rustial-renderer-wgpu 0.0.1

// ---------------------------------------------------------------------------
//! # GPU buffer batching -- merges per-tile geometry into per-page draw calls
//!
//! The batch module is the CPU-side counterpart to the tile atlas
//! ([`crate::gpu::tile_atlas`]).  Where the atlas packs *textures* into
//! shared GPU pages, this module packs *geometry* into shared vertex and
//! index buffers so that every tile sharing an atlas page can be drawn
//! with a single `draw_indexed` call.
//!
//! ## Tile batches ([`build_tile_batches`])
//!
//! Each visible tile is a textured quad (4 vertices, 6 indices).  The
//! builder looks up the tile's [`AtlasRegion`](super::tile_atlas::AtlasRegion),
//! computes the world-space quad corners (camera-relative, f32), remaps
//! the UVs into the atlas sub-rectangle, and appends the vertices and
//! indices into a per-page accumulator.  After iterating all visible tiles,
//! each non-empty page accumulator is uploaded as a single vertex + index
//! buffer pair ([`TileBatch`]).
//!
//! The renderer issues one `draw_indexed` per non-empty page, binding the
//! page's atlas texture.  This reduces draw calls from N (one per tile) to
//! P (one per atlas page, typically 1-2).
//!
//! ## Terrain batches ([`build_terrain_batches`])
//!
//! Same grouping strategy, but the input is [`TerrainMeshData`] instead of
//! tile quads.  Each terrain mesh has its own vertex count and index list;
//! the builder rebases indices and remaps UVs identically to tiles.
//!
//! ## Vector batches ([`build_vector_batch`])
//!
//! Vector layers are already pre-tessellated by the engine into a single
//! [`VectorMeshData`] per layer.  This function converts the f64 world
//! positions to camera-relative f32 and uploads a single vertex + index
//! buffer pair.  No atlas grouping is needed because vector geometry is
//! not textured.
//!
//! ## Memory pattern
//!
//! Tile and vector batch buffers are now **cached across frames** using
//! invalidation-driven cache keys.  Tile batches are keyed by the
//! visible tile set (target/actual pairs), quantised camera origin, and
//! active projection.  Vector batches are keyed by layer vertex/index
//! counts and quantised camera origin.  When the key matches the
//! previous frame, existing GPU buffers are reused without any
//! allocation or upload.  When the key differs (e.g. camera movement
//! or tile set change), new buffers are created with
//! `BufferUsages::VERTEX` / `INDEX` and the cache is updated.
//!
//! Terrain batches (for the legacy CPU-displaced fallback path) and
//! hillshade batches remain per-frame temporaries because the shared-grid
//! terrain path handles its own caching through `SharedTerrainGridMesh`
//! and `CachedHeightTexture`.
// ---------------------------------------------------------------------------

use crate::gpu::terrain_vertex::TerrainVertex;
use crate::gpu::tile_atlas::TileAtlas;
use crate::gpu::vector_vertex::VectorVertex;
use crate::gpu::vertex::TileVertex;
use glam::DVec3;
use rustial_engine::{
    CameraProjection, PreparedHillshadeRaster, TerrainMeshData, TileId, VectorMeshData, VisibleTile,
};
use wgpu::util::DeviceExt;

/// Maximum ancestor depth for renderer-side fallback texture search.
const MAX_FALLBACK_DEPTH: u8 = 8;

pub(crate) fn find_terrain_texture_actual(
    tile: TileId,
    visible_tiles: &[VisibleTile],
) -> Option<TileId> {
    if let Some(vt) = visible_tiles
        .iter()
        .find(|vt| vt.target == tile && vt.data.is_some())
    {
        return Some(vt.actual);
    }

    let mut current = tile;
    let mut depth = 0u8;
    while depth < MAX_FALLBACK_DEPTH {
        let Some(parent) = current.parent() else {
            break;
        };
        if let Some(vt) = visible_tiles
            .iter()
            .find(|vt| vt.target == parent && vt.data.is_some())
        {
            return Some(vt.actual);
        }
        if visible_tiles
            .iter()
            .any(|vt| vt.actual == parent && vt.data.is_some())
        {
            return Some(parent);
        }
        current = parent;
        depth += 1;
    }

    None
}

// ---------------------------------------------------------------------------
// TileBatch
// ---------------------------------------------------------------------------

/// GPU buffers for all flat tile quads that share a single atlas page.
///
/// Produced by [`build_tile_batches`] and consumed by
/// [`WgpuMapRenderer::render_tile_batches`](crate::WgpuMapRenderer).
/// One draw call per non-`None` batch.
pub struct TileBatch {
    /// Merged vertex buffer (4 [`TileVertex`] per tile).
    pub vertex_buffer: wgpu::Buffer,
    /// Merged index buffer (6 `u32` indices per tile).
    pub index_buffer: wgpu::Buffer,
    /// Total number of indices to draw.
    pub index_count: u32,
}

/// Per-page tile draw buffers split by depth behavior.
///
/// Opaque tiles (full opacity) are drawn first with depth writes enabled.
/// Translucent tiles (fade transitions and fallback softening) are drawn
/// afterward with depth writes disabled so same-plane cross-fades do not
/// self-occlude.
pub struct TilePageBatches {
    /// Full-opacity tiles for this atlas page.
    pub opaque: Option<TileBatch>,
    /// Fading or fallback tiles for this atlas page.
    pub translucent: Option<TileBatch>,
}

fn tile_opacity_for(vt: &VisibleTile) -> f32 {
    let fallback_zoom_delta = if vt.actual.zoom > vt.target.zoom {
        (vt.actual.zoom - vt.target.zoom) as f32
    } else {
        vt.target.zoom.saturating_sub(vt.actual.zoom) as f32
    };
    let base_opacity = if vt.target == vt.actual {
        1.0_f32
    } else {
        (0.9_f32 - 0.12_f32 * fallback_zoom_delta).clamp(0.55_f32, 0.9_f32)
    };
    base_opacity * vt.fade_opacity
}

fn tile_requires_blended_pass(vt: &VisibleTile) -> bool {
    tile_opacity_for(vt) < 0.999_999
}

fn build_tile_batch(
    device: &wgpu::Device,
    verts: Vec<TileVertex>,
    idxs: Vec<u32>,
) -> Option<TileBatch> {
    if verts.is_empty() {
        return None;
    }

    let vertex_buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
        label: Some("tile_batch_vb"),
        contents: bytemuck::cast_slice(&verts),
        usage: wgpu::BufferUsages::VERTEX,
    });
    let index_buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
        label: Some("tile_batch_ib"),
        contents: bytemuck::cast_slice(&idxs),
        usage: wgpu::BufferUsages::INDEX,
    });
    Some(TileBatch {
        vertex_buffer,
        index_buffer,
        index_count: idxs.len() as u32,
    })
}

