Add bank-account exercise

config.json

@@ -738,6 +738,14 @@
         "practices": [],
         "prerequisites": [],
         "difficulty": 3
+      },
+      {
+        "slug": "bank-account",
+        "name": "Bank Account",
+        "uuid": "27f36909-8005-4537-80b4-4c81a3325b5b",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 3
       }
     ]
   },

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

@@ -0,0 +1,10 @@
+# Instructions
+
+Your task is to implement bank accounts supporting opening/closing, withdrawals, and deposits of money.
+
+As bank accounts can be accessed in many different ways (internet, mobile phones, automatic charges), your bank software must allow accounts to be safely accessed from multiple threads/processes (terminology depends on your programming language) in parallel.
+For example, there may be many deposits and withdrawals occurring in parallel; you need to ensure there are no [race conditions][wikipedia] between when you read the account balance and set the new balance.
+
+It should be possible to close an account; operations against a closed account must fail.
+
+[wikipedia]: https://en.wikipedia.org/wiki/Race_condition#In_software

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

@@ -0,0 +1,20 @@
+# Introduction
+
+After years of filling out forms and waiting, you've finally acquired your banking license.
+This means you are now officially eligible to open your own bank, hurray!
+
+Your first priority is to get the IT systems up and running.
+After a day of hard work, you can already open and close accounts, as well as handle withdrawals and deposits.
+
+Since you couldn't be bothered writing tests, you invite some friends to help test the system.
+However, after just five minutes, one of your friends claims they've lost money!
+While you're confident your code is bug-free, you start looking through the logs to investigate.
+
+Ah yes, just as you suspected, your friend is at fault!
+They shared their test credentials with another friend, and together they conspired to make deposits and withdrawals from the same account _in parallel_.
+Who would do such a thing?
+
+While you argue that it's physically _impossible_ for someone to access their account in parallel, your friend smugly notifies you that the banking rules _require_ you to support this.
+Thus, no parallel banking support, no go-live signal.
+Sighing, you create a mental note to work on this tomorrow.
+This will set your launch date back at _least_ one more day, but well...

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

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

exercises/practice/bank-account/.meta/example.coffee

@@ -0,0 +1,29 @@
+class BankAccount
+  constructor: ->
+    @_open = false
+
+  open: ->
+    throw Error 'account already open' if @_open
+    @_open = true
+    @_balance = 0
+
+  close: ->
+    throw Error 'account not open' unless @_open
+    @_open = false
+
+  balance: ->
+    throw Error 'account not open' unless @_open
+    @_balance
+
+  deposit: (money) ->
+    throw Error 'account not open' unless @_open
+    throw Error "amount must be greater than 0" unless money > 0
+    @_balance += money
+
+  withdraw: (money) ->
+    throw Error 'account not open' unless @_open
+    throw Error "amount must be greater than 0" unless money > 0
+    throw Error "amount must be less than balance" unless money <= @_balance
+    @_balance -= money
+
+module.exports = BankAccount

exercises/practice/bank-account/.meta/tests.toml

@@ -0,0 +1,61 @@
+# This is an auto-generated file.
+#
+# Regenerating this file via `configlet sync` will:
+# - Recreate every `description` key/value pair
+# - Recreate every `reimplements` key/value pair, where they exist in problem-specifications
+# - Remove any `include = true` key/value pair (an omitted `include` key implies inclusion)
+# - Preserve any other key/value pair
+#
+# As user-added comments (using the # character) will be removed when this file
+# is regenerated, comments can be added via a `comment` key.
+
+[983a1528-4ceb-45e5-8257-8ce01aceb5ed]
+description = "Newly opened account has zero balance"
+
+[e88d4ec3-c6bf-4752-8e59-5046c44e3ba7]
+description = "Single deposit"
+
+[3d9147d4-63f4-4844-8d2b-1fee2e9a2a0d]
+description = "Multiple deposits"
+
+[08f1af07-27ae-4b38-aa19-770bde558064]
+description = "Withdraw once"
+
+[6f6d242f-8c31-4ac6-8995-a90d42cad59f]
+description = "Withdraw twice"
+
+[45161c94-a094-4c77-9cec-998b70429bda]
+description = "Can do multiple operations sequentially"
+
+[f9facfaa-d824-486e-8381-48832c4bbffd]
+description = "Cannot check balance of closed account"
+
+[7a65ba52-e35c-4fd2-8159-bda2bde6e59c]
+description = "Cannot deposit into closed account"
+
+[a0a1835d-faae-4ad4-a6f3-1fcc2121380b]
+description = "Cannot deposit into unopened account"
+
+[570dfaa5-0532-4c1f-a7d3-0f65c3265608]
+description = "Cannot withdraw from closed account"
+
+[c396d233-1c49-4272-98dc-7f502dbb9470]
+description = "Cannot close an account that was not opened"
+
+[c06f534f-bdc2-4a02-a388-1063400684de]
+description = "Cannot open an already opened account"
+
+[0722d404-6116-4f92-ba3b-da7f88f1669c]
+description = "Reopened account does not retain balance"
+
+[ec42245f-9361-4341-8231-a22e8d19c52f]
+description = "Cannot withdraw more than deposited"
+
+[4f381ef8-10ef-4507-8e1d-0631ecc8ee72]
+description = "Cannot withdraw negative"
+
+[d45df9ea-1db0-47f3-b18c-d365db49d938]
+description = "Cannot deposit negative"
+
+[ba0c1e0b-0f00-416f-8097-a7dfc97871ff]
+description = "Can handle concurrent transactions"

