#!/usr/bin/perl

# PLANAR polygon AREA calculation in entered units
# If vertices are travelled in wrong direction, the result
# will be negative, and sign change will show the result.

# Data pairs: Easting, Northing
@poly = ( 613.69, 548.36,
	  669.00, 510.00,
	  593.27, 440.77,
	  398.07, 497.69,
	  425.46, 593.46,
	  517.44, 533.63 );

# test data.. should yield area=1.0000
#@poly = ( 0, 0,
#	  1, 0,
#	  1, 1,
#	  0, 1 );

# Step index by TWO (2) !
# $poly[$i+0] = y
# $poly[$i+1] = x

# Number of numbers in the poly array
$N = 1+$#poly;

# Separate the corner coordinate vector into X and Y components
@x=();
@y=();
for ($i = 0; $i < $N; $i += 2) {
    $ii = $i/2;

    $x[$ii] = $poly[$i+0];
    $y[$ii] = $poly[$i+1];

#    $j = ($i + 2) % $N;
#    $x = $poly[$j+1] - $poly[$i+1];
#    $y = $poly[$j+0] - $poly[$i+0];
#
#    $s = sqrt($x*$x + $y*$y);
#    printf( "v[%d] = { %12.3f, %12.3f }; s = %12.3f\n", $ii, $x, $y, $s );

}

$N = $N / 2;

$area = 0.0;  
$ji = 0.0;
$ij = 0.0;

for ($i = 0; $i < $N; ++$i) {
    $j = ($i + 1) % $N;
    $area += $x[$i] * $y[$j] - $x[$j] * $y[$i];
}

$area *= 0.5;

printf ( "Area = %.6f\n", abs($area) );