Implement bank account exercise

config.json

@@ -1555,6 +1555,14 @@
         ],
         "difficulty": 3
       },
+      {
+        "slug": "bank-account",
+        "name": "Bank Account",
+        "uuid": "48b91d4d-2fd4-4941-8f3f-030872daec88",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 3
+      },
       {
         "slug": "gilded-rose",
         "name": "Gilded Rose",

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 is 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 +1,15 @@
+{
+  "authors": ["meatball133"],
+  "files": {
+    "solution": [
+      "bank_account.rb"
+    ],
+    "test": [
+      "bank_account_test.rb"
+    ],
+    "example": [
+      ".meta/example.rb"
+    ]
+  },
+  "blurb": "Simulate a bank account supporting opening/closing, withdraws, and deposits of money. Watch out for concurrent transactions!"
+}

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

@@ -0,0 +1,48 @@
+class BankAccount
+  def initialize
+    @balance = 0
+    @status = false
+  end
+
+  def balance
+    unless @status
+      raise ArgumentError.new("You can't check the balance of a closed account")
+    end
+    @balance
+  end
+
+  def open
+    if @status
+      raise ArgumentError.new("You can't open an already open account")
+    end
+    @status = true
+  end
+
+  def close
+    unless @status
+      raise ArgumentError.new("You can't close an already closed account")
+    end
+    @balance = 0
+    @status = false
+  end
+
+  def deposit(amount)
+    if amount < 0
+      raise ArgumentError.new("You can't deposit a negative amount")
+    elsif !@status
+      raise ArgumentError.new("You can't deposit money into a closed account")
+    end
+    @balance += amount
+  end
+
+  def withdraw(amount)
+    if amount < 0
+      raise ArgumentError.new("You can't withdraw a negative amount")
+    elsif amount > @balance
+      raise ArgumentError.new("You can't withdraw more than you have")
+    elsif !@status
+      raise ArgumentError.new("You can't withdraw money into a closed account")
+    end
+    @balance -= amount
+  end
+end

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

@@ -0,0 +1,62 @@
+# 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"
+include = false

exercises/practice/bank-account/bank_account.rb

@@ -0,0 +1,7 @@
+=begin
+Write your code for the 'Bank Account' exercise in this file. Make the tests in
+`bank_account_test.rb` pass.
+
+To get started with TDD, see the `README.md` file in your
+`ruby/bank-account` directory.
+=end

exercises/practice/bank-account/bank_account_test.rb

@@ -0,0 +1,135 @@
+require 'minitest/autorun'
+require_relative 'bank_account'
+
+class BankAccountTest < Minitest::Test
+  def test_newly_opened_account_has_zero_balance
+    # skip
+    bank_account = BankAccount.new
+    bank_account.open
+    assert_equal bank_account.balance, 0
+  end
+
+  def test_single_deposit
+    skip
+    bank_account = BankAccount.new
+    bank_account.open
+    bank_account.deposit(100)
+    assert_equal bank_account.balance, 100
+  end
+
+  def test_multiple_deposits
+    skip
+    bank_account = BankAccount.new
+    bank_account.open
+    bank_account.deposit(100)
+    bank_account.deposit(50)
+    assert_equal bank_account.balance, 150
+  end
+
+  def test_withdraw_once
+    skip
+    bank_account = BankAccount.new
+    bank_account.open
+    bank_account.deposit(100)
+    bank_account.withdraw(75)
+    assert_equal bank_account.balance, 25
+  end
+
+  def test_withdraw_twice
+    skip
+    bank_account = BankAccount.new
+    bank_account.open
+    bank_account.deposit(100)
+    bank_account.withdraw(80)
+    bank_account.withdraw(20)
+    assert_equal bank_account.balance, 0
+  end
+
+  def test_can_do_multiple_operations_sequentially
+    skip
+    bank_account = BankAccount.new
+    bank_account.open
+    bank_account.deposit(100)
+    bank_account.deposit(110)
+    bank_account.withdraw(200)
+    bank_account.deposit(60)
+    bank_account.withdraw(50)
+    assert_equal bank_account.balance, 20
+  end
+
+  def test_cannot_check_balance_of_closed_account
+    skip
+    bank_account = BankAccount.new
+    bank_account.open
+    bank_account.close
+    assert_raises(ArgumentError, "You can't check the balance of a closed account") { bank_account.balance }
+  end
+
+  def test_cannot_deposit_into_closed_account
+    skip
+    bank_account = BankAccount.new
+    bank_account.open
+    bank_account.close
+    assert_raises(ArgumentError, "You can't deposit money into a closed account") { bank_account.deposit(50) }
+  end
+
+  def test_cannot_deposit_into_unopened_account
+    skip
+    bank_account = BankAccount.new
+    assert_raises(ArgumentError, "You can't deposit money into a closed account") { bank_account.deposit(50) }
+  end
+
+  def test_cannot_withdraw_from_closed_account
+    skip
+    bank_account = BankAccount.new
+    bank_account.open
+    bank_account.close
+    assert_raises(ArgumentError, "You can't withdraw money into a closed account") { bank_account.withdraw(50) }
+  end
+
+  def test_cannot_close_an_account_that_was_not_opened
+    skip
+    bank_account = BankAccount.new
+    assert_raises(ArgumentError, "You can't close an already closed account") { bank_account.close }
+  end
+
+  def test_cannot_open_an_already_opened_account
+    skip
+    bank_account = BankAccount.new
+    bank_account.open
+    assert_raises(ArgumentError, "You can't open an already open account") { bank_account.open }
+  end
+
+  def test_reopened_account_does_not_retain_balance
+    skip
+    bank_account = BankAccount.new
+    bank_account.open
+    bank_account.deposit(50)
+    bank_account.close
+    bank_account.open
+    assert_equal bank_account.balance, 0
+  end
+
+  def test_cannot_withdraw_more_than_deposited
+    skip
+    bank_account = BankAccount.new
+    bank_account.open
+    bank_account.deposit(25)
+    assert_raises(ArgumentError, "You can't withdraw more than you have") { bank_account.withdraw(50) }
+  end
+
+  def test_cannot_withdraw_negative
+    skip
+    bank_account = BankAccount.new
+    bank_account.open
+    bank_account.deposit(100)
+    assert_raises(ArgumentError, "You can't withdraw a negative amount") { bank_account.withdraw(-50) }
+  end
+
+  def test_cannot_deposit_negative
+    skip
+    bank_account = BankAccount.new
+    bank_account.open
+    assert_raises(ArgumentError, "You can't deposit a negative amount") { bank_account.deposit(-50) }
+  end
+end