Shop Problem

Here is the PHP Solution to an actual interview question:

Create a class called Shop. It should contain two methods:

buy($now, $quantity, $expiry)

and 

sell($now, $quantity)

Method buy accepts 3 parameters:

• now – an integer that represents the present time
• quantity – an integer for the amount currently being brought into the shop
• expiry – an integer that represents the time at which this shipment expires

Method sell accepts 2 parameters:

• now – an integer that represents the present time
• quantity – an integer for the amount currently being sold from the store

This shop does not sell expired goods and tries to sell from the stock that is going to expire first. The task is to implement this class.

<?php

class Shop
{

  public $queue;
  public $apples;
  public $quantity;

  public function __construct()
  {
    $this->queue  = new SplMinHeap();
    $this->apples = array();
  }

  public function check_apples($now)
  {
    while ($this->queue->top() <= $now)
    {
      $this->delete_current_apple();
    }
  }
  
  private function debug($msg){
    return;
    echo "<div style=\"border:1px solid black;\"><pre>";
    var_dump($msg);
    echo "</pre></div>";
  }

  public function buy($now, $quantity, $expiry)
  {
    $this->queue->insert(intval($expiry));
    $this->apples[intval($expiry)] = intval($quantity);
    $this->quantity                += intval($quantity);
    $this->check_apples($now);
    return $this->quantity;
  }

  private function get_current_apple_quantity()
  {
    $cur = intval($this->queue->top());
    return intval($this->apples[$cur]);
  }

  private function delete_current_apple()
  {
    $cur            = intval($this->queue->top());
    $this->queue->extract();
    $this->quantity -= $this->apples[$cur];
    unset($this->apples[$cur]);
  }

  private function remove_apples($req_quantity)
  {
    $quantity = intval($req_quantity);
    $this->debug("Removing $quantity from " . $this->get_current_apple_quantity() . "<br />");
    $this->debug($this->quantity);
    if ($this->get_current_apple_quantity() > $quantity)
    {
      $cur                = $this->queue->top();
      $this->apples[$cur] -= $quantity;
      $this->quantity     -= $quantity;
    }
    $this->debug($this->quantity);
  }

  private function sell_helper($req_quantity)
  {
    $quantity     = intval($req_quantity);
    $cur_quantity = 0;
    do
    {
      $this->debug("Starting with " . $this->quantity);
      $cur_apple_count = $this->get_current_apple_quantity();
      $cur_quantity    += $cur_apple_count;
      $quantity_this_round = ($quantity >= $cur_quantity)?$cur_quantity:($cur_quantity - $quantity);

      if ($quantity_this_round >= $cur_apple_count)
      {
        $this->debug("deleting $quantity_this_round >= $cur_apple_count");
        $this->delete_current_apple();
      }
      else
      {
        if ($quantity_this_round < $cur_apple_count)
        {
          $this->remove_apples($cur_apple_count - $quantity_this_round);
        }
        else{
          $this->debug("unreachable code");
        }
        $cur_quantity = $quantity;
      }
      $this->debug($this->quantity);
      $this->debug("while ($cur_quantity < $quantity);<br />");
    }
    while ($cur_quantity < $quantity);
  }

  public function sell($now, $quantity)
  {
    $this->check_apples($now);
    $this->sell_helper($quantity);
    return $this->quantity;
  }

}