/// Build tile batches grouped by atlas page.
///
/// Returns a `Vec` indexed by atlas page number.  Entries are `None` for
/// pages with no visible tiles this frame.  The renderer iterates this
/// vector, binds the corresponding atlas page texture, and issues one
/// `draw_indexed` per `Some` entry.
///
/// # Arguments
///
/// * `device`        -- WGPU device for buffer allocation.
/// * `visible_tiles` -- The visible tile set for this frame (from engine).
/// * `atlas`         -- The tile texture atlas (for region lookups).
/// * `camera_origin` -- World-space camera origin subtracted from positions
///   to produce camera-relative f32 coordinates.
///
/// # Skipped tiles
///
/// Tiles whose `actual` id is not present in the atlas are silently
/// skipped.  This happens when a tile's texture has not finished
/// uploading yet -- the tile simply does not render this frame.
pub fn build_tile_batches(
    device: &wgpu::Device,
    visible_tiles: &[VisibleTile],
    atlas: &TileAtlas,
    camera_origin: DVec3,
    projection: CameraProjection,
) -> Vec<TilePageBatches> {
    if atlas.page_count() == 0 {
        return Vec::new();
    }

    // Pre-allocate per-page accumulators.  Typical visible tile counts are
    // 20-60 at moderate zoom; at 4 verts / 6 indices per tile this is tiny.
    let page_count = atlas.page_count();
    let mut page_verts_opaque: Vec<Vec<TileVertex>> = vec![Vec::new(); page_count];
    let mut page_idxs_opaque: Vec<Vec<u32>> = vec![Vec::new(); page_count];
    let mut page_verts_translucent: Vec<Vec<TileVertex>> = vec![Vec::new(); page_count];
    let mut page_idxs_translucent: Vec<Vec<u32>> = vec![Vec::new(); page_count];

    for vt in visible_tiles {
        let region = match atlas.get(&vt.actual) {
            Some(r) => r,
            None => continue,
        };

        // Geometry tile: for child-fallback entries (actual.zoom > target.zoom)
        // we use the child's own grid-cell bounds so the quad covers only
        // the appropriate sub-region of the target tile.  For all other
        // cases the target tile bounds are used.
        let geometry_tile = if vt.actual.zoom > vt.target.zoom {
            vt.actual
        } else {
            vt.target
        };
        let [sw, se, ne, nw] = projected_tile_corners(geometry_tile, projection, camera_origin);

        let texture_region = vt.texture_region();

        let (page_verts, page_idxs) = if tile_requires_blended_pass(vt) {
            (&mut page_verts_translucent, &mut page_idxs_translucent)
        } else {
            (&mut page_verts_opaque, &mut page_idxs_opaque)
        };

        // Base vertex index for this quad's indices.
        let base = page_verts[region.page].len() as u32;
        let tile_opacity = tile_opacity_for(vt);

        // Quad vertices: CCW winding (BL, BR, TR, TL).
        // UVs are first mapped to the parent sub-rect, then remapped into
        // the atlas sub-rectangle with half-texel inset.
        page_verts[region.page].extend_from_slice(&[
            TileVertex {
                position: [sw.x as f32, sw.y as f32, sw.z as f32],
                uv: region.remap_uv(texture_region.u_min, texture_region.v_max),
                opacity: tile_opacity,
            },
            TileVertex {
                position: [se.x as f32, se.y as f32, se.z as f32],
                uv: region.remap_uv(texture_region.u_max, texture_region.v_max),
                opacity: tile_opacity,
            },
            TileVertex {
                position: [ne.x as f32, ne.y as f32, ne.z as f32],
                uv: region.remap_uv(texture_region.u_max, texture_region.v_min),
                opacity: tile_opacity,
            },
            TileVertex {
                position: [nw.x as f32, nw.y as f32, nw.z as f32],
                uv: region.remap_uv(texture_region.u_min, texture_region.v_min),
                opacity: tile_opacity,
            },
        ]);

        // Two triangles per quad: (0,1,2) and (0,2,3).
        page_idxs[region.page].extend_from_slice(&[
            base,
            base + 1,
            base + 2,
            base,
            base + 2,
            base + 3,
        ]);
    }

    page_verts_opaque
        .into_iter()
        .zip(page_idxs_opaque)
        .zip(
            page_verts_translucent
                .into_iter()
                .zip(page_idxs_translucent),
        )
        .map(
            |((opaque_verts, opaque_idxs), (translucent_verts, translucent_idxs))| {
                TilePageBatches {
                    opaque: build_tile_batch(device, opaque_verts, opaque_idxs),
                    translucent: build_tile_batch(device, translucent_verts, translucent_idxs),
                }
            },
        )
        .collect()
}

fn projected_tile_corners(
    tile: TileId,
    projection: CameraProjection,
    camera_origin: DVec3,
) -> [glam::DVec3; 4] {
    let southwest =
        glam::DVec3::from_array(projection.project_tile_corner(&tile, 0.0, 1.0)) - camera_origin;
    let southeast =
        glam::DVec3::from_array(projection.project_tile_corner(&tile, 1.0, 1.0)) - camera_origin;
    let northeast =
        glam::DVec3::from_array(projection.project_tile_corner(&tile, 1.0, 0.0)) - camera_origin;
    let northwest =
        glam::DVec3::from_array(projection.project_tile_corner(&tile, 0.0, 0.0)) - camera_origin;
    [southwest, southeast, northeast, northwest]
}

// ---------------------------------------------------------------------------
// TerrainBatch
// ---------------------------------------------------------------------------

/// GPU buffers for all terrain meshes that share a single atlas page.
///
/// Produced by [`build_terrain_batches`] and consumed by
/// [`WgpuMapRenderer::render_terrain_batches`](crate::WgpuMapRenderer).
pub struct TerrainBatch {
    /// Merged vertex buffer ([`TerrainVertex`] with position + UV + normal).
    pub vertex_buffer: wgpu::Buffer,
    /// Merged index buffer (`u32` indices).
    pub index_buffer: wgpu::Buffer,
    /// Total number of indices to draw.
    pub index_count: u32,
}

/// GPU buffers for all terrain hillshade overlay meshes that share a single atlas page.
pub struct HillshadeBatch {
    /// Merged vertex buffer ([`TerrainVertex`] with position + UV + normal).
    pub vertex_buffer: wgpu::Buffer,
    /// Merged index buffer (`u32` indices).
    pub index_buffer: wgpu::Buffer,
    /// Total number of indices to draw.
    pub index_count: u32,
}

