Add prime-factors exercise (#372)

config.json

@@ -454,6 +454,17 @@
           "strings"
         ]
       },
+      {
+        "slug": "prime-factors",
+        "name": "Prime Factors",
+        "uuid": "cb79cb42-7efe-49fe-98c4-68dbdc32f2a3",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 3,
+        "topics": [
+          "math"
+        ]
+      },
       {
         "slug": "largest-series-product",
         "name": "Largest Series Product",

exercises/practice/prime-factors/.docs/instructions.md

@@ -0,0 +1,36 @@
+# Instructions
+
+Compute the prime factors of a given natural number.
+
+A prime number is only evenly divisible by itself and 1.
+
+Note that 1 is not a prime number.
+
+## Example
+
+What are the prime factors of 60?
+
+- Our first divisor is 2.
+  2 goes into 60, leaving 30.
+- 2 goes into 30, leaving 15.
+  - 2 doesn't go cleanly into 15.
+    So let's move on to our next divisor, 3.
+- 3 goes cleanly into 15, leaving 5.
+  - 3 does not go cleanly into 5.
+    The next possible factor is 4.
+  - 4 does not go cleanly into 5.
+    The next possible factor is 5.
+- 5 does go cleanly into 5.
+- We're left only with 1, so now, we're done.
+
+Our successful divisors in that computation represent the list of prime factors of 60: 2, 2, 3, and 5.
+
+You can check this yourself:
+
+```text
+2 * 2 * 3 * 5
+= 4 * 15
+= 60
+```
+
+Success!

exercises/practice/prime-factors/.meta/config.json

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "keiravillekode"
+  ],
+  "files": {
+    "solution": [
+      "prime-factors.el"
+    ],
+    "test": [
+      "prime-factors-test.el"
+    ],
+    "example": [
+      ".meta/example.el"
+    ]
+  },
+  "blurb": "Compute the prime factors of a given natural number.",
+  "source": "The Prime Factors Kata by Uncle Bob",
+  "source_url": "https://web.archive.org/web/20221026171801/http://butunclebob.com/ArticleS.UncleBob.ThePrimeFactorsKata"
+}

exercises/practice/prime-factors/.meta/example.el

@@ -0,0 +1,19 @@
+;;; prime-factors.el --- Prime Factors (exercism)  -*- lexical-binding: t; -*-
+
+;;; Commentary:
+
+;;; Code:
+
+(require 'cl-lib)
+
+(defun factors-from (start value)
+  (unless (= value 1)
+    (cl-loop for p from start
+      until (= 0 (% value p))
+      finally return (cons p (factors-from p (/ value p))))))
+
+(defun factors (value)
+  (factors-from 2 value))
+
+(provide 'prime-factors)
+;;; prime-factors.el ends here

exercises/practice/prime-factors/.meta/tests.toml

@@ -0,0 +1,46 @@
+# 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.
+
+[924fc966-a8f5-4288-82f2-6b9224819ccd]
+description = "no factors"
+
+[17e30670-b105-4305-af53-ddde182cb6ad]
+description = "prime number"
+
+[238d57c8-4c12-42ef-af34-ae4929f94789]
+description = "another prime number"
+
+[f59b8350-a180-495a-8fb1-1712fbee1158]
+description = "square of a prime"
+
+[756949d3-3158-4e3d-91f2-c4f9f043ee70]
+description = "product of first prime"
+
+[bc8c113f-9580-4516-8669-c5fc29512ceb]
+description = "cube of a prime"
+
+[7d6a3300-a4cb-4065-bd33-0ced1de6cb44]
+description = "product of second prime"
+
+[073ac0b2-c915-4362-929d-fc45f7b9a9e4]
+description = "product of third prime"
+
+[6e0e4912-7fb6-47f3-a9ad-dbcd79340c75]
+description = "product of first and second prime"
+
+[00485cd3-a3fe-4fbe-a64a-a4308fc1f870]
+description = "product of primes and non-primes"
+
+[02251d54-3ca1-4a9b-85e1-b38f4b0ccb91]
+description = "product of primes"
+
+[070cf8dc-e202-4285-aa37-8d775c9cd473]
+description = "factors include a large prime"

exercises/practice/prime-factors/prime-factors-test.el

@@ -0,0 +1,60 @@
+;;; prime-factors-test.el --- Tests for Prime Factors (exercism)  -*- lexical-binding: t; -*-
+
+;;; Commentary:
+
+;;; Code:
+
+(load-file "prime-factors.el")
+(declare-function factors "prime-factors.el" (value))
+
+
+(ert-deftest no-factors ()
+  (should (equal '() (factors 1))))
+
+
+(ert-deftest prime-number ()
+  (should (equal '(2) (factors 2))))
+
+
+(ert-deftest another-prime-number ()
+  (should (equal '(3) (factors 3))))
+
+
+(ert-deftest square-of-a-prime ()
+  (should (equal '(3 3) (factors 9))))
+
+
+(ert-deftest product-of-first-prime ()
+  (should (equal '(2 2) (factors 4))))
+
+
+(ert-deftest cube-of-a-prime ()
+  (should (equal '(2 2 2) (factors 8))))
+
+
+(ert-deftest product-of-second-prime ()
+  (should (equal '(3 3 3) (factors 27))))
+
+
+(ert-deftest product-of-third-prime ()
+  (should (equal '(5 5 5 5) (factors 625))))
+
+
+(ert-deftest product-of-first-and-second-prime ()
+  (should (equal '(2 3) (factors 6))))
+
+
+(ert-deftest product-of-primes-and-non-primes ()
+  (should (equal '(2 2 3) (factors 12))))
+
+
+(ert-deftest product-of-primes ()
+  (should (equal '(5 17 23 461) (factors 901255))))
+
+
+(ert-deftest factors-include-a-large-prime ()
+  (should (equal '(11 9539 894119) (factors 93819012551))))
+
+
+(provide 'prime-factors-test)
+;;; prime-factors-test.el ends here

exercises/practice/prime-factors/prime-factors.el

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+;;; prime-factors.el --- Prime Factors (exercism)  -*- lexical-binding: t; -*-
+
+;;; Commentary:
+
+;;; Code:
+
+
+(defun factors (value)
+  (error "Delete this S-Expression and write your own implementation"))
+
+
+(provide 'prime-factors)
+;;; prime-factors.el ends here