Blurst #1 2013 Calculate Position

gistfile1.rb

class Trading::CalculatePosition

  def self.execute(user, contender, options={})
    position = user.positions.where(contender_id: contender.id).first    

    contracts = user.contracts
                    .select{|c| c.contender_id == contender.id}
                    .sort{|x,y| x.created_at <=> y.created_at }

    if contracts.empty? 
      position.destroy if position
      return
    end

    position = user.positions.build(contender: contender) if position.nil?
    calculate(user, position, contracts)
    position.save
  end

  def self.calculate(user, position, contracts)
    net_shares = 0
    net_value = 0
    position.shares = 0

    contracts.each do |contract|
      if contract.seller == user
        if position.short?
          position.shares -= contract.shares
          net_shares -= contract.shares
          net_value -= contract.value
        else
          position.shares -= contract.shares
          if position.short? # so went from long to short
            net_shares = position.shares
            net_value = contract.price * position.shares
          end
        end
      else
        if position.long?
          position.shares += contract.shares
          net_shares += contract.shares
          net_value += contract.value         
        else
          position.shares += contract.shares
          if position.long? # so went from short to long
            net_shares = position.shares
            net_value = contract.price * position.shares
          end
        end
      end
    end

    position.average_price = (net_shares == 0) ? 0 : (net_value / net_shares).round(2)
    position.save
  end   

end