/* [Gear Parameters] */
gear_module = 2.0;               // [0.5:0.1:10.0]
teeth_pinion = 20;               // [6:1:100]
teeth_gear = 40;                 // [10:1:150]
pressure_angle = 20.0;           // [14.5:0.5:25.0]
face_width = 8.0;                // [2.0:0.5:50.0]

/* [Bore & Keyway] */
bore_diameter = 6.0;             // [2.0:0.5:25.0]
keyway_width = 2.0;              // [0.5:0.5:10.0]
keyway_depth = 1.2;              // [0.2:0.1:5.0]

/* [Display & Animation] */
rotation_angle = 0.0;            // [0:1:360]
show_pitch_circles = false;
show_base_plate = false;
pinion_color = "Gold";
gear_color = "DodgerBlue";
base_color = "LightSlateGray";

/* [Quality] */
$fn = 64;

// --- Calculations ---
rp1 = gear_module * teeth_pinion / 2; // Pitch radius 1
rp2 = gear_module * teeth_gear / 2;   // Pitch radius 2
center_distance = rp1 + rp2;

// Involute math helpers
function inv(a) = tan(a) - (a * PI / 180); // Involute function (angle in degrees, result in radians)
function inv_angle(inv_val) = 
    // Newton-Raphson approximation for inv_angle in radians
    let(
        a0 = pow(3 * inv_val, 1/3),
        a1 = a0 - (tan(a0 * 180 / PI) - a0 - inv_val) / pow(tan(a0 * 180 / PI), 2),
        a2 = a1 - (tan(a1 * 180 / PI) - a1 - inv_val) / pow(tan(a1 * 180 / PI), 2),
        a3 = a2 - (tan(a2 * 180 / PI) - a2 - inv_val) / pow(tan(a2 * 180 / PI), 2)
    ) a3 * 180 / PI;

// Single gear 2D profile
module spur_gear_2d(teeth, m, alpha) {
    pitch_radius = m * teeth / 2;
    base_radius = pitch_radius * cos(alpha);
    addendum = 1.0 * m;
    dedendum = 1.25 * m;
    tip_radius = pitch_radius + addendum;
    root_radius = max(pitch_radius - dedendum, 0.1);
    
    // Half tooth thickness angle at pitch circle (in degrees)
    pitch_half_angle = (360 / (4 * teeth));
    inv_alpha = (tan(alpha) - (alpha * PI / 180)) * 180 / PI;

    // Number of sample points along involute profile
    num_pts = 8;
    
    // Build single tooth profile points
    r_start = max(root_radius, base_radius);
    
    // Involute points for right flank
    right_flank = [
        for (i = [0:num_pts])
            let(
                r = r_start + (tip_radius - r_start) * (i / num_pts),
                alpha_r = acos(base_radius / r),
                inv_r = (tan(alpha_r) - (alpha_r * PI / 180)) * 180 / PI,
                theta = pitch_half_angle + inv_alpha - inv_r
            ) [r * cos(theta), r * sin(theta)]
    ];
    
    // Root radial extension if root < base
    right_root = (root_radius < base_radius) ? [
        let(theta_base = pitch_half_angle + inv_alpha)
        [root_radius * cos(theta_base), root_radius * sin(theta_base)]
    ] : [];

    // Left flank points (mirrored Y)
    left_flank = [
        for (i = [num_pts:-1:0])
            let(
                r = r_start + (tip_radius - r_start) * (i / num_pts),
                alpha_r = acos(base_radius / r),
                inv_r = (tan(alpha_r) - (alpha_r * PI / 180)) * 180 / PI,
                theta = pitch_half_angle + inv_alpha - inv_r
            ) [r * cos(-theta), r * sin(-theta)]
    ];
    
    left_root = (root_radius < base_radius) ? [
        let(theta_base = pitch_half_angle + inv_alpha)
        [root_radius * cos(-theta_base), root_radius * sin(-theta_base)]
    ] : [];

    tooth_pts = concat(
        [[root_radius * cos(0), 0]],
        right_root,
        right_flank,
        left_flank,
        left_root
    );

    union() {
        // Root core circle
        circle(r = root_radius, $fn = teeth * 4);
        
