A loan calculator that allows additional conditions such as

• Adding principals at specific paying periods. For example, I get a loan of $10000 at 8% interest for a 25 year term, find the changes in payment amount if I pay additional principals $2000 at paying period 15 and 27.
• Set initial payment amount. For example, I get a loan of $10000 at 8% interest for a 25 year term. With an initial periodically payment of $100, find periodic payment amount at period 15 when I pay additional principal of $3000.