$fn = $preview ? 32 : 64;

eraser_width = 125.6;
eraser_height = 51;
eraser_depth = 24;
frame_margin = 10;
frame_thickness = 2;
nub_diameter = 2;
screw_holes = true;
screw_hole_diameter = 5;
holder_offset = 60;
pen_diameter = 17.7;
pen_count = 6;

module clasp(diameter) {
    difference() {
        cube([
            diameter + frame_thickness * 2,
            frame_margin,
            diameter * 0.74 + frame_thickness * 2 + nub_diameter / 2
        ]);
        translate([
            frame_thickness,
            -1,
            frame_thickness + diameter / 2
        ]) cube([
            diameter,
            frame_margin * 4 + 2,
            diameter
        ]);
        translate([
            diameter / 2 + frame_thickness,
            -1,
            diameter / 2 + frame_thickness
        ]) rotate([-90, 0, 0]) cylinder(
            h=frame_margin + 2,
            r=diameter / 2
        );
    }
    translate([
        frame_thickness,
        frame_margin / 2,
        frame_thickness + diameter * 0.74
    ]) sphere(nub_diameter / 2);
    translate([
        frame_thickness + diameter,
        frame_margin / 2,
        frame_thickness + diameter * 0.74
    ]) sphere(nub_diameter / 2);
}

// Eraser shelf
difference() {
    cube([
        eraser_width + frame_thickness * 2,
        eraser_height + frame_thickness * 2,
        eraser_depth + frame_thickness * 2
    ]);
    translate([
        frame_thickness,
        frame_thickness,
        frame_thickness
    ]) cube([
        eraser_width,
        eraser_height + frame_thickness + 1,
        eraser_depth + frame_thickness + 1
    ]);
    if (screw_holes) {
        translate([
        frame_thickness + frame_margin,
            eraser_height + frame_thickness * 2 - frame_margin,
            -1
        ]) cylinder(
            h=frame_thickness + 2,
            r=screw_hole_diameter / 2
        );
        translate([
            eraser_width + frame_thickness - frame_margin,
            eraser_height + frame_thickness * 2 - frame_margin,
            -1
        ]) cylinder(
            h=frame_thickness + 2,
            r=screw_hole_diameter / 2
        );
    }
}

// Pen holder frame
difference() {
    translate([
        0,
        -holder_offset,
        0
    ]) cube([
        eraser_width + frame_thickness * 2,
        holder_offset,
        frame_thickness
    ]);
    translate([
        frame_margin,
        -holder_offset + frame_margin,
        -1
    ]) cube([
        eraser_width + frame_thickness * 2 - frame_margin * 2,
        holder_offset - frame_margin,
        frame_thickness + 2
    ]);
}

// Lower screw bar
difference() {
    union() {
        translate([
            0,
            -holder_offset * 0.618 + frame_margin * 0.375,
            0,
        ]) cube([
            eraser_width,
            frame_margin / 2,
            frame_thickness
        ]);
        translate([
            eraser_width / 2 + frame_thickness,
            -holder_offset * 0.618 + frame_margin * 0.625,
            0,
        ]) cylinder(
            h=frame_thickness,
            r=frame_margin / 2
        );
    }
    translate([
        eraser_width / 2 + frame_thickness,
        -holder_offset * 0.618 + frame_margin * 0.625,
        -1,
    ]) cylinder(
        h=frame_thickness + 2,
        r=screw_hole_diameter / 2
    );
}

// Pen holder clasps
translate([
    (eraser_width - pen_count * (pen_diameter + frame_thickness) + frame_thickness) / 2,
    -holder_offset,
    0
]) for(i = [0:pen_count - 1]) {
    translate([(pen_diameter + frame_thickness) * i, 0]) clasp(pen_diameter);
}