Trading kata

README.md

I have an array stock_prices_yesterday where:

The indices are the time in minutes past trade opening time, which was 9:30am local time. The values are the price in dollars of Apple stock at that time. For example, the stock cost $500 at 10:30am, so stock_prices_yesterday[60] = 500.

Write an efficient algorithm for computing the best profit I could have made from 1 purchase and 1 sale of 1 Apple stock yesterday.

No "shorting"—you must buy before you sell. You may not buy and sell in the same time step (at least 1 minute must pass).

Source interviewcake.com

appl.rb

class Trader
  attr_accessor :stock_prices

  RIDICULOUSLY_HIGH_PRICE = 100_000_000_000

  def initialize(stock_prices)
    @stock_prices = stock_prices
    @min_buy_value = RIDICULOUSLY_HIGH_PRICE
    @max_profit = - 8000
  end

  def max_profit
    stock_prices.each.with_index do |sell_value, current_time|
      update_max_profit(sell_value, current_time)
    end
    @max_profit
  end

  private

  def update_max_profit(sell_value, time)
    current_profit = sell_value - update_min_buy_value(time)
    @max_profit = [@max_profit, current_profit].max
  end

  def update_min_buy_value(time)
    if time > 0
      @min_buy_value = [@min_buy_value, stock_prices[time - 1]].min
    else
      @min_buy_value
    end
  end

end

appl_test.rb

require 'minitest/autorun'
require_relative 'appl.rb'

class TestTrader < Minitest::Test

  def test_max_profit_return_0_for_a_contstant_stock_price
    stock_prices = [500, 500, 500, 500, 500]
    jerome = Trader.new(stock_prices)
    assert_equal 0, jerome.max_profit
  end

  def test_max_profit_return_delta_when_one_price_is_higher
    stock_prices = [500, 500, 500, 600, 500]
    jerome = Trader.new(stock_prices)
    assert_equal 100, jerome.max_profit
  end

  def test_max_profit_is_negative_when_stock_goes_only_down
    stock_prices = [900, 800, 700, 600, 500]
    jerome = Trader.new(stock_prices)
    assert_equal -100, jerome.max_profit
  end

  def test_max_profit_when_min_after_max
    stock_prices = [500, 500, 500, 600, 400]
    jerome = Trader.new(stock_prices)
    assert_equal 100, jerome.max_profit
  end

end