Add bank-account exercise (#115)

config.json

@@ -585,6 +585,18 @@
         ],
         "difficulty": 2
       },
+      {
+        "slug": "bank-account",
+        "name": "Bank Account",
+        "uuid": "a3964ef3-f2d8-4982-9240-d253345b0988",
+        "practices": [
+          "floating-point-numbers"
+        ],
+        "prerequisites": [
+          "floating-point-numbers"
+        ],
+        "difficulty": 4
+      },
       {
         "slug": "go-counting",
         "name": "Go Counting",

exercises/practice/bank-account/.docs/instructions.md

@@ -0,0 +1,12 @@
+# Instructions
+
+Simulate a bank account supporting opening/closing, withdrawals, and deposits of money.
+Watch out for concurrent transactions!
+
+A bank account can be accessed in multiple ways.
+Clients can make deposits and withdrawals using the internet, mobile phones, etc.
+Shops can charge against the account.
+
+Create an account that can be accessed from multiple threads/processes (terminology depends on your programming language).
+
+It should be possible to close an account; operations against a closed account must fail.

exercises/practice/bank-account/.meta/config.json

@@ -0,0 +1,17 @@
+{
+  "authors": [
+    "ErikSchierboom"
+  ],
+  "files": {
+    "solution": [
+      "src/bank_account.cljs"
+    ],
+    "test": [
+      "test/bank_account_test.cljs"
+    ],
+    "example": [
+      ".meta/src/example.cljs"
+    ]
+  },
+  "blurb": "Simulate a bank account supporting opening/closing, withdraws, and deposits of money. Watch out for concurrent transactions!"
+}

exercises/practice/bank-account/.meta/src/example.cljs

@@ -0,0 +1,17 @@
+(ns bank-account)
+
+(defrecord BankAccount [balance open?])
+
+(defn open-account []
+  (atom (BankAccount. 0 true)))
+
+(defn close-account [bank-account]
+  (swap! bank-account assoc :open? false))
+
+(defn get-balance [bank-account]
+  (when (:open? @bank-account)
+    (:balance @bank-account)))
+
+(defn update-balance [bank-account adjustment]
+  (locking bank-account
+    (swap! bank-account assoc :balance (+ (get-balance bank-account) adjustment))))

exercises/practice/bank-account/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/bank-account/src/bank_account.cljs

@@ -0,0 +1,17 @@
+(ns bank-account)
+
+(defn open-account [] ;; <- arglist goes here
+  ;; your code goes here
+  )
+
+(defn close-account [] ;; <- arglist goes here
+  ;; your code goes here
+  )
+
+(defn get-balance [] ;; <- arglist goes here
+  ;; your code goes here
+  )
+
+(defn update-balance [] ;; <- arglist goes here
+  ;; your code goes here
+  )

exercises/practice/bank-account/test/bank_account_test.cljs

@@ -0,0 +1,44 @@
+(ns bank-account-test
+  (:require
+   [clojure.test :refer [deftest testing is use-fixtures]]
+   [bank-account]))
+
+(defn pcalls
+  "Executes the no-arg fns in parallel, returning a lazy sequence of
+  their values"
+  {:added "1.0"
+   :static true}
+  [& fns] (pmap #(%) fns))
+
+#_(defn shutdown-agents-fixture [f]
+  (f)
+  (shutdown-agents))
+
+;(use-fixtures :once shutdown-agents-fixture)
+
+(deftest initial-account-state
+  (testing "Accounts are opened with a balance of 0"
+    (is (= 0 (-> (bank-account/open-account)
+                 (bank-account/get-balance))))))
+
+(deftest increment-and-get-balance
+  (testing "Adding money to the account works"
+    (let [account (bank-account/open-account)]
+      (is (= 0 (bank-account/get-balance account)))
+      (bank-account/update-balance account 10)
+      (is (= 10 (bank-account/get-balance account))))))
+
+(deftest increment-decrement-and-get-balance
+  (testing "Taking money out of the account works"
+    (let [account (bank-account/open-account)]
+      (is (= 0 (bank-account/get-balance account)))
+      (bank-account/update-balance account 10)
+      (is (= 10 (bank-account/get-balance account)))
+      (bank-account/update-balance account -10)
+      (is (= 0 (bank-account/get-balance account))))))
+
+(deftest closed-accounts-are-nil
+  (testing "Closing an account makes it nil"
+    (let [account (bank-account/open-account)]
+      (bank-account/close-account account)
+      (is (nil? (bank-account/get-balance account))))))