$fn = 150;

number_of_pens = 8;
pen_radius = 8;
pen_length = 115;
side_height = 40;
circle_spacing = 3;
wall_offset_y = 13;
wall_offset_x = 10;
circle_offset_y = 0;
height = (number_of_pens*pen_radius*2)+(number_of_pens+1)*circle_spacing;
width = (2*pen_radius);
width_circle_spacing = 1;
d = wall_offset_y - 3;
p = 5;
k = height/2;
h = (d^2 - (height/2 - p)^2)/(2*d);
R = sqrt(pow(h,2)+pow((height/2)-p,2));
echo(R);
echo(h);
difference(){
    group(){
        linear_extrude(height) group(){ 
            difference() {
                translate([-12,0]) square([12,10]);
                rotate([0,0,-45]) translate([-16,-11]) square([20,10]);
            } 
            translate([0,4]) square([3,1]);
            translate([0,10]) square([3,1]);
            translate([3,10]) square([1,3]);
            translate([3,2]) square([1,3]);
            translate([-pen_length-wall_offset_x,10]) square([pen_length+wall_offset_x,3]); //inner wall
            translate([-pen_length-wall_offset_x,0]) square([3,wall_offset_y+width+3+width_circle_spacing*2]); //bottom 
            translate([-pen_length-wall_offset_x,wall_offset_y+width+width_circle_spacing*2]) square([pen_length+3,3]); //outer wall
            translate([-wall_offset_x,12]) square([3,width+3+width_circle_spacing*2]);
        };

        translate([-pen_length+3-wall_offset_x,13,0]) cube([side_height,width+width_circle_spacing*2,3]);
        translate([-pen_length+3-wall_offset_x,13,height-circle_spacing]) cube([side_height,width+width_circle_spacing*2,3]); 
    }    

    for(i = [1:1:number_of_pens]) {
      position = pen_radius+circle_spacing+((i-1)*circle_spacing)+((i-1)*pen_radius*2);
      translate([-5-wall_offset_x, +wall_offset_y+width_circle_spacing+circle_offset_y+pen_radius, position]) rotate([0,90,0]) cylinder(10, pen_radius, pen_radius);  
    }
    translate([-pen_length-wall_offset_x,h,height/2]) rotate([0,90,0]) cylinder(h = 10, r = R);
}