//! # Mesh Module
//!
//! Defines `Mesh`, a persistent GPU geometry container. Mesh data is uploaded to the GPU once at creation time
//! and remains valid across all frames until dropped. It holds no rendering knowledge—only raw geometric data.

use crate::vertex::Vertex;
use wgpu::util::DeviceExt;

/// Persistent GPU geometry: vertex positions, optional indices, and draw call counters.
/// Created once via `Mesh::new()` during scene setup; referenced by Renderer for every frame.
pub struct Mesh {
    /// GPU buffer containing vertex attribute data (position, UV, color).
    pub vertex_buffer: wgpu::Buffer,
    /// Optional GPU buffer for indexed drawing. Present when the mesh uses index-based rendering instead of simple vertex iteration.
    pub index_buffer: Option<wgpu::Buffer>,
    /// Number of vertices in the mesh. Used as `0..num_vertices` for non-indexed draws.
    pub num_vertices: u32,
    /// Number of indices in the index buffer. Used as `0..num_indices` for indexed draws.
    pub num_indices: u32,
}

impl Mesh {
    /// Creates a new Mesh by uploading vertex and optional index data to GPU buffers.
    /// Inputs: device (GPU command source), vertices (CPU-side vertex array), indices (optional CPU-side index array).
    /// Returns a Mesh with two GPU buffers ready for rendering. Called at scene initialization time only.
    pub fn new(device: &wgpu::Device, vertices: &[Vertex], indices: Option<&[u16]>) -> Self {
        let vertex_buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
            label: Some("Mesh Vertex Buffer"),
            contents: bytemuck::cast_slice(vertices),
            usage: wgpu::BufferUsages::VERTEX,
        });

        // Create optional index buffer and count indices if provided
        let (index_buffer, num_indices) = if let Some(data) = indices {
            let buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
                label: Some("Mesh Index Buffer"),
                usage: wgpu::BufferUsages::INDEX,
                contents: bytemuck::cast_slice(data),
            });
            (Some(buffer), data.len() as u32)
        } else {
            (None, 0)
        };
        Self {
            vertex_buffer,
            index_buffer,
            num_vertices: vertices.len() as u32,
            num_indices,
        }
    }
}