/// Build terrain batches grouped by atlas page.
///
/// Same grouping strategy as [`build_tile_batches`], but the input is
/// variable-size [`TerrainMeshData`] instead of fixed 4-vertex tile quads.
///
/// Each terrain mesh's positions are converted from f64 world-space to
/// camera-relative f32.  UVs are remapped into the atlas sub-rectangle.
/// Indices are rebased so that multiple meshes sharing a page form a
/// single contiguous vertex/index stream.
///
/// # Skipped meshes
///
/// Meshes whose `tile` id is not present in the atlas are silently
/// skipped (same rationale as tile batches).
pub fn build_terrain_batches(
    device: &wgpu::Device,
    terrain_meshes: &[TerrainMeshData],
    atlas: &TileAtlas,
    camera_origin: DVec3,
    visible_tiles: &[VisibleTile],
) -> Vec<Option<TerrainBatch>> {
    if atlas.page_count() == 0 || terrain_meshes.is_empty() {
        return Vec::new();
    }

    let page_count = atlas.page_count();
    let mut page_verts: Vec<Vec<TerrainVertex>> = vec![Vec::new(); page_count];
    let mut page_idxs: Vec<Vec<u32>> = vec![Vec::new(); page_count];

    for mesh in terrain_meshes {
        let actual_tile =
            find_terrain_texture_actual(mesh.tile, visible_tiles).unwrap_or(mesh.tile);
        let texture_region = rustial_engine::VisibleTile {
            target: mesh.tile,
            actual: actual_tile,
            data: None,
            fade_opacity: 1.0,
        }
        .texture_region();

        let region = match atlas.get(&actual_tile) {
            Some(r) => r,
            None => continue,
        };

        debug_assert_eq!(
            mesh.positions.len(),
            mesh.uvs.len(),
            "TerrainMeshData positions/uvs length mismatch for tile {:?}",
            mesh.tile,
        );
        debug_assert_eq!(
            mesh.positions.len(),
            mesh.normals.len(),
            "TerrainMeshData positions/normals length mismatch for tile {:?}",
            mesh.tile,
        );

        let vert_count = mesh.positions.len();
        let base = page_verts[region.page].len() as u32;
        page_verts[region.page].reserve(vert_count);

        for i in 0..vert_count {
            let pos = &mesh.positions[i];
            let uv = &mesh.uvs[i];
            let normal = &mesh.normals[i];

            let mapped_u =
                texture_region.u_min + (texture_region.u_max - texture_region.u_min) * uv[0];
            let mapped_v =
                texture_region.v_min + (texture_region.v_max - texture_region.v_min) * uv[1];

            page_verts[region.page].push(TerrainVertex {
                position: [
                    (pos[0] - camera_origin.x) as f32,
                    (pos[1] - camera_origin.y) as f32,
                    (pos[2] - camera_origin.z) as f32,
                ],
                uv: region.remap_uv(mapped_u, mapped_v),
                normal: *normal,
            });
        }

        page_idxs[region.page].reserve(mesh.indices.len());
        for idx in &mesh.indices {
            debug_assert!(
                (*idx as usize) < vert_count,
                "TerrainMeshData index {} out of bounds (vertex_count={}) for tile {:?}",
                idx,
                vert_count,
                mesh.tile,
            );
            page_idxs[region.page].push(base + idx);
        }
    }

    page_verts
        .into_iter()
        .zip(page_idxs)
        .map(|(verts, idxs)| {
            if verts.is_empty() {
                return None;
            }
            let vertex_buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
                label: Some("terrain_batch_vb"),
                contents: bytemuck::cast_slice(&verts),
                usage: wgpu::BufferUsages::VERTEX,
            });
            let index_buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
                label: Some("terrain_batch_ib"),
                contents: bytemuck::cast_slice(&idxs),
                usage: wgpu::BufferUsages::INDEX,
            });
            Some(TerrainBatch {
                vertex_buffer,
                index_buffer,
                index_count: idxs.len() as u32,
            })
        })
        .collect()
}

/// Build terrain hillshade batches grouped by the dedicated hillshade atlas.
pub fn build_hillshade_batches(
    device: &wgpu::Device,
    terrain_meshes: &[TerrainMeshData],
    hillshade_rasters: &[PreparedHillshadeRaster],
    atlas: &TileAtlas,
    camera_origin: DVec3,
) -> Vec<Option<HillshadeBatch>> {
    if atlas.page_count() == 0 || terrain_meshes.is_empty() || hillshade_rasters.is_empty() {
        return Vec::new();
    }

    let available: std::collections::HashSet<TileId> =
        hillshade_rasters.iter().map(|r| r.tile).collect();
    let page_count = atlas.page_count();
    let mut page_verts: Vec<Vec<TerrainVertex>> = vec![Vec::new(); page_count];
    let mut page_idxs: Vec<Vec<u32>> = vec![Vec::new(); page_count];

    for mesh in terrain_meshes {
        if !available.contains(&mesh.tile) {
            continue;
        }

        let region = match atlas.get(&mesh.tile) {
            Some(r) => r,
            None => continue,
        };

        let vert_count = mesh.positions.len();
        let base = page_verts[region.page].len() as u32;
        page_verts[region.page].reserve(vert_count);

        for i in 0..vert_count {
            let pos = &mesh.positions[i];
            let uv = &mesh.uvs[i];
            let normal = &mesh.normals[i];

            page_verts[region.page].push(TerrainVertex {
                position: [
                    (pos[0] - camera_origin.x) as f32,
                    (pos[1] - camera_origin.y) as f32,
                    (pos[2] - camera_origin.z) as f32,
                ],
                uv: region.remap_uv(uv[0], uv[1]),
                normal: *normal,
            });
        }

        page_idxs[region.page].reserve(mesh.indices.len());
        for idx in &mesh.indices {
            page_idxs[region.page].push(base + idx);
        }
    }

    page_verts
        .into_iter()
        .zip(page_idxs)
        .map(|(verts, idxs)| {
            if verts.is_empty() {
                return None;
            }
            let vertex_buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
                label: Some("hillshade_batch_vb"),
                contents: bytemuck::cast_slice(&verts),
                usage: wgpu::BufferUsages::VERTEX,
            });
            let index_buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
                label: Some("hillshade_batch_ib"),
                contents: bytemuck::cast_slice(&idxs),
                usage: wgpu::BufferUsages::INDEX,
            });
            Some(HillshadeBatch {
                vertex_buffer,
                index_buffer,
                index_count: idxs.len() as u32,
            })
        })
        .collect()
}

// ---------------------------------------------------------------------------
// VectorBatchEntry
/// GPU buffers for a single vector layer's tessellated mesh.
///
/// Produced by [`build_vector_batch`] and consumed by
/// [`WgpuMapRenderer::render_vector_batches`](crate::WgpuMapRenderer).
///
/// Vector geometry is not textured (each vertex carries its own RGBA
/// colour), so there is no atlas grouping -- one entry per layer.
pub struct VectorBatchEntry {
    /// Vertex buffer ([`VectorVertex`] with position + colour).
    pub vertex_buffer: wgpu::Buffer,
    /// Index buffer (`u32` indices).
    pub index_buffer: wgpu::Buffer,
    /// Total number of indices to draw.
    pub index_count: u32,
}

