Debugging Guide for Nethercore ZX

Debug ZX games using the built-in inspection system and testing tools. Press F4 to open the debug panel during development.

Debug System Overview

The debug inspection system provides:

• Live value editing (sliders, checkboxes, color pickers)
• Read-only watches for monitoring
• Grouped organization for complex games
• Frame control (pause, step, time scale)
• Zero overhead in release builds (compiles out)

Reference nethercore/include/zx.rs lines 1370-1478 for complete FFI signatures.

F4 Debug Panel

Press F4 during gameplay to toggle the debug panel. The panel displays all registered values and provides interactive controls.

Keyboard Shortcuts

Key	Action
F4	Toggle debug panel
F5	Pause/unpause game
F6	Step one frame (while paused)
F7	Decrease time scale
F8	Increase time scale

Registering Editable Values

Register values during init() to make them editable in the F4 panel:

static mut PLAYER_SPEED: f32 = 5.0;
static mut GRAVITY: f32 = 25.0;
static mut GOD_MODE: u8 = 0;
static mut ENEMY_COUNT: i32 = 5;

#[no_mangle]
pub extern "C" fn init() {
    unsafe {
        // Basic registration
        debug_register_f32(b"Speed".as_ptr(), 5, &PLAYER_SPEED);
        debug_register_f32(b"Gravity".as_ptr(), 7, &GRAVITY);
        debug_register_bool(b"God Mode".as_ptr(), 8, &GOD_MODE);
        debug_register_i32(b"Enemies".as_ptr(), 7, &ENEMY_COUNT);
    }
}

Range-Constrained Values (Sliders)

Use range variants for slider UI with min/max bounds:

static mut DIFFICULTY: i32 = 2;
static mut SPEED: f32 = 5.0;

fn init() {
    unsafe {
        debug_register_i32_range(b"Difficulty".as_ptr(), 10, &DIFFICULTY, 1, 5);
        debug_register_f32_range(b"Speed".as_ptr(), 5, &SPEED, 0.0, 20.0);
    }
}

Compound Types

Register vectors, colors, and rects:

static mut PLAYER_POS: [f32; 3] = [0.0, 0.0, 0.0];
static mut TINT_COLOR: [u8; 4] = [255, 255, 255, 255];

fn init() {
    unsafe {
        debug_register_vec3(b"Position".as_ptr(), 8, PLAYER_POS.as_ptr());
        debug_register_color(b"Tint".as_ptr(), 4, TINT_COLOR.as_ptr());
    }
}

Watch Functions (Read-Only)

Use watch functions to display values without allowing editing:

static mut FRAME_COUNT: u32 = 0;
static mut FPS: f32 = 0.0;
static mut PLAYER_VEL: [f32; 2] = [0.0, 0.0];

fn init() {
    unsafe {
        debug_watch_u32(b"Frame".as_ptr(), 5, &FRAME_COUNT);
        debug_watch_f32(b"FPS".as_ptr(), 3, &FPS);
        debug_watch_vec2(b"Velocity".as_ptr(), 8, PLAYER_VEL.as_ptr());
    }
}

Organizing with Groups

Group related values for a cleaner debug panel:

fn init() {
    unsafe {
        debug_group_begin(b"Player".as_ptr(), 6);
        debug_register_vec3(b"Position".as_ptr(), 8, PLAYER_POS.as_ptr());
        debug_register_f32(b"Health".as_ptr(), 6, &PLAYER_HEALTH);
        debug_register_f32(b"Speed".as_ptr(), 5, &PLAYER_SPEED);
        debug_group_end();

        debug_group_begin(b"Physics".as_ptr(), 7);
        debug_register_f32(b"Gravity".as_ptr(), 7, &GRAVITY);
        debug_register_f32(b"Friction".as_ptr(), 8, &FRICTION);
        debug_group_end();

        debug_group_begin(b"Debug".as_ptr(), 5);
        debug_register_bool(b"God Mode".as_ptr(), 8, &GOD_MODE);
        debug_register_bool(b"Show Hitboxes".as_ptr(), 13, &SHOW_HITBOXES);
        debug_group_end();
    }
}

Groups are collapsible in the UI - click the header to expand/collapse.

