#!/bin/bash
# loancalc--Given a principal loan amount, interest rate, and 
#   duration of loan (years), calculates the per-payment amount

#  Formula is M = P * ( J / (1 - (1 + J) ^ -N)),
#   where P = principal, J = monthly interest rate, N = duration (months).
#
#  Users typically enter P, I (annual interest rate), and L (length, years).

source ../1/library.sh         # Start by sourcing the script library.

if test $Argc -ne 3  {
  echo "Usage: $0 principal interest loan-duration-years" > !2
  exit 1
}

setglobal P = $1,  I = $2,   L = $3
setglobal J = $[scriptbc -p 8 $I / '(' 12 '*' 100 ')']
setglobal N = "$shExpr(' $L * 12 ')"
setglobal M = $[scriptbc -p 8 $P '*' '(' $J / '('1 - '('1 + $J')' '^' -$N')' ')']

# Now a little prettying up of the value:

setglobal dollars = $[echo $M | cut -d. -f1]
setglobal cents = $[echo $M | cut -d. -f2 | cut -c1-2]

cat << """
A $L-year loan at $I% interest with a principal amount of $[nicenumber $P 1] 
results in a payment of '$'$dollars.$cents each month for the duration of
the loan ($N payments).
"""

exit 0