/// Build a vector batch entry from engine-tessellated mesh data.
///
/// Returns `None` if the mesh has no indices (nothing to draw).
///
/// # Arguments
///
/// * `device`        -- WGPU device for buffer allocation.
/// * `mesh`          -- Tessellated vector geometry from the engine.
/// * `camera_origin` -- Subtracted from positions for camera-relative f32.
///
/// # Panics (debug only)
///
/// Debug-asserts that `positions.len() == colors.len()`.  A mismatch would
/// cause the `zip` to silently truncate vertices while indices still
/// reference the original (larger) range, producing GPU-side out-of-bounds
/// reads.
pub fn build_vector_batch(
    device: &wgpu::Device,
    mesh: &VectorMeshData,
    camera_origin: DVec3,
) -> Option<VectorBatchEntry> {
    if mesh.indices.is_empty() {
        return None;
    }

    debug_assert_eq!(
        mesh.positions.len(),
        mesh.colors.len(),
        "VectorMeshData positions/colors length mismatch ({} vs {})",
        mesh.positions.len(),
        mesh.colors.len(),
    );

    let vertices: Vec<VectorVertex> = mesh
        .positions
        .iter()
        .zip(mesh.colors.iter())
        .map(|(pos, color)| VectorVertex {
            position: [
                (pos[0] - camera_origin.x) as f32,
                (pos[1] - camera_origin.y) as f32,
                (pos[2] - camera_origin.z) as f32,
            ],
            color: *color,
        })
        .collect();

    let vertex_buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
        label: Some("vector_batch_vb"),
        contents: bytemuck::cast_slice(&vertices),
        usage: wgpu::BufferUsages::VERTEX,
    });
    let index_buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
        label: Some("vector_batch_ib"),
        contents: bytemuck::cast_slice(&mesh.indices),
        usage: wgpu::BufferUsages::INDEX,
    });

    Some(VectorBatchEntry {
        vertex_buffer,
        index_buffer,
        index_count: mesh.indices.len() as u32,
    })
}

// ---------------------------------------------------------------------------
// Fill-extrusion batches
// ---------------------------------------------------------------------------

/// GPU buffers for a single fill-extrusion layer's tessellated mesh.
///
/// Like [`VectorBatchEntry`] but with per-vertex normals for lighting.
pub struct FillExtrusionBatchEntry {
    /// Vertex buffer ([`FillExtrusionVertex`](super::fill_extrusion_vertex::FillExtrusionVertex)).
    pub vertex_buffer: wgpu::Buffer,
    /// Index buffer (`u32` indices).
    pub index_buffer: wgpu::Buffer,
    /// Total number of indices to draw.
    pub index_count: u32,
}

/// Build a fill-extrusion batch entry from engine-tessellated mesh data.
///
/// Returns `None` if the mesh has no indices or no normals.
pub fn build_fill_extrusion_batch(
    device: &wgpu::Device,
    mesh: &VectorMeshData,
    camera_origin: DVec3,
) -> Option<FillExtrusionBatchEntry> {
    use crate::gpu::fill_extrusion_vertex::FillExtrusionVertex;

    if mesh.indices.is_empty() || mesh.normals.is_empty() {
        return None;
    }

    debug_assert_eq!(
        mesh.positions.len(),
        mesh.normals.len(),
        "FillExtrusion positions/normals length mismatch ({} vs {})",
        mesh.positions.len(),
        mesh.normals.len(),
    );

    let vertices: Vec<FillExtrusionVertex> = mesh
        .positions
        .iter()
        .zip(mesh.normals.iter())
        .zip(mesh.colors.iter())
        .map(|((pos, normal), color)| FillExtrusionVertex {
            position: [
                (pos[0] - camera_origin.x) as f32,
                (pos[1] - camera_origin.y) as f32,
                (pos[2] - camera_origin.z) as f32,
            ],
            normal: *normal,
            color: *color,
        })
        .collect();

    let vertex_buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
        label: Some("fill_extrusion_batch_vb"),
        contents: bytemuck::cast_slice(&vertices),
        usage: wgpu::BufferUsages::VERTEX,
    });
    let index_buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
        label: Some("fill_extrusion_batch_ib"),
        contents: bytemuck::cast_slice(&mesh.indices),
        usage: wgpu::BufferUsages::INDEX,
    });

    Some(FillExtrusionBatchEntry {
        vertex_buffer,
        index_buffer,
        index_count: mesh.indices.len() as u32,
    })
}

// ---------------------------------------------------------------------------
// Fill batches
// ---------------------------------------------------------------------------

/// GPU buffers for a dedicated fill layer.
pub struct FillBatchEntry {
    /// Vertex buffer ([`FillVertex`]).
    pub vertex_buffer: wgpu::Buffer,
    /// Index buffer (`u32` indices).
    pub index_buffer: wgpu::Buffer,
    /// Total number of indices to draw.
    pub index_count: u32,
    /// Fill-specific parameter buffer for the fill-params uniform.
    pub fill_params_buffer: wgpu::Buffer,
    /// Bind group that holds view-proj (binding 0) + fill params (binding 1).
    pub bind_group: wgpu::BindGroup,
}

/// Build a fill batch entry from engine-tessellated mesh data.
///
/// Returns `None` if the mesh has no indices.
pub fn build_fill_batch(
    device: &wgpu::Device,
    mesh: &VectorMeshData,
    camera_origin: DVec3,
    uniform_buffer: &wgpu::Buffer,
    fill_bind_group_layout: &wgpu::BindGroupLayout,
) -> Option<FillBatchEntry> {
    use crate::gpu::fill_vertex::FillVertex;
    use crate::pipeline::fill_pipeline::FillParamsUniform;

    if mesh.indices.is_empty() {
        return None;
    }

    let vertices: Vec<FillVertex> = mesh
        .positions
        .iter()
        .zip(mesh.colors.iter())
        .map(|(pos, color)| FillVertex {
            position: [
                (pos[0] - camera_origin.x) as f32,
                (pos[1] - camera_origin.y) as f32,
                (pos[2] - camera_origin.z) as f32,
            ],
            color: *color,
        })
        .collect();

    let vertex_buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
        label: Some("fill_batch_vb"),
        contents: bytemuck::cast_slice(&vertices),
        usage: wgpu::BufferUsages::VERTEX,
    });
    let index_buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
        label: Some("fill_batch_ib"),
        contents: bytemuck::cast_slice(&mesh.indices),
        usage: wgpu::BufferUsages::INDEX,
    });

    let fill_params = FillParamsUniform {
        fill_translate: mesh.fill_translate,
        fill_opacity: mesh.fill_opacity,
        fill_antialias: if mesh.fill_antialias { 1.0 } else { 0.0 },
        outline_color: mesh.fill_outline_color,
    };
    let fill_params_buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
        label: Some("fill_params_buf"),
        contents: bytemuck::bytes_of(&fill_params),
        usage: wgpu::BufferUsages::UNIFORM | wgpu::BufferUsages::COPY_DST,
    });

    let bind_group = device.create_bind_group(&wgpu::BindGroupDescriptor {
        label: Some("fill_batch_bg"),
        layout: fill_bind_group_layout,
        entries: &[
            wgpu::BindGroupEntry {
                binding: 0,
                resource: uniform_buffer.as_entire_binding(),
            },
            wgpu::BindGroupEntry {
                binding: 1,
                resource: fill_params_buffer.as_entire_binding(),
            },
        ],
    });

    Some(FillBatchEntry {
        vertex_buffer,
        index_buffer,
        index_count: mesh.indices.len() as u32,
        fill_params_buffer,
        bind_group,
    })
}

