Add collatz-conjecture exercise (#118)

config.json

@@ -585,6 +585,19 @@
         ],
         "difficulty": 2
       },
+      {
+        "slug": "collatz-conjecture",
+        "name": "Collatz Conjecture",
+        "uuid": "99c2ede9-23a8-40a1-a54e-f33fddb7d881",
+        "practices": [
+          "numbers"
+        ],
+        "prerequisites": [
+          "numbers",
+          "conditionals"
+        ],
+        "difficulty": 2
+      },
       {
         "slug": "grade-school",
         "name": "Grade School",

exercises/practice/collatz-conjecture/.docs/instructions.md

@@ -0,0 +1,29 @@
+# Instructions
+
+The Collatz Conjecture or 3x+1 problem can be summarized as follows:
+
+Take any positive integer n.
+If n is even, divide n by 2 to get n / 2.
+If n is odd, multiply n by 3 and add 1 to get 3n + 1.
+Repeat the process indefinitely.
+The conjecture states that no matter which number you start with, you will always reach 1 eventually.
+
+Given a number n, return the number of steps required to reach 1.
+
+## Examples
+
+Starting with n = 12, the steps would be as follows:
+
+0. 12
+1. 6
+2. 3
+3. 10
+4. 5
+5. 16
+6. 8
+7. 4
+8. 2
+9. 1
+
+Resulting in 9 steps.
+So for input n = 12, the return value would be 9.

exercises/practice/collatz-conjecture/.meta/config.json

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "ErikSchierboom"
+  ],
+  "files": {
+    "solution": [
+      "src/collatz_conjecture.cljs"
+    ],
+    "test": [
+      "test/collatz_conjecture_test.cljs"
+    ],
+    "example": [
+      ".meta/src/example.cljs"
+    ]
+  },
+  "blurb": "Calculate the number of steps to reach 1 using the Collatz conjecture.",
+  "source": "An unsolved problem in mathematics named after mathematician Lothar Collatz",
+  "source_url": "https://en.wikipedia.org/wiki/3x_%2B_1_problem"
+}

exercises/practice/collatz-conjecture/.meta/src/example.cljs

@@ -0,0 +1,12 @@
+(ns collatz-conjecture)
+
+(defn collatz-helper [n]
+  (cond (= 1 n)   1
+        (even? n) (/ n 2)
+        :else     (inc (* 3 n))))
+
+(defn collatz [n]
+  (if (> n 0)
+    (count (take-while #(not= 1 %)
+                       (iterate collatz-helper n)))
+    (throw (new js/Error "Just defined for numbers greater than 0."))))

exercises/practice/collatz-conjecture/.meta/tests.toml

@@ -0,0 +1,21 @@
+# This is an auto-generated file. Regular comments will be removed when this
+# file is regenerated. Regenerating will not touch any manually added keys,
+# so comments can be added in a "comment" key.
+
+[540a3d51-e7a6-47a5-92a3-4ad1838f0bfd]
+description = "zero steps for one"
+
+[3d76a0a6-ea84-444a-821a-f7857c2c1859]
+description = "divide if even"
+
+[754dea81-123c-429e-b8bc-db20b05a87b9]
+description = "even and odd steps"
+
+[ecfd0210-6f85-44f6-8280-f65534892ff6]
+description = "large number of even and odd steps"
+
+[7d4750e6-def9-4b86-aec7-9f7eb44f95a3]
+description = "zero is an error"
+
+[c6c795bf-a288-45e9-86a1-841359ad426d]
+description = "negative value is an error"

exercises/practice/collatz-conjecture/deps.edn

@@ -0,0 +1,10 @@
+{:deps
+ {org.clojure/clojure {:mvn/version "1.10.1"}
+  org.clojure/clojurescript {:mvn/version "1.10.773"}}
+
+ :aliases
+ {:test
+  {:extra-paths ["test"]
+   :extra-deps
+   {olical/cljs-test-runner {:mvn/version "3.8.0"}}
+   :main-opts ["-m" "cljs-test-runner.main"]}}}

exercises/practice/collatz-conjecture/src/collatz_conjecture.cljs

@@ -0,0 +1,5 @@
+(ns collatz-conjecture)
+
+(defn collatz [num] ;; <- arglist goes here
+  ;; your code goes here
+)

exercises/practice/collatz-conjecture/test/collatz_conjecture_test.cljs

@@ -0,0 +1,29 @@
+(ns collatz-conjecture-test
+  (:require [clojure.test :refer [deftest is testing]]
+            [collatz-conjecture :refer [collatz]]))
+
+(deftest steps-for-1
+  (testing "zero steps for one"
+    (is (= 0 (collatz 1)))))
+
+(deftest steps-for-16
+  (testing "divide if even"
+    (is (= 4 (collatz 16)))))
+
+(deftest steps-for-12
+  (testing "even and odd steps"
+    (is (= 9 (collatz 12)))))
+
+(deftest steps-for-1000000
+  (testing "Large number of even and odd steps"
+    (is (= 152 (collatz 1000000)))))
+
+(deftest steps-for-0
+  (testing "zero is an error"
+    (is (thrown? js/Error
+                 (collatz 0)))))
+
+(deftest steps-for-negative
+  (testing "negative value is an error"
+    (is (thrown? js/Error
+                 (collatz -15)))))