Demonstration of how test.check works. Generative testing in Clojure

test.check-interest-rate.clj

(ns secland.compount-interest-test
  (:require [clojure.spec.alpha :as s]
            [clojure.test.check.clojure-test :refer [defspec]]
            [clojure.test.check.generators :as gen]
            [clojure.test.check.properties :as prop]
            [secland.finances.compound-interest :as comp-interest]))

(defn- define-rate-range [min max]
  (gen/fmap #(BigDecimal. (/ % 1000.)) (gen/large-integer* {:min min :max max})))

(def generator-interest-type
  (gen/let [conv-type (gen/elements (vec (s/describe ::comp-interest/type)))
            rate-values (case conv-type
                          :month->year (define-rate-range 1 150)
                          :month->day (define-rate-range 1 150)
                          :year->month (define-rate-range 10 4000)
                          :day->year (define-rate-range 1 4)
                          :year->day (define-rate-range 10 4000)
                          :day->month (define-rate-range 1 4))
            base (gen/elements (vec (s/describe ::comp-interest/base)))
            precision (gen/large-integer* {:min 2 :max 20})]
    {:base base
     :type conv-type
     :rate rate-values
     :precision precision}))


(defn find-the-reverse-operation [operation-type]
  (operation-type {:month->year :year->month
                   :month->day :day->month
                   :year->month :month->year
                   :day->year :year->day
                   :year->day :day->year
                   :day->month :month->day}))

(defspec conversion-to-interest-rates 1000
  (prop/for-all [interest-type generator-interest-type]
                (let [interest-rate-forward (comp-interest/converter interest-type)
                      interest-rate-backward (comp-interest/converter (assoc interest-type
                                                                          :type (find-the-reverse-operation (:type interest-type))
                                                                          :rate interest-rate-forward))]
                  (.equals interest-rate-backward (:rate interest-type))
                  (decimal? interest-rate-forward)
                  (= (.scale interest-rate-forward) (:precision interest-type)))))