//! # Vertex Module
//!
//! Defines `Vertex`, the CPU-side data layout that is sent to the GPU as vertex attribute buffers.
//! The struct's field order and offsets must exactly match the shader input attributes defined in
//! PipelineCache::build_pipeline() — any mismatch will corrupt GPU rendering output.
//!
//! ## Technical Points
//! - `#[repr(C)]` ensures fields are laid out contiguously without Rust padding reordering, matching C ABI.
//! - `bytemuck::Pod + bytemuck::Zeroable` enables safe `cast_slice()` conversion for GPU buffer uploads.

/// Per-vertex attribute tuple: position (3D), normal (3D), texture coordinate (2D), color (RGBA).
/// Must match the vertex buffer layout in PipelineCache::build_pipeline() byte-for-byte.
#[repr(C)]
#[derive(Copy, Clone, Debug, bytemuck::Pod, bytemuck::Zeroable)]
pub struct Vertex {
    /// XYZ coordinates of this vertex in world space. Offset: 0 bytes.
    pub position: [f32; 3],
    /// XYZ coordinates of the vertex normal. Offset: 12 bytes.
    pub normal: [f32; 3],
    /// UV texture coordinates. Offset: 24 bytes
    pub uv: [f32; 2],
    /// RGBA color values. Offset: 32 bytes.
    pub color: [f32; 4],
}

impl Default for Vertex {
    // Default values for stability
    fn default() -> Self {
        Self {
            // Default position at center (0, 0)
            position: [0.0, 0.0, 0.0],

            // Normal pointing upward (standard for lighting calculations)
            normal: [0.0, 1.0, 0.0],

            // UV coordinates at the origin of the texture
            uv: [0.0, 0.0],

            // Opaque white color by default
            color: [1.0, 1.0, 1.0, 1.0],
        }
    }
}