#!/usr/bin/perl
use warnings;
use strict;
use Tk;

# by zentara@zentara.net 
# There was an advanced perl programming contest awhile 
# back to simulate a roller coaster. This was my entry. 

# This approach is a very simple method to simulate 
# the roller-coaster motion. I didn't waste any time 
# on actually computing the velocities or positions, 
# rather I relied on the Law of Conservation of Energy, 
# which says that the sum of the Kinetic and Potential 
# Energies will remain constant. The initial total energy  
# is the potential energy at the highest point, and that 
# is released into kinetic energy( i.e. velocity) as the 
# coaster drops.  So as the coaster drops,  
# it's KE increases, and as the coaster climbs it's KE 
# decreases, and stalls if it climbs past it's start height. 
#  
# I figured this was the best approach, since we don't have  
# an actual equation to differentiate to get the instantaneous 
# values. So this is not mathematically exact, it ignores the 
# constants in favor of better animation. 
# After all, it's just a visual simulation,   


$|++;

my %segs =();
my $count = 0;       # holds how many segments there are 
my $action = 0;      # action flag 
my $cur_loc = 1;     # holds current segment in case of stop run 
my $cur_ke  = 0;     # initial kinetic energy 
my $g  = 10;         # gravitational acceleration 
my $m = 1;           # mass of coaster  
                     # (left in for possible future Lorentz transforms :-)) 

my $velocity = 0;    # speedometer  :-) 

while(<DATA>){
    $count++;
    chomp;
    next if /^\s*#/ || /^\s*$/; #skip comments and blank lines  
    my ($x, $y) = split(/\s+/, $_);
    $segs{$count} = [$x, (400 - $y) ]; #inverse coords   
}


my $mw = tkinit;
$mw->geometry("650x450+100+100");

my $canvas = $mw->Canvas(-width => 620, -height => 420,
                         -bg => 'black')->pack();

my @s;
 foreach (1..$count){
   my @str = ( ${$segs{$_}}[0], ${$segs{$_}}[1] );
   push @s, @str;
}

my $curve = $canvas->createLine( @s,
                          -width => 5,
                          -smooth => 1,
                          -fill => 'lightgreen');

my $coaster = $canvas->createOval(${$segs{1}}[0] - 5,${$segs{1}}[1] - 5,
                                  ${$segs{1}}[0] + 5 ,${$segs{1}}[1] + 5,
                                  -fill => 'yellow');

my $group = $canvas->createGroup([${$segs{1}}[0],${$segs{1}}[1]],
                                          -members => [$coaster]);

my $subframe = $mw->Frame(-background =>'gray50')->pack(-fill => 'x');

$subframe->Button(-text =>'Exit',
            -background => 'hotpink',
            -activebackground => 'red',
            -command => sub{ exit }
            )->pack(-side=>'left',-padx=>40);

$subframe->Button(-text =>'Slower',
            -background => 'lightyellow',
            -activebackground => 'yellow',
            -command => sub{ if($g < 3)
                         { print chr(07);return}
                         else{$g *= .8; print "$g\n"; } } #not too slow :-) 
            )->pack(-side=>'left',-padx=>40);

$subframe->Label(-text =>'Velocity ',
            -background => 'black',
            -foreground => 'green',
            )->pack(-side=>'left');
       
            
$subframe->Label(-textvariable => \$velocity,
            -width => 5,
            -background => 'black',
            -foreground => 'green',
            )->pack(-side=>'left');

$subframe->Button(-text =>'Faster',
            -background => 'lightgreen',
            -activebackground => 'green',
            -command => sub{ $g *= 1.25; print "$g\n"; }
            )->pack(-side=>'left',-padx=>40);

my $actbut;
$actbut = $subframe->Button(-text =>'Start',
              -background =>'lightsteelblue',
              -activebackground =>'lightskyblue',
              -command => sub { &action($cur_loc,$cur_ke) },
             )->pack(-side=>'right',-padx=>40);

MainLoop;

####################################################################### 
sub action {

  if( $actbut->cget(-text) eq 'Start'){
            $actbut->configure(-text=>'Stop');
            $action = 1;
    }else{
            $actbut->configure(-text=>'Start');
            $action = 0;
    }

my ($cur_seg,$cur_ke,) = @_;
my $dt;

#   potential energy at start 
  my $pe_max = $m*$g*(${$segs{1}}[1]);

#Energy is conserved so at any point 
# pe_max = pe_current + ke_current  
# so  
# ke_current = pe_max - pe_current (both directly proportion to y) 

while( $action ){

    my $cur = $cur_seg;
    my $cur_next = $cur_seg + 1;
     if($cur_next > $count ){ $action = 0;
                              $actbut->configure(-text=>'Start');
                              $cur_next = 1;
                             }
                             
    my $x1 = ${$segs{$cur}}[0];
    my $y1 = ${$segs{$cur}}[1];

    my $x2 = ${$segs{$cur_next}}[0];
    my $y2 = ${$segs{$cur_next}}[1];

     my $dx = $x2 - $x1;
     my $dy = $y2 - $y1;
     my $distance = sqrt( $dx**2 + $dy**2);

# compute average y of segment 
     my $y_ave = ($y2 + $y1)/2;

    $cur_ke = $m*$g*$y_ave - $pe_max;

#print "dy->$dy\tcur_ke->$cur_ke\n"; 

$canvas->move($group, $dx,$dy);
$canvas->update;

   if($cur_next > $count ){ $cur_seg = 1}
         else {$cur_seg++}

# velocity is proportional to the sqrt of ke 
# and time is inverse to velocity         
# $v = $distance/$seconds  
# $seconds = $distance/$v  

  if($cur_ke >= 0){
     my $v = sqrt(2*$cur_ke/$m);
     $dt =  $distance/$v;  # constant 2 added for visual effect  
         if($cur_loc >= $count){
              $cur_loc = 1;
              $cur_ke = 0;
              $dt = .001;
              $velocity = 0;
             }
       $velocity = sprintf('%.1f',$v);
       $velocity = sprintf('%.1f',$v);

       #select(undef,undef,undef,$dt);    
       $mw->after($dt*1000,$canvas->update);
       #it's not the best to use a blocking select in Tk 
       #but it's all in microseconds....so no problem 
       #select takes seconds , after takes microseconds 
     }else{
         my $message =
        'Uh oh, Out of KE, Get out and push. Hit Start to get moving.' ;
          $mw->messageBox(
             -background => 'lightyellow',
             -icon       => 'error',
             -message    => $message,
             -type       => 'OK'
              );
      &restart;
      }

$cur_loc = $cur_seg;
} #end of while loop 

if($cur_loc >= $count){
      $cur_loc = 1;
      $cur_ke = 0;
      $dt = .001;
      $velocity = 0;
  }
}
###################################################### 
sub restart{
  $actbut->configure(-text=>'Start');

$action = 0;      # action flag 
$cur_loc = 1;     # holds current segment in case of stop run 
$cur_ke  = 0;     # initial kinetic energy 
$g = 10;

}
#################################################### 

__DATA__
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4.71503160037539 374.563732945664
6.06061 369.017
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34.1461008917288 281.852959297317
36.3636 276.526
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139.394 217.937
145.455 220.74
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157.576 226.994
163.636 230.329
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175.758 237.145
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200 250
206.061 252.8
212.121 255.359
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224.242 259.62
230.303 261.263
236.364 262.546
242.424 263.445
248.485 263.941
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521.212 38.9591
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600 250