products'
 
 class PalindromesTest < Minitest::Test
+  def test_smallest_palindrome_from_single_digit_factors
+    palindromes = Palindromes.new(max_factor: 9)
+    palindromes.generate
+    smallest = palindromes.smallest
+    assert_equal 1, smallest.value
+    assert_equal [[1, 1]], smallest.factors
+  end
+
   def test_largest_palindrome_from_single_digit_factors
+    skip
     palindromes = Palindromes.new(max_factor: 9)
     palindromes.generate
     largest = palindromes.largest
@@ -45,4 +54,69 @@ def test_smallest_palindrome_from_triple_digit_factors
     assert_equal 10_201, smallest.value
     assert_equal [[101, 101]], smallest.factors
   end
+
+  def test_smallest_palindrome_from_four_digit_factors
+    skip
+    palindromes = Palindromes.new(min_factor: 1000, max_factor: 9999)
+    palindromes.generate
+    smallest = palindromes.smallest
+    assert_equal 1_002_001, smallest.value
+    assert_equal [[1001, 1001]], smallest.factors
+  end
+
+  def test_largest_palindrome_from_four_digit_factors
+    skip
+    palindromes = Palindromes.new(min_factor: 1000, max_factor: 9999)
+    palindromes.generate
+    largest = palindromes.largest
+    assert_equal 99_000_099, largest.value
+    assert_equal [[9901, 9999]], largest.factors
+  end
+
+  def test_empty_for_smallest_if_no_palindrome_in_range
+    skip
+    palindromes = Palindromes.new(min_factor: 1002, max_factor: 1003)
+    palindromes.generate
+    smallest = palindromes.smallest
+    assert_nil smallest.value
+    assert_empty smallest.factors
+  end
+
+  def test_empty_for_largest_if_no_palindrome_in_range
+    skip
+    palindromes = Palindromes.new(min_factor: 15, max_factor: 15)
+    palindromes.generate
+    largest = palindromes.largest
+    assert_nil largest.value
+    assert_empty largest.factors
+  end
+
+  def test_error_for_smallest_if_min_more_than_max
+    skip
+    error = assert_raises(ArgumentError) do
+      palindromes = Palindromes.new(min_factor: 10_000, max_factor: 1)
+      palindromes.generate
+      palindromes.smallest
+    end
+    assert_equal "min must be <= max", error.message
+  end
+
+  def test_error_for_largest_if_min_more_than_max
+    skip
+    error = assert_raises(ArgumentError) do
+      palindromes = Palindromes.new(min_factor: 2, max_factor: 1)
+      palindromes.generate
+      palindromes.smallest
+    end
+    assert_equal "min must be <= max", error.message
+  end
+
+  def test_smallest_palindrome_does_not_use_smallest_factors
+    skip
+    palindromes = Palindromes.new(min_factor: 3215, max_factor: 4000)
+    palindromes.generate
+    smallest = palindromes.smallest
+    assert_equal 10_988_901, smallest.value
+    assert_equal [[3297, 3333]], smallest.factors
+  end
 end