#!/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
}

setvar P = "$1",  I = "$2",   L = "$3"
setvar J = "$(scriptbc -p 8 $I / \( 12 \* 100 \) )"
setvar N = """$(( $L * 12 ))"
setvar M = "$(scriptbc -p 8 $P \* \( $J / \(1 - \(1 + $J\) \^ -$N\) \) )"

# Now a little prettying up of the value:

setvar dollars = "$(echo $M | cut -d. -f1)"
setvar cents = "$(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