// ---------------------------------------------------------------------------
// Fill-pattern batches
// ---------------------------------------------------------------------------

/// GPU buffers for a single patterned fill layer.
pub struct FillPatternBatchEntry {
    /// Vertex buffer ([`FillPatternVertex`]).
    pub vertex_buffer: wgpu::Buffer,
    /// Index buffer (`u32` indices).
    pub index_buffer: wgpu::Buffer,
    /// Total number of indices to draw.
    pub index_count: u32,
    /// Bind group 0: view-proj (binding 0) + fill params (binding 1).
    pub uniform_bind_group: wgpu::BindGroup,
    /// Bind group 1: pattern texture (binding 0) + sampler (binding 1).
    pub texture_bind_group: wgpu::BindGroup,
    /// Retained pattern texture (must outlive the bind group).
    pub _pattern_texture: wgpu::Texture,
    /// Retained pattern texture view.
    pub _pattern_texture_view: wgpu::TextureView,
    /// Retained fill-params buffer.
    pub _fill_params_buffer: wgpu::Buffer,
}

/// Build a fill-pattern batch entry from engine mesh data.
///
/// Returns `None` if the mesh has no indices, no pattern UVs, or no pattern
/// image.
#[allow(clippy::too_many_arguments)]
pub fn build_fill_pattern_batch(
    device: &wgpu::Device,
    queue: &wgpu::Queue,
    mesh: &VectorMeshData,
    camera_origin: DVec3,
    uniform_buffer: &wgpu::Buffer,
    uniform_bgl: &wgpu::BindGroupLayout,
    texture_bgl: &wgpu::BindGroupLayout,
    pattern_sampler: &wgpu::Sampler,
) -> Option<FillPatternBatchEntry> {
    use crate::gpu::fill_pattern_vertex::FillPatternVertex;
    use crate::pipeline::fill_pipeline::FillParamsUniform;

    let pattern = mesh.fill_pattern.as_ref()?;
    if mesh.indices.is_empty() || mesh.fill_pattern_uvs.is_empty() {
        return None;
    }

    let vertices: Vec<FillPatternVertex> = (0..mesh.positions.len())
        .map(|i| {
            let pos = &mesh.positions[i];
            FillPatternVertex {
                position: [
                    (pos[0] - camera_origin.x) as f32,
                    (pos[1] - camera_origin.y) as f32,
                    (pos[2] - camera_origin.z) as f32,
                ],
                color: mesh.colors[i],
                uv: if i < mesh.fill_pattern_uvs.len() {
                    mesh.fill_pattern_uvs[i]
                } else {
                    [0.0, 0.0]
                },
            }
        })
        .collect();

    let vertex_buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
        label: Some("fill_pattern_batch_vb"),
        contents: bytemuck::cast_slice(&vertices),
        usage: wgpu::BufferUsages::VERTEX,
    });
    let index_buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
        label: Some("fill_pattern_batch_ib"),
        contents: bytemuck::cast_slice(&mesh.indices),
        usage: wgpu::BufferUsages::INDEX,
    });

    // Upload pattern image to GPU texture.
    let texture_size = wgpu::Extent3d {
        width: pattern.width,
        height: pattern.height,
        depth_or_array_layers: 1,
    };
    let pattern_texture = device.create_texture(&wgpu::TextureDescriptor {
        label: Some("fill_pattern_tex"),
        size: texture_size,
        mip_level_count: 1,
        sample_count: 1,
        dimension: wgpu::TextureDimension::D2,
        format: wgpu::TextureFormat::Rgba8UnormSrgb,
        usage: wgpu::TextureUsages::TEXTURE_BINDING | wgpu::TextureUsages::COPY_DST,
        view_formats: &[],
    });

    queue.write_texture(
        wgpu::TexelCopyTextureInfo {
            texture: &pattern_texture,
            mip_level: 0,
            origin: wgpu::Origin3d::ZERO,
            aspect: wgpu::TextureAspect::All,
        },
        &pattern.data,
        wgpu::TexelCopyBufferLayout {
            offset: 0,
            bytes_per_row: Some(pattern.width * 4),
            rows_per_image: Some(pattern.height),
        },
        texture_size,
    );

    let pattern_texture_view = pattern_texture.create_view(&wgpu::TextureViewDescriptor::default());

    // Fill params uniform.
    let fill_params = FillParamsUniform {
        fill_translate: mesh.fill_translate,
        fill_opacity: mesh.fill_opacity,
        fill_antialias: if mesh.fill_antialias { 1.0 } else { 0.0 },
        outline_color: mesh.fill_outline_color,
    };
    let fill_params_buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
        label: Some("fill_pattern_params_buf"),
        contents: bytemuck::bytes_of(&fill_params),
        usage: wgpu::BufferUsages::UNIFORM | wgpu::BufferUsages::COPY_DST,
    });

    // Bind group 0: uniforms.
    let uniform_bind_group = device.create_bind_group(&wgpu::BindGroupDescriptor {
        label: Some("fill_pattern_uniform_bg"),
        layout: uniform_bgl,
        entries: &[
            wgpu::BindGroupEntry {
                binding: 0,
                resource: uniform_buffer.as_entire_binding(),
            },
            wgpu::BindGroupEntry {
                binding: 1,
                resource: fill_params_buffer.as_entire_binding(),
            },
        ],
    });

    // Bind group 1: texture + sampler.
    let texture_bind_group = device.create_bind_group(&wgpu::BindGroupDescriptor {
        label: Some("fill_pattern_texture_bg"),
        layout: texture_bgl,
        entries: &[
            wgpu::BindGroupEntry {
                binding: 0,
                resource: wgpu::BindingResource::TextureView(&pattern_texture_view),
            },
            wgpu::BindGroupEntry {
                binding: 1,
                resource: wgpu::BindingResource::Sampler(pattern_sampler),
            },
        ],
    });

    Some(FillPatternBatchEntry {
        vertex_buffer,
        index_buffer,
        index_count: mesh.indices.len() as u32,
        uniform_bind_group,
        texture_bind_group,
        _pattern_texture: pattern_texture,
        _pattern_texture_view: pattern_texture_view,
        _fill_params_buffer: fill_params_buffer,
    })
}

// ---------------------------------------------------------------------------
// Line batches
// ---------------------------------------------------------------------------

/// GPU buffers for a single line layer's tessellated mesh with dash data.
pub struct LineBatchEntry {
    /// Vertex buffer ([`LineVertex`]).
    pub vertex_buffer: wgpu::Buffer,
    /// Index buffer (`u32` indices).
    pub index_buffer: wgpu::Buffer,
    /// Total number of indices to draw.
    pub index_count: u32,
    /// Line style parameters `[dash_length, gap_length, cap_round, 0]`.
    pub line_params: [f32; 4],
}

