#!/usr/bin/perl
#
# reptax.perl: Assessing the effects of repeated taxation.
#
# Gordon Irlam - 2006.
#
# Eg: reptax.perl 22 1.9 1.8 2.5

if ($#ARGV != 3) {
    die "Usage: reptax.perl tax_rate yield real_yield_growth_rate inflation_rate";
}

$tax_rate = $ARGV[0] / 100.0;
$yield = $ARGV[1] / 100.0;
$growth_rate = $ARGV[2] / 100.0;
$inflation_rate = $ARGV[3] / 100.0;

printf "Tax rate = %.1f%%\n", $tax_rate * 100.0;
printf "Yield = %.1f%%\n", $yield * 100.0;
printf "Real yield growth rate = %.1f%%\n", $growth_rate * 100.0;
printf "Inflation rate = %.1f%%\n", $inflation_rate * 100.0;
printf "\n";
print "       yearly sale        final year sale\n";
printf "\n";
print "       real   real          real   real\n";
print "year increase rate        increase rate\n";
$repeat = 1.0;
$single = 1.0;
$s_basis = 1.0;
$cpi = 1.0;
for ($n = 1; $n <= 40; $n++) {
    $repeat = (1.0 + ($yield + $growth_rate + $inflation_rate) * (1.0 - $tax_rate)) * $repeat;
    $s_basis = $s_basis + $yield * (1.0 - $tax_rate) * $single;
    $single = (1.0 + $yield * (1.0 - $tax_rate) + $growth_rate + $inflation_rate) * $single;
    $cpi = (1.0 + $inflation_rate) * $cpi;
    $r_grown = $repeat / $cpi - 1.0;
    $s_grown = ($s_basis + (1.0 - $tax_rate) * ($single - $s_basis)) / $cpi - 1.0;
    $r_annual = (1.0 + $r_grown) ** (1.0 / $n) - 1.0;
    $s_annual = (1.0 + $s_grown) ** (1.0 / $n) - 1.0;
    if (($n <= 10) || (($n % 5) == 0)) {
        printf " %2d   %5.1f%% %4.1f%%         %5.1f%% %4.1f%%\n", $n, $r_grown * 100.0, $r_annual * 100.0, $s_grown * 100.0, $s_annual * 100.0;
    }
}