Add palindrome-products exercise

config.json

@@ -990,6 +990,14 @@
         "prerequisites": [],
         "difficulty": 8
       },
+      {
+        "slug": "palindrome-products",
+        "name": "Palindrome Products",
+        "uuid": "ddbf1590-ce3d-4ab5-94c5-08ecbee79764",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 8
+      },
       {
         "slug": "zebra-puzzle",
         "name": "Zebra Puzzle",

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": [
+    "keiravillekode"
+  ],
+  "files": {
+    "solution": [
+      "palindrome-products.el"
+    ],
+    "test": [
+      "palindrome-products-test.el"
+    ],
+    "example": [
+      ".meta/example.el"
+    ]
+  },
+  "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.el

@@ -0,0 +1,74 @@
+;;; palindrome-products.el --- Palindrome Products (exercism)  -*- lexical-binding: t; -*-
+
+;;; Commentary:
+
+;;; Code:
+
+(require 'cl-lib)
+
+
+(defun is-palindrome (n)
+  (if (= 0 (% n 10))
+    (= 0 n)
+    (let ((acc 0)
+          (digit 0))
+      (cl-loop while (< acc n)
+               do (setq digit (% n 10))
+               do (setq n (/ n 10))
+
+               when (= acc n)
+               do (cl-return t)
+
+               do (setq acc (+ (* 10 acc) digit))
+
+               when (= acc n)
+               do (cl-return t)
+
+               finally return nil))))
+
+
+(defun smallest (min-factor max-factor)
+  (when (< max-factor min-factor)
+    (error "min must be <= max"))
+
+  (let ((value nil)
+        (factors nil))
+    (cl-loop for first from min-factor to max-factor
+      do (cl-loop for second from first to max-factor
+                  for product = (* first second)
+
+                  do (if (and value (< value product)) (cl-return))
+
+                  when (is-palindrome product)
+                  do (cond ((or (null factors) (< product value))
+                            (setq value product)
+                            (setq factors `((,first ,second))))
+                           ((= product value)
+                            (push `(,first ,second) factors)))))
+    (list :value value :factors (reverse factors))))
+
+
+(defun largest (min-factor max-factor)
+  (when (< max-factor min-factor)
+    (error "min must be <= max"))
+
+  (let ((value nil)
+        (factors nil))
+    (cl-loop for second from max-factor downto min-factor
+      do (cl-loop for first from second downto min-factor
+                  for product = (* first second)
+
+                  do (if (and value (< product value)) (cl-return))
+
+                  when (is-palindrome product)
+                  do (cond ((or (null factors) (< value product))
+                            (setq value product)
+                            (setq factors `((,first ,second))))
+                           ((= product value)
+                            (push `(,first ,second) factors)))))
+    (list :value value :factors (reverse factors))))
+
+
+(provide 'palindrome-products)
+;;; palindrome-products.el ends here
+

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-test.el

@@ -0,0 +1,97 @@
+;;; palindrome-products-test.el --- Tests for Palindrome Products (exercism)  -*- lexical-binding: t; -*-
+
+;;; Commentary:
+
+;;; Code:
+
+
+(load-file "palindrome-products.el")
+(declare-function smallest "palindrome-products.el" (min-factor max-factor))
+(declare-function largest "palindrome-products.el" (min-factor max-factor))
+
+
+(ert-deftest find-the-smallest-palindrome-from-single-digit-factors ()
+  (should (equal '(:value 1
+                   :factors ((1 1)))
+                 (smallest 1 9))))
+
+
+(ert-deftest find-the-largest-palindrome-from-single-digit-factors ()
+  (should (equal '(:value 9
+                   :factors ((1 9) (3 3)))
+                 (largest 1 9))))
+
+
+(ert-deftest find-the-smallest-palindrome-from-double-digit-factors ()
+  (should (equal '(:value 121
+                   :factors ((11 11)))
+                 (smallest 10 99))))
+
+
+(ert-deftest find-the-largest-palindrome-from-double-digit-factors ()
+  (should (equal '(:value 9009
+                   :factors ((91 99)))
+                 (largest 10 99))))
+
+
+(ert-deftest find-the-smallest-palindrome-from-triple-digit-factors ()
+  (should (equal '(:value 10201
+                   :factors ((101 101)))
+                 (smallest 100 999))))
+
+
+(ert-deftest find-the-largest-palindrome-from-triple-digit-factors ()
+  (should (equal '(:value 906609
+                   :factors ((913 993)))
+                 (largest 100 999))))
+
+
+(ert-deftest find-the-smallest-palindrome-from-four-digit-factors ()
+  (should (equal '(:value 1002001
+                   :factors ((1001 1001)))
+                 (smallest 1000 9999))))
+
+
+(ert-deftest find-the-largest-palindrome-from-four-digit-factors ()
+  (should (equal '(:value 99000099
+                   :factors ((9901 9999)))
+                 (largest 1000 9999))))
+
+
+(ert-deftest empty-result-for-smallest-if-no-palindrome-in-the-range ()
+  (should (equal '(:value nil
+                   :factors ())
+                 (smallest 1002 1003))))
+
+
+(ert-deftest empty-result-for-largest-if-no-palindrome-in-the-range ()
+  (should (equal '(:value nil
+                   :factors ())
+                 (largest 15 15))))
+
+
+(ert-deftest error-result-for-smallest-if-min-is-more-than-max ()
+  (should-error (smallest 10000 1)))
+
+
+(ert-deftest error-result-for-largest-if-min-is-more-than-max ()
+  (should-error (largest 2 1)))
+
+
+(ert-deftest smallest-product-does-not-use-the-smallest-factor ()
+  (should (equal '(:value 10988901
+                   :factors ((3297 3333)))
+                 (smallest 3215 4000))))
+
+(ert-deftest find-the-smallest-palindrome-from-six-digit-factors ()
+  (should (equal '(:value 50067176005
+                   :factors ((223619 223895)))
+                 (smallest 223617 244818))))
+
+(ert-deftest find-the-largest-palindrome-from-six-digit-factors ()
+  (should (equal '(:value 59842824895
+                   :factors ((244445 244811)))
+                 (largest 223617 244818))))
+
+(provide 'palindrome-products-test)
+;;; palindrome-products-test.el ends here

exercises/practice/palindrome-products/palindrome-products.el

@@ -0,0 +1,17 @@
+;;; palindrome-products.el --- Palindrome Products (exercism)  -*- lexical-binding: t; -*-
+
+;;; Commentary:
+
+;;; Code:
+
+
+(defun smallest (min-factor max-factor)
+  (error "Delete this S-Expression and write your own implementation"))
+
+(defun largest (min-factor max-factor)
+  (error "Delete this S-Expression and write your own implementation"))
+
+
+(provide 'palindrome-products)
+;;; palindrome-products.el ends here
+