/// Build a line batch entry from engine-tessellated mesh data.
///
/// Returns `None` if the mesh has no indices or no line distances.
pub fn build_line_batch(
    device: &wgpu::Device,
    mesh: &VectorMeshData,
    camera_origin: DVec3,
) -> Option<LineBatchEntry> {
    use crate::gpu::line_vertex::LineVertex;

    if mesh.indices.is_empty() || mesh.line_distances.is_empty() {
        return None;
    }

    let vertices: Vec<LineVertex> = (0..mesh.positions.len())
        .map(|i| {
            let pos = &mesh.positions[i];
            LineVertex {
                position: [
                    (pos[0] - camera_origin.x) as f32,
                    (pos[1] - camera_origin.y) as f32,
                    (pos[2] - camera_origin.z) as f32,
                ],
                color: mesh.colors[i],
                line_normal: mesh.line_normals[i],
                line_distance: mesh.line_distances[i],
                cap_join: if i < mesh.line_cap_joins.len() {
                    mesh.line_cap_joins[i]
                } else {
                    0.0
                },
            }
        })
        .collect();

    let vertex_buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
        label: Some("line_batch_vb"),
        contents: bytemuck::cast_slice(&vertices),
        usage: wgpu::BufferUsages::VERTEX,
    });
    let index_buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
        label: Some("line_batch_ib"),
        contents: bytemuck::cast_slice(&mesh.indices),
        usage: wgpu::BufferUsages::INDEX,
    });

    Some(LineBatchEntry {
        vertex_buffer,
        index_buffer,
        index_count: mesh.indices.len() as u32,
        line_params: mesh.line_params,
    })
}

// ---------------------------------------------------------------------------
// Line-pattern batches
// ---------------------------------------------------------------------------

/// GPU buffers for a single patterned line layer.
pub struct LinePatternBatchEntry {
    /// Vertex buffer ([`LinePatternVertex`]).
    pub vertex_buffer: wgpu::Buffer,
    /// Index buffer (`u32` indices).
    pub index_buffer: wgpu::Buffer,
    /// Total number of indices to draw.
    pub index_count: u32,
    /// Bind group 0: view-proj + line-style uniforms.
    pub uniform_bind_group: wgpu::BindGroup,
    /// Bind group 1: pattern texture (binding 0) + sampler (binding 1).
    pub texture_bind_group: wgpu::BindGroup,
    /// Retained pattern texture (must outlive the bind group).
    pub _pattern_texture: wgpu::Texture,
    /// Retained pattern texture view.
    pub _pattern_texture_view: wgpu::TextureView,
    /// Line style parameters `[dash_length, gap_length, cap_round, 0]`.
    pub line_params: [f32; 4],
}

/// Build a line-pattern batch entry from engine mesh data.
///
/// Returns `None` if the mesh has no indices, no pattern UVs, or no pattern
/// image.
#[allow(clippy::too_many_arguments)]
pub fn build_line_pattern_batch(
    device: &wgpu::Device,
    queue: &wgpu::Queue,
    mesh: &VectorMeshData,
    camera_origin: DVec3,
    uniform_buffer: &wgpu::Buffer,
    uniform_bgl: &wgpu::BindGroupLayout,
    texture_bgl: &wgpu::BindGroupLayout,
    pattern_sampler: &wgpu::Sampler,
) -> Option<LinePatternBatchEntry> {
    use crate::gpu::line_pattern_vertex::LinePatternVertex;

    let pattern = mesh.line_pattern.as_ref()?;
    if mesh.indices.is_empty() || mesh.line_pattern_uvs.is_empty() {
        return None;
    }

    let vertices: Vec<LinePatternVertex> = (0..mesh.positions.len())
        .map(|i| {
            let pos = &mesh.positions[i];
            LinePatternVertex {
                position: [
                    (pos[0] - camera_origin.x) as f32,
                    (pos[1] - camera_origin.y) as f32,
                    (pos[2] - camera_origin.z) as f32,
                ],
                color: mesh.colors[i],
                line_normal: if i < mesh.line_normals.len() {
                    mesh.line_normals[i]
                } else {
                    [0.0, 0.0]
                },
                line_distance: if i < mesh.line_distances.len() {
                    mesh.line_distances[i]
                } else {
                    0.0
                },
                cap_join: if i < mesh.line_cap_joins.len() {
                    mesh.line_cap_joins[i]
                } else {
                    0.0
                },
                uv: if i < mesh.line_pattern_uvs.len() {
                    mesh.line_pattern_uvs[i]
                } else {
                    [0.0, 0.0]
                },
            }
        })
        .collect();

    let vertex_buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
        label: Some("line_pattern_batch_vb"),
        contents: bytemuck::cast_slice(&vertices),
        usage: wgpu::BufferUsages::VERTEX,
    });
    let index_buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
        label: Some("line_pattern_batch_ib"),
        contents: bytemuck::cast_slice(&mesh.indices),
        usage: wgpu::BufferUsages::INDEX,
    });

    // Upload pattern image to GPU texture.
    let texture_size = wgpu::Extent3d {
        width: pattern.width,
        height: pattern.height,
        depth_or_array_layers: 1,
    };
    let pattern_texture = device.create_texture(&wgpu::TextureDescriptor {
        label: Some("line_pattern_tex"),
        size: texture_size,
        mip_level_count: 1,
        sample_count: 1,
        dimension: wgpu::TextureDimension::D2,
        format: wgpu::TextureFormat::Rgba8UnormSrgb,
        usage: wgpu::TextureUsages::TEXTURE_BINDING | wgpu::TextureUsages::COPY_DST,
        view_formats: &[],
    });

    queue.write_texture(
        wgpu::TexelCopyTextureInfo {
            texture: &pattern_texture,
            mip_level: 0,
            origin: wgpu::Origin3d::ZERO,
            aspect: wgpu::TextureAspect::All,
        },
        &pattern.data,
        wgpu::TexelCopyBufferLayout {
            offset: 0,
            bytes_per_row: Some(pattern.width * 4),
            rows_per_image: Some(pattern.height),
        },
        texture_size,
    );

    let pattern_texture_view = pattern_texture.create_view(&wgpu::TextureViewDescriptor::default());

    // Bind group 0: uniforms (shared view-proj + line-style).
    let uniform_bind_group = device.create_bind_group(&wgpu::BindGroupDescriptor {
        label: Some("line_pattern_uniform_bg"),
        layout: uniform_bgl,
        entries: &[wgpu::BindGroupEntry {
            binding: 0,
            resource: uniform_buffer.as_entire_binding(),
        }],
    });

    // Bind group 1: texture + sampler.
    let texture_bind_group = device.create_bind_group(&wgpu::BindGroupDescriptor {
        label: Some("line_pattern_texture_bg"),
        layout: texture_bgl,
        entries: &[
            wgpu::BindGroupEntry {
                binding: 0,
                resource: wgpu::BindingResource::TextureView(&pattern_texture_view),
            },
            wgpu::BindGroupEntry {
                binding: 1,
                resource: wgpu::BindingResource::Sampler(pattern_sampler),
            },
        ],
    });

    Some(LinePatternBatchEntry {
        vertex_buffer,
        index_buffer,
        index_count: mesh.indices.len() as u32,
        uniform_bind_group,
        texture_bind_group,
        _pattern_texture: pattern_texture,
        _pattern_texture_view: pattern_texture_view,
        line_params: mesh.line_params,
    })
}

