Stock Picker

📌 Overview

This project implements a Ruby method called stock_picker that receives an array of stock prices, one per hypothetical day, and returns the best pair of days to buy and sell to maximize profit.

Return value

The method returns an array with two indices [buy_day, sell_day], or an empty array [] when no profitable transaction exists.

Algorithm summary

We use a single-pass algorithm (O(n) time) that tracks the minimum price seen so far (min_price/min_day) and the maximum profit (max_profit). For each day we compute the profit if we sold that day and update the best pair when a larger profit is found. On ties, the first profitable pair is kept.

Key points

• Buy must happen before sell.
• Returns [] when no profitable trade is possible.
• Time complexity: O(n). Space complexity: O(1).

Examples

stock_picker([17, 3, 6, 9, 15, 8, 6, 1, 10]) # => [1, 4]
stock_picker([2, 3, 10, 6, 4, 8, 1])         # => [0, 2]
stock_picker([10, 9, 8, 7])                  # => []

Testing (RSpec)

The repository includes specs in spec/stock_picker_spec.rb. To run the tests locally:

bundle install
bundle exec rspec --format documentation

How it works (code summary)

1. Initialize min_price with the first price and max_profit = 0.
2. Iterate prices with index; for each price compute profit = price - min_price.
3. If profit > max_profit, update max_profit and best_days.
4. If price < min_price, update min_price and min_day.
5. Return best_days if max_profit is positive, otherwise return [].