main.clj

(ns problem1.core)

(defn- real-numbers-below [x] (range 1 x))

(defn- multiple-of [x num] (zero? (mod x num))
(defn- multiple-of-three [x] (multiple-of x 3))
(defn- multiple-of-five  [x] (multiple-of x 5))
(defn multiple-of-three-or-five [x] 
  (or (multiple-of-three x) 
      (multiple-of-five x)))


(defn multiples-of-three-and-five-below [x]
  (filter multiple-of-three-or-five (real-numbers-below x)))

(defn sum-of-multiples [x]
  (reduce + (multiples-of-three-and-five-below x)))

main_test.clj

(ns problem1.core-test
  (:require [clojure.test :refer :all]
            [problem1.core :refer :all]))

(deftest multiples-of-n
  (testing "3 should be a multiple of three"
    (is (multiple-of-three 3)))
  
  (testing "4 should not be a multiple of three"
    (is (not (multiple-of-three 4))))
  
  (testing "5 should be a multiple of five"
    (is (multiple-of-five 5)))
  
  (testing "6 should not be a multiple of five"
    (is (not (multiple-of-five 6))))

  (testing "below 10 the multiples should be 3 5 6 and 9"
    (is (= (multiples-of-three-and-five-below 10), [3 5 6 9])))
  
  (testing "the sum of all multiples below 10 should be 23"
    (is (= (sum-of-multiples 10), 23)))
  
  ; SPOILERS!!!
  (testing "the sum of all multiples below 1000 should be something"
    (is (= (sum-of-multiples 1000), 233168)))
)