// ---------------------------------------------------------------------------
// Circle batches
// ---------------------------------------------------------------------------

/// GPU buffers for a circle layer rendered via SDF quads.
pub struct CircleBatchEntry {
    /// Vertex buffer ([`CircleVertex`]).
    pub vertex_buffer: wgpu::Buffer,
    /// Index buffer (`u32` indices).
    pub index_buffer: wgpu::Buffer,
    /// Total number of indices to draw.
    pub index_count: u32,
}

/// Build a circle batch from engine circle instance data.
///
/// Each circle becomes a screen-aligned quad (4 vertices, 6 indices).
/// Returns `None` if there are no circle instances.
pub fn build_circle_batch(
    device: &wgpu::Device,
    mesh: &VectorMeshData,
    camera_origin: DVec3,
) -> Option<CircleBatchEntry> {
    use crate::gpu::circle_vertex::CircleVertex;

    if mesh.circle_instances.is_empty() {
        return None;
    }

    let instance_count = mesh.circle_instances.len();
    let mut vertices: Vec<CircleVertex> = Vec::with_capacity(instance_count * 4);
    let mut indices: Vec<u32> = Vec::with_capacity(instance_count * 6);

    let offsets: [[f32; 2]; 4] = [[-1.0, 1.0], [1.0, 1.0], [-1.0, -1.0], [1.0, -1.0]];

    for (i, ci) in mesh.circle_instances.iter().enumerate() {
        let cx = (ci.center[0] - camera_origin.x) as f32;
        let cy = (ci.center[1] - camera_origin.y) as f32;
        let cz = (ci.center[2] - camera_origin.z) as f32;
        let params = [ci.radius, ci.stroke_width, ci.blur, 0.0];

        for offset in &offsets {
            vertices.push(CircleVertex {
                position: [cx, cy, cz],
                quad_offset: *offset,
                color: ci.color,
                stroke_color: ci.stroke_color,
                params,
            });
        }

        let base = (i as u32) * 4;
        indices.extend_from_slice(&[base, base + 2, base + 1, base + 1, base + 2, base + 3]);
    }

    let vertex_buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
        label: Some("circle_batch_vb"),
        contents: bytemuck::cast_slice(&vertices),
        usage: wgpu::BufferUsages::VERTEX,
    });
    let index_buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
        label: Some("circle_batch_ib"),
        contents: bytemuck::cast_slice(&indices),
        usage: wgpu::BufferUsages::INDEX,
    });

    Some(CircleBatchEntry {
        vertex_buffer,
        index_buffer,
        index_count: indices.len() as u32,
    })
}

// ---------------------------------------------------------------------------
// Heatmap batches
// ---------------------------------------------------------------------------

/// GPU buffers for a heatmap layer.
pub struct HeatmapBatchEntry {
    /// Vertex buffer ([`HeatmapVertex`]).
    pub vertex_buffer: wgpu::Buffer,
    /// Index buffer (`u32` indices).
    pub index_buffer: wgpu::Buffer,
    /// Total number of indices to draw.
    pub index_count: u32,
}

/// Build a heatmap batch from engine heatmap point data.
///
/// Each point becomes a screen-aligned quad.  Returns `None` if there
/// are no heatmap points.
pub fn build_heatmap_batch(
    device: &wgpu::Device,
    mesh: &VectorMeshData,
    camera_origin: DVec3,
) -> Option<HeatmapBatchEntry> {
    use crate::gpu::heatmap_vertex::HeatmapVertex;

    if mesh.heatmap_points.is_empty() {
        return None;
    }

    let point_count = mesh.heatmap_points.len();
    let mut vertices: Vec<HeatmapVertex> = Vec::with_capacity(point_count * 4);
    let mut indices: Vec<u32> = Vec::with_capacity(point_count * 6);

    let offsets: [[f32; 2]; 4] = [[-1.0, 1.0], [1.0, 1.0], [-1.0, -1.0], [1.0, -1.0]];

    for (i, pt) in mesh.heatmap_points.iter().enumerate() {
        let cx = (pt[0] - camera_origin.x) as f32;
        let cy = (pt[1] - camera_origin.y) as f32;
        let cz = 0.0f32;
        let weight = pt[2] as f32;
        let radius = pt[3] as f32;
        let params = [weight, radius, mesh.heatmap_intensity, 0.0];

        for offset in &offsets {
            vertices.push(HeatmapVertex {
                position: [cx, cy, cz],
                quad_offset: *offset,
                params,
            });
        }

        let base = (i as u32) * 4;
        indices.extend_from_slice(&[base, base + 2, base + 1, base + 1, base + 2, base + 3]);
    }

    let vertex_buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
        label: Some("heatmap_batch_vb"),
        contents: bytemuck::cast_slice(&vertices),
        usage: wgpu::BufferUsages::VERTEX,
    });
    let index_buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
        label: Some("heatmap_batch_ib"),
        contents: bytemuck::cast_slice(&indices),
        usage: wgpu::BufferUsages::INDEX,
    });

    Some(HeatmapBatchEntry {
        vertex_buffer,
        index_buffer,
        index_count: indices.len() as u32,
    })
}

// ---------------------------------------------------------------------------
// Placeholder quad batches
// ---------------------------------------------------------------------------

/// Build a single merged [`VectorBatchEntry`] containing quads for all
/// loading placeholders.
///
/// Each placeholder becomes a flat coloured quad (4 vertices, 6 indices)
/// at Z = -0.01 (slightly behind the tile plane) so that loaded tiles
/// naturally occlude them via depth test.  The colour includes a shimmer
/// opacity multiplier derived from the placeholder's animation phase and
/// the active [`PlaceholderStyle`](rustial_engine::PlaceholderStyle).
///
/// Returns `None` when `placeholders` is empty.
pub fn build_placeholder_batches(
    device: &wgpu::Device,
    placeholders: &[rustial_engine::LoadingPlaceholder],
    style: &rustial_engine::PlaceholderStyle,
    camera_origin: DVec3,
) -> Option<VectorBatchEntry> {
    if placeholders.is_empty() {
        return None;
    }

    let quad_count = placeholders.len();
    let mut vertices: Vec<VectorVertex> = Vec::with_capacity(quad_count * 4);
    let mut indices: Vec<u32> = Vec::with_capacity(quad_count * 6);

    for ph in placeholders {
        let opacity = style.shimmer_opacity(ph.animation_phase);
        let color = [
            style.background_color[0],
            style.background_color[1],
            style.background_color[2],
            style.background_color[3] * opacity,
        ];

        let min = ph.bounds.min.position;
        let max = ph.bounds.max.position;
        let ox = camera_origin.x;
        let oy = camera_origin.y;
        // Place slightly behind the tile plane so loaded tiles occlude.
        let z: f32 = -0.01;

        let base = vertices.len() as u32;
        // v0 = SW, v1 = SE, v2 = NE, v3 = NW  (CCW front face)
        vertices.push(VectorVertex {
            position: [(min.x - ox) as f32, (min.y - oy) as f32, z],
            color,
        });
        vertices.push(VectorVertex {
            position: [(max.x - ox) as f32, (min.y - oy) as f32, z],
            color,
        });
        vertices.push(VectorVertex {
            position: [(max.x - ox) as f32, (max.y - oy) as f32, z],
            color,
        });
        vertices.push(VectorVertex {
            position: [(min.x - ox) as f32, (max.y - oy) as f32, z],
            color,
        });
        indices.extend_from_slice(&[base, base + 1, base + 2, base, base + 2, base + 3]);
    }

    let vertex_buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
        label: Some("placeholder_batch_vb"),
        contents: bytemuck::cast_slice(&vertices),
        usage: wgpu::BufferUsages::VERTEX,
    });
    let index_buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
        label: Some("placeholder_batch_ib"),
        contents: bytemuck::cast_slice(&indices),
        usage: wgpu::BufferUsages::INDEX,
    });

