Mortgage calculation related functions

mortgage.py

def spitzer(k,r,p):
  return k-p*(r/12.0)

def months(k,r,p):
	total = 0
	while k > 0:
    k -= spitzer(k,r,p)
    total += 1
	return total

def binomial(a, n):
  if n < 0:
   	return 1.0 / binomial(a, -n)
  sum = 1.
  pow = 1. * n
  term = 1.
  cof = 1.
  for i in range(1, 10):
    cof *= pow / i
    pow -= 1.
    term *= a
    sum += cof * term
  return sum

def monthlyRefund(interest, months, sum):
  interest /= 100.
  return sum * interest / 12. / (1.0 - binomial(interest / 12., -months))

def waste(interest, months, sum):
  return monthlyRefund(interest, months, sum)*months-sum

def gracewaste(interest, months, grace, sum):
  return waste(interest,months-grace,sum)+sum*(interest/100./12.)*grace

def split(interest, low_interest, months, grace, sum, low_sum):
	splitWaste = waste(low_interest,grace,low_sum)+gracewaste(interest,months,grace,sum)
	return monthlyRefund(low_interest,grace,low_sum)+sum*interest/100./12.,monthlyRefund(interest,months-grace,sum),splitWaste,waste(interest,months,sum+low_sum)-splitWaste