Frame Control

Query debug state to respect pause/time scale:

fn update() {
    unsafe {
        // Skip update when paused via debug panel
        if debug_is_paused() != 0 {
            return;
        }

        // Apply time scale to delta_time
        let dt = delta_time() * debug_get_time_scale();

        // Use scaled dt for all time-based logic
        PLAYER_VEL_Y += GRAVITY * dt;
        PLAYER_X += PLAYER_VEL_X * dt;
    }
}

Time Scale Values:

• F7 decreases: 0.5x, 0.25x, 0.1x
• F8 increases: 2x, 4x, 8x
• Default: 1.0x

Multiplayer Sync Testing

Test rollback determinism with CLI flags:

Sync Test Mode

Runs two parallel simulations and compares state:

nether run --sync-test

This launches two game instances in parallel:

1. Both receive identical inputs
2. State is compared after each frame
3. Any divergence is reported as a desync

Desync causes:

• Using external randomness instead of random()
• Reading wall-clock time instead of delta_time()/tick_count()
• Uninitialized memory
• Modifying state in render() instead of update()

P2P Test Mode

Tests actual network rollback with two instances:

nether run --p2p-test

Creates a local two-player session:

1. Player 1 window (local)
2. Player 2 window (local)
3. GGRS netcode active between them

Use this to verify:

• Input synchronization works
• Rollbacks don't cause visual glitches
• State serialization is correct

Common Debugging Patterns

Visualizing Hitboxes

static mut SHOW_HITBOXES: u8 = 0;

fn init() {
    unsafe {
        debug_register_bool(b"Hitboxes".as_ptr(), 8, &SHOW_HITBOXES);
    }
}

fn render() {
    unsafe {
        if SHOW_HITBOXES != 0 {
            // Draw wireframe boxes around collision volumes
            for entity in entities.iter() {
                draw_rect(
                    entity.x - entity.half_w,
                    entity.y - entity.half_h,
                    entity.half_w * 2.0,
                    entity.half_h * 2.0,
                    0x00FF0080, // Semi-transparent green
                );
            }
        }
    }
}

Debug HUD

fn render() {
    unsafe {
        // Show frame counter and FPS
        let frame = tick_count();
        // Draw debug text at top-left
        draw_text_str(&format!("Frame: {}", frame), 10.0, 10.0, 16.0, 0xFFFFFFFF);
    }
}

Slow Motion for Animation Debugging

Use time scale controls (F7/F8) to:

1. Slow animations to 0.25x
2. Check interpolation smoothness
3. Verify state transitions
4. Frame-step with F6 for exact positioning

Debugging Desyncs

When --sync-test reports a desync:

1. Check random usage - Every random() call must happen in same order
2. Verify state location - All game state in static variables?
3. Review render() - No state modifications allowed
4. Check floating point - Consistent operations across runs
5. Inspect conditionals - Same branches taken each simulation

Add debug watches to narrow down which variable diverges first.

Helper Functions

Rust helper functions in zx.rs:

// String-based helpers
debug_f32("speed", &SPEED);        // debug_register_f32 wrapper
debug_i32("count", &COUNT);        // debug_register_i32 wrapper
debug_bool("enabled", &ENABLED);   // debug_register_bool wrapper
debug_group("Player");             // debug_group_begin wrapper
debug_group_close();               // debug_group_end wrapper

Available Registration Functions

Function	Type	UI Control
debug_register_i8/i16/i32	Signed integers	Number input
debug_register_u8/u16/u32	Unsigned integers	Number input
debug_register_f32	Float	Number input
debug_register_bool	Boolean (u8)	Checkbox
debug_register_*_range	Numbers	Slider
debug_register_vec2/vec3	Float arrays	Multi-input
debug_register_rect	4 i16	Multi-input
debug_register_color	4 u8 RGBA	Color picker
debug_register_fixed_*	Fixed-point	Decimal display

Performance Notes

• Debug functions compile to no-ops in release builds
• Registration happens once in init(), not per-frame
• Watches read memory directly, no runtime cost
• F4 panel rendering uses separate draw path

Additional Resources

• examples/debug-setup-rust.md - Complete Rust debug setup
• references/sync-test-checklist.md - Desync debugging checklist
• nethercore/include/zx.rs - Debug FFI (lines 1370-1478)
• nethercore/docs/book/src/api/debug.md - Full API reference