c:\test>868223-gen -N=1000 | 868223-plot -RETAIN=50

##</code><code>##

#! perl -slw
use strict;
use Data::Dump qw[ pp ];
use List::Util qw[ max reduce ];
use GD;

use constant {
    SCALE => 1000,
    OFFSET => 100,
};


sub rgb2n { unpack 'N', pack 'CCCC', 0, @_ }
sub rgba2n { unpack 'N', pack 'C4', pop, @_ }

sub morton {
    my( $x, $y ) = @_;
    my $z = 0;
    for my $i ( 0 .. 31 ) {
        $z |= ( $x & 1<<$i )<<$i | ( $y & 1<<$i)<<( $i + 1 );
    }
    return $z;
}

our $RETAIN //= 20;

my( $maxX, $maxY ) = (0,0);

my @points = sort{
    $maxX = max( $maxX, $a->[0], $b->[0] );
    $maxY = max( $maxY, $a->[1], $b->[1] );

    $a->[0] <=> $b->[0] || $a->[1] <=> $b->[1];
} map[ split ], <>;

my $img = GD::Image->new( $maxX*SCALE+(OFFSET*2), $maxY*SCALE+(OFFSET*2), 1 );

$img->filledRectangle( 0,0, $img->getBounds, rgb2n( 255,255,255 ) );

my $last;
for my $pt ( @points ) {
    my( $x, $y, $dy ) = map{ $_*SCALE+OFFSET } @{ $pt };
    $dy -= OFFSET;

    $img->filledEllipse(
        $x, $y, $dy, $dy, rgba2n( 255, 0, 0, 90 )
    );
    $img->line( $x-10, $y, $x+10, $y, 0 );
    $img->line( $x, $y-10, $x, $y+10, 0 );
    if( $last ) {
        $img->line( $last->[0], $last->[1], $x, $y, rgb2n( 0,0,255 ) );
    }
    $last = [ $x, $y ];
}

my @ordered = map{
    [ morton( int( $_->[0]*SCALE ), int( $_->[1]*SCALE ) ), $_ ]
} @points;

while( @ordered > $RETAIN ) {
    my( $nearest, $nDiff ) = ( 0, 1e308 );
    for my $i ( 0 .. $#ordered - 1 ) {
        my $diff = abs( $ordered[ $i+1 ] - $ordered[ $i ] );
        ( $nearest, $nDiff ) = ( $i, $diff ) if $diff < $nDiff;
    }
    my $discard = ( 
        $ordered[ $nearest ][ 1 ][ 2 ] 
        < 
        $ordered[ $nearest+1 ][ 1 ][ 2 ] ) ? $nearest+1 : $nearest;
    splice @ordered, $discard, 1;
}

$last = undef;
for my $pt ( map $_->[1], @ordered ) {
    my( $x, $y, $dy ) = map{ $_*SCALE } @{ $pt };
    $x += OFFSET;

    $img->filledEllipse(
        $x, $y, $dy, $dy, rgba2n( 0, 255, 0, 90 )
    );
    $img->line( $x-10, $y, $x+10, $y, 0 );
    $img->line( $x, $y-10, $x, $y+10, 0 );
    if( $last ) {
        $img->line( $last->[0], $last->[1], $x, $y, rgb2n( 0, 255, 0 ) );
    }
    $last = [ $x, $y ];
}


$img->flipVertical;

open PNG, '>:raw', "$0.png";
print PNG $img->png;
close PNG;
system 1, "$0.png";