Add difference-of-squares exercise (#122)

config.json

@@ -315,6 +315,18 @@
         "difficulty": 5,
         "topics": null
       },
+      {
+        "slug": "difference-of-squares",
+        "name": "Difference Of Squares",
+        "uuid": "0a83a4b3-446b-424e-bd56-2b53c26a956d",
+        "practices": [
+          "numbers"
+        ],
+        "prerequisites": [
+          "numbers"
+        ],
+        "difficulty": 1
+      },
       {
         "slug": "crypto-square",
         "name": "Crypto Square",

exercises/practice/difference-of-squares/.docs/instructions.md

@@ -0,0 +1,14 @@
+# Instructions
+
+Find the difference between the square of the sum and the sum of the squares of the first N natural numbers.
+
+The square of the sum of the first ten natural numbers is
+(1 + 2 + ... + 10)² = 55² = 3025.
+
+The sum of the squares of the first ten natural numbers is
+1² + 2² + ... + 10² = 385.
+
+Hence the difference between the square of the sum of the first ten natural numbers and the sum of the squares of the first ten natural numbers is 3025 - 385 = 2640.
+
+You are not expected to discover an efficient solution to this yourself from first principles; research is allowed, indeed, encouraged.
+Finding the best algorithm for the problem is a key skill in software engineering.

exercises/practice/difference-of-squares/.meta/config.json

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "ErikSchierboom"
+  ],
+  "files": {
+    "solution": [
+      "src/difference_of_squares.cljs"
+    ],
+    "test": [
+      "test/difference_of_squares_test.cljs"
+    ],
+    "example": [
+      ".meta/src/example.cljs"
+    ]
+  },
+  "blurb": "Find the difference between the square of the sum and the sum of the squares of the first N natural numbers.",
+  "source": "Problem 6 at Project Euler",
+  "source_url": "https://projecteuler.net/problem=6"
+}

exercises/practice/difference-of-squares/.meta/src/example.cljs

@@ -0,0 +1,12 @@
+(ns difference-of-squares)
+
+(defn- sum [xs] (reduce + xs))
+
+(defn sum-of-squares [n]
+  (sum (map #(int (Math/pow % 2)) (range 0 (inc n)))))
+
+(defn square-of-sum [n]
+  (int (Math/pow (sum (range 0 (inc n))) 2)))
+
+(defn difference [x]
+  (- (square-of-sum x) (sum-of-squares x)))

exercises/practice/difference-of-squares/.meta/tests.toml

@@ -0,0 +1,30 @@
+# 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.
+
+[e46c542b-31fc-4506-bcae-6b62b3268537]
+description = "square of sum 1"
+
+[9b3f96cb-638d-41ee-99b7-b4f9c0622948]
+description = "square of sum 5"
+
+[54ba043f-3c35-4d43-86ff-3a41625d5e86]
+description = "square of sum 100"
+
+[01d84507-b03e-4238-9395-dd61d03074b5]
+description = "sum of squares 1"
+
+[c93900cd-8cc2-4ca4-917b-dd3027023499]
+description = "sum of squares 5"
+
+[94807386-73e4-4d9e-8dec-69eb135b19e4]
+description = "sum of squares 100"
+
+[44f72ae6-31a7-437f-858d-2c0837adabb6]
+description = "difference of squares 1"
+
+[005cb2bf-a0c8-46f3-ae25-924029f8b00b]
+description = "difference of squares 5"
+
+[b1bf19de-9a16-41c0-a62b-1f02ecc0b036]
+description = "difference of squares 100"

exercises/practice/difference-of-squares/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/difference-of-squares/src/difference_of_squares.cljs

@@ -0,0 +1,13 @@
+(ns difference-of-squares)
+
+(defn difference [] ;; <- arglist goes here
+  ;; your code goes here
+)
+
+(defn sum-of-squares [] ;; <- arglist goes here
+  ;; your code goes here
+)
+
+(defn square-of-sum [] ;; <- arglist goes here
+  ;; your code goes here
+)

exercises/practice/difference-of-squares/test/difference_of_squares_test.cljs

@@ -0,0 +1,30 @@
+(ns difference-of-squares-test
+  (:require [clojure.test :refer [deftest is]]
+            [difference-of-squares :as dos]))
+
+(deftest square-of-sum-to-5
+  (is (= 225 (dos/square-of-sum 5))))
+
+(deftest sum-of-squares-to-5
+  (is (= 55 (dos/sum-of-squares 5))))
+
+(deftest difference-of-squares-to-5
+  (is (= 170 (dos/difference 5))))
+
+(deftest square-of-sum-to-10
+  (is (= 3025 (dos/square-of-sum 10))))
+
+(deftest sum-of-squares-to-10
+  (is (= 385 (dos/sum-of-squares 10))))
+
+(deftest difference-of-squares-to-10
+  (is (= 2640 (dos/difference 10))))
+
+(deftest square-of-sum-to-100
+  (is (= 25502500 (dos/square-of-sum 100))))
+
+(deftest sum-of-squares-to-100
+  (is (= 338350 (dos/sum-of-squares 100))))
+
+(deftest difference-of-squares-to-100
+  (is (= 25164150 (dos/difference 100))))