exercises/practice/bank-account/bank-account.coffee

@@ -0,0 +1,14 @@
+class BankAccount
+  constructor: (args) ->
+
+  open: () ->
+
+  close: () ->
+
+  balance: () ->
+
+  deposit: (args) ->
+
+  withdraw: (args) ->
+
+module.exports = BankAccount

exercises/practice/bank-account/bank-account.spec.coffee

@@ -0,0 +1,103 @@
+BankAccount = require './bank-account'
+
+describe 'BankAccount', ->
+  it 'Newly opened account has zero balance', ->
+    bankAccount = new BankAccount
+    bankAccount.open()
+    expect(bankAccount.balance()).toEqual 0
+
+  xit "Single deposit", ->
+    bankAccount = new BankAccount
+    bankAccount.open()
+    bankAccount.deposit 100
+    expect(bankAccount.balance()).toEqual 100
+
+  xit "Multiple deposits", ->
+    bankAccount = new BankAccount
+    bankAccount.open()
+    bankAccount.deposit 100
+    bankAccount.deposit 50
+    expect(bankAccount.balance()).toEqual 150
+
+  xit "Withdraw once", ->
+    bankAccount = new BankAccount
+    bankAccount.open()
+    bankAccount.deposit 100
+    bankAccount.withdraw 75
+    expect(bankAccount.balance()).toEqual 25
+
+  xit "Withdraw twice", ->
+    bankAccount = new BankAccount
+    bankAccount.open()
+    bankAccount.deposit 100
+    bankAccount.withdraw 80
+    bankAccount.withdraw 20
+    expect(bankAccount.balance()).toEqual 0
+
+  xit "Can do multiple operations sequentially", ->
+    bankAccount = new BankAccount
+    bankAccount.open()
+    bankAccount.deposit 100
+    bankAccount.deposit 110
+    bankAccount.withdraw 200
+    bankAccount.deposit 60
+    bankAccount.withdraw 50
+    expect(bankAccount.balance()).toEqual 20
+
+  xit "Cannot check balance of closed account", ->
+    bankAccount = new BankAccount
+    bankAccount.open()
+    bankAccount.close()
+    expect( -> bankAccount.balance()).toThrow "account not open"
+
+  xit "Cannot deposit into closed account", ->
+    bankAccount = new BankAccount
+    bankAccount.open()
+    bankAccount.close()
+    expect( -> bankAccount.deposit 50).toThrow "account not open"
+
+  xit "Cannot deposit into unopened account", ->
+    bankAccount = new BankAccount
+    expect( -> bankAccount.deposit 50).toThrow "account not open"
+
+  xit "Cannot withdraw from closed account", ->
+    bankAccount = new BankAccount
+    bankAccount.open()
+    bankAccount.close()
+    expect( -> bankAccount.withdraw 50).toThrow "account not open"
+
+  xit "Cannot close an account that was not opened", ->
+    bankAccount = new BankAccount
+    expect( -> bankAccount.close()).toThrow "account not open"
+
+  xit "Cannot open an already opened account", ->
+    bankAccount = new BankAccount
+    bankAccount.open()
+    expect( -> bankAccount.open()).toThrow "account already open"
+
+
+  xit "Reopened account does not retain balance", ->
+    bankAccount = new BankAccount
+    bankAccount.open()
+    bankAccount.deposit 50
+    bankAccount.close()
+    bankAccount.open()
+    expect(bankAccount.balance()).toEqual 0
+
+  xit "Cannot withdraw more than deposited", ->
+    bankAccount = new BankAccount
+    bankAccount.open()
+    bankAccount.deposit 25
+    expect( -> bankAccount.withdraw 50).toThrow "amount must be less than balance"
+
+  xit "Cannot withdraw negative", ->
+    bankAccount = new BankAccount
+    bankAccount.open()
+    bankAccount.deposit 100
+    expect( -> bankAccount.withdraw -50).toThrow "amount must be greater than 0"
+
+  xit "Cannot deposit negative", ->
+    bankAccount = new BankAccount
+    bankAccount.open()
+    expect( -> bankAccount.deposit -50).toThrow "amount must be greater than 0"
+