        // Teeth array
        for (k = [0:teeth-1]) {
            rotate(k * 360 / teeth)
            polygon(tooth_pts);
        }
    }
}

// 2D Keyway bore cutout
module bore_with_keyway_2d(d, kw, kd) {
    union() {
        circle(d = d, $fn = 48);
        translate([-kw / 2, d / 2 - 0.1])
        square([kw, kd + 0.1]);
    }
}

// Complete 3D Spur Gear
module spur_gear_solid(teeth, m, alpha, width, bore_d, kw, kd, gear_col) {
    color(gear_col)
    difference() {
        linear_extrude(height = width, center = true, convexity = 10)
        difference() {
            spur_gear_2d(teeth, m, alpha);
            bore_with_keyway_2d(bore_d, kw, kd);
        }
        
        // Chamfers on rim edges for realistic manufacturing look
        pitch_r = m * teeth / 2;
        tip_r = pitch_r + 1.0 * m;
        
        translate([0, 0, width / 2])
        difference() {
            cylinder(r = tip_r + 2, h = 0.6, center = false);
            cylinder(r1 = tip_r - 0.6, r2 = tip_r, h = 0.61, center = false);
        }
        
        translate([0, 0, -width / 2 - 0.01])
        difference() {
            cylinder(r = tip_r + 2, h = 0.6, center = false);
            cylinder(r1 = tip_r, r2 = tip_r - 0.6, h = 0.61, center = false);
        }
    }
}

// --- Assembly ---

// Pinion (Gear 1: 20T) at Origin
rotate([0, 0, rotation_angle])
spur_gear_solid(
    teeth = teeth_pinion,
    m = gear_module,
    alpha = pressure_angle,
    width = face_width,
    bore_d = bore_diameter,
    kw = keyway_width,
    kd = keyway_depth,
    gear_col = pinion_color
);

// Driven Gear (Gear 2: 40T) at Center Distance
// Gear ratio is 20/40 = 0.5. Rotation is opposite direction.
// Offset phase ensures perfect tooth-in-gap mesh at contact point.
gear2_mesh_phase = 180 / teeth_gear; // 4.5 deg shift to align tooth with gap
translate([center_distance, 0, 0])
rotate([0, 0, 180 - (rotation_angle * (teeth_pinion / teeth_gear)) + gear2_mesh_phase])
spur_gear_solid(
    teeth = teeth_gear,
    m = gear_module,
    alpha = pressure_angle,
    width = face_width,
    bore_d = bore_diameter,
    kw = keyway_width,
    kd = keyway_depth,
    gear_col = gear_color
);

// Optional Pitch Circle visualization (dashed/thin)
if (show_pitch_circles) {
    color("LimeGreen") {
        translate([0, 0, face_width / 2 + 0.2])
        difference() {
            cylinder(r = rp1 + 0.3, h = 0.2, center = true);
            cylinder(r = rp1 - 0.3, h = 0.3, center = true);
        }
        translate([center_distance, 0, face_width / 2 + 0.2])
        difference() {
            cylinder(r = rp2 + 0.3, h = 0.2, center = true);
            cylinder(r = rp2 - 0.3, h = 0.3, center = true);
        }
    }
}

// Optional Base Plate with Shaft Pins
if (show_base_plate) {
    color(base_color) {
        translate([center_distance / 2, 0, -face_width / 2 - 3])
        difference() {
            hull() {
                cylinder(r = bore_diameter + 6, h = 3, center = true);
                translate([center_distance, 0, 0])
                cylinder(r = bore_diameter + 6, h = 3, center = true);
            }
            // Mounting holes
            cylinder(d = 3, h = 5, center = true);
            translate([center_distance, 0, 0])
            cylinder(d = 3, h = 5, center = true);
        }
    }
}