$fn=64;

// --- PARAMETERS ---
grid_x = 4;               // Number of units in X direction
grid_y = 4;               // Number of units in Y direction
unit_size = 42.0;         // Size of a single grid unit (42mm for standard modular bins)
clearance = 0.6;          // Clearance to easily fit over the bin
wall_thickness = 2.0;     // Thickness of the outer lip
top_thickness = 2.0;      // Thickness of the top cover
lip_height = 8.0;         // Depth of the lip that goes over the bin
corner_radius = 4.0;      // Outer corner radius
snap_ridge = 0.6;         // Depth of the inner snap ridge

// --- CALCULATED DIMENSIONS ---
in_w = (grid_x * unit_size) + clearance;
in_d = (grid_y * unit_size) + clearance;
out_w = in_w + (2 * wall_thickness);
out_d = in_d + (2 * wall_thickness);

// --- MAIN OBJECT ---
union() {
    difference() {
        // Main outer body
        rounded_rect_centered(out_w, out_d, lip_height + top_thickness, corner_radius + wall_thickness);
        
        // Subtracted inner void for the snap fit
        inner_void();
    }
    
    // Top reinforcing ridge for stiffness and stacking
    translate([0, 0, lip_height + top_thickness])
    difference() {
        rounded_rect_centered(out_w, out_d, 1.5, corner_radius + wall_thickness);
        translate([0, 0, -0.1])
        rounded_rect_centered(out_w - 6, out_d - 6, 1.7, corner_radius + wall_thickness - 3);
    }
}

// --- MODULES ---

// Creates the profiled inner cavity to allow a snap-on fit without supports
module inner_void() {
    lip_w = in_w - (2 * snap_ridge);
    lip_d = in_d - (2 * snap_ridge);
    
    // 1. Bottom entry flare (helps guide the lid onto the bin)
    hull() {
        translate([0, 0, -0.1]) 
            rounded_rect_centered(in_w + 2, in_d + 2, 0.1, corner_radius + 1);
        translate([0, 0, 1.2]) 
            rounded_rect_centered(lip_w, lip_d, 0.1, corner_radius);
    }
    
    // 2. Tight snap lip
    translate([0, 0, 1.2]) 
        rounded_rect_centered(lip_w, lip_d, 1.0, corner_radius);
        
    // 3. 45-degree transition to the looser main cavity (printable without supports)
    hull() {
        translate([0, 0, 2.2]) 
            rounded_rect_centered(lip_w, lip_d, 0.1, corner_radius);
        translate([0, 0, 2.2 + snap_ridge]) 
            rounded_rect_centered(in_w, in_d, 0.1, corner_radius + snap_ridge);
    }
    
    // 4. Main looser cavity
    translate([0, 0, 2.2 + snap_ridge]) 
        rounded_rect_centered(in_w, in_d, lip_height, corner_radius + snap_ridge);
}

// Helper module for a centered rounded rectangle
module rounded_rect_centered(w, d, h, r) {
    translate([-w/2, -d/2, 0])
    hull() {
        translate([r, r, 0]) cylinder(r=r, h=h);
        translate([w-r, r, 0]) cylinder(r=r, h=h);
        translate([w-r, d-r, 0]) cylinder(r=r, h=h);
        translate([r, d-r, 0]) cylinder(r=r, h=h);
    }
}