    Some(VectorBatchEntry {
        vertex_buffer,
        index_buffer,
        index_count: indices.len() as u32,
    })
}

// ---------------------------------------------------------------------------
// Symbol batches
// ---------------------------------------------------------------------------

/// Prepared GPU buffers for one symbol batch.
pub struct SymbolBatchEntry {
    /// Vertex buffer ([`SymbolVertex`]).
    pub vertex_buffer: wgpu::Buffer,
    /// Index buffer (`u32` indices).
    pub index_buffer: wgpu::Buffer,
    /// Total number of indices to draw.
    pub index_count: u32,
}

/// Build a symbol batch from placed symbols and a populated glyph atlas.
///
/// When `PlacedSymbol::glyph_quads` is populated (via
/// [`layout_symbol_glyphs`](rustial_engine::symbols::layout_symbol_glyphs))
/// the pre-computed glyph positions are used directly.  Otherwise the
/// function falls back to a naive character-by-character left-to-right
/// layout using atlas advance metrics.
pub fn build_symbol_batch(
    device: &wgpu::Device,
    symbols: &[rustial_engine::symbols::PlacedSymbol],
    atlas: &rustial_engine::symbols::GlyphAtlas,
    camera_origin: DVec3,
    render_em_px: f32,
) -> Option<SymbolBatchEntry> {
    use crate::gpu::symbol_vertex::SymbolVertex;

    let atlas_dims = atlas.dimensions();
    if atlas_dims[0] == 0 || atlas_dims[1] == 0 {
        return None;
    }

    let atlas_w = atlas_dims[0] as f32;
    let atlas_h = atlas_dims[1] as f32;

    let mut vertices: Vec<SymbolVertex> = Vec::new();
    let mut indices: Vec<u32> = Vec::new();

    for symbol in symbols {
        if !symbol.visible || symbol.opacity <= 0.0 {
            continue;
        }
        if symbol.text.as_ref().is_none_or(|t| t.is_empty()) {
            continue;
        }

        // World-space anchor, camera-relative.
        let ax = (symbol.world_anchor[0] - camera_origin.x) as f32;
        let ay = (symbol.world_anchor[1] - camera_origin.y) as f32;
        let az = (symbol.world_anchor[2] - camera_origin.z) as f32;

        let scale = symbol.size_px / render_em_px.max(1.0);

        let color = [
            symbol.fill_color[0],
            symbol.fill_color[1],
            symbol.fill_color[2],
            symbol.fill_color[3] * symbol.opacity,
        ];
        let halo_color = symbol.halo_color;
        let params = [1.0_f32, 1.0, 0.0, 0.0];

        if !symbol.glyph_quads.is_empty() {
            // ---- Pre-computed glyph layout path ----
            for quad in &symbol.glyph_quads {
                let entry = match atlas.get(&symbol.font_stack, quad.codepoint) {
                    Some(e) => e,
                    None => continue,
                };

                let gw = entry.size[0] as f32 * scale;
                let gh = entry.size[1] as f32 * scale;
                let bearing_x = entry.bearing_x as f32 * scale;
                let bearing_y = entry.bearing_y as f32 * scale;

                // Glyph quad corners: quad.x/y are the pen position (pixels);
                // bearing adjusts to the glyph bitmap origin.
                let x0 = quad.x + bearing_x;
                let y0 = quad.y + bearing_y;
                let x1 = x0 + gw;
                let y1 = y0 - gh;

                let u0 = entry.origin[0] as f32 / atlas_w;
                let v0 = entry.origin[1] as f32 / atlas_h;
                let u1 = (entry.origin[0] + entry.size[0]) as f32 / atlas_w;
                let v1 = (entry.origin[1] + entry.size[1]) as f32 / atlas_h;

                let base = vertices.len() as u32;
                vertices.push(SymbolVertex {
                    position: [ax, ay, az],
                    glyph_offset: [x0, y0],
                    tex_coord: [u0, v0],
                    color,
                    halo_color,
                    params,
                });
                vertices.push(SymbolVertex {
                    position: [ax, ay, az],
                    glyph_offset: [x1, y0],
                    tex_coord: [u1, v0],
                    color,
                    halo_color,
                    params,
                });
                vertices.push(SymbolVertex {
                    position: [ax, ay, az],
                    glyph_offset: [x0, y1],
                    tex_coord: [u0, v1],
                    color,
                    halo_color,
                    params,
                });
                vertices.push(SymbolVertex {
                    position: [ax, ay, az],
                    glyph_offset: [x1, y1],
                    tex_coord: [u1, v1],
                    color,
                    halo_color,
                    params,
                });
                indices.extend_from_slice(&[
                    base,
                    base + 2,
                    base + 1,
                    base + 1,
                    base + 2,
                    base + 3,
                ]);
            }
        } else {
            // ---- Fallback: naive character-by-character layout ----
            let text = symbol.text.as_deref().unwrap_or("");
            let mut cursor_x: f32 = 0.0;
            for codepoint in text.chars() {
                let entry = match atlas.get(&symbol.font_stack, codepoint) {
                    Some(e) => e,
                    None => continue,
                };

                let gw = entry.size[0] as f32 * scale;
                let gh = entry.size[1] as f32 * scale;
                let bearing_x = entry.bearing_x as f32 * scale;
                let bearing_y = entry.bearing_y as f32 * scale;

                let x0 = cursor_x + bearing_x;
                let y0 = bearing_y;
                let x1 = x0 + gw;
                let y1 = y0 - gh;

                let u0 = entry.origin[0] as f32 / atlas_w;
                let v0 = entry.origin[1] as f32 / atlas_h;
                let u1 = (entry.origin[0] + entry.size[0]) as f32 / atlas_w;
                let v1 = (entry.origin[1] + entry.size[1]) as f32 / atlas_h;

                let base = vertices.len() as u32;
                vertices.push(SymbolVertex {
                    position: [ax, ay, az],
                    glyph_offset: [x0, y0],
                    tex_coord: [u0, v0],
                    color,
                    halo_color,
                    params,
                });
                vertices.push(SymbolVertex {
                    position: [ax, ay, az],
                    glyph_offset: [x1, y0],
                    tex_coord: [u1, v0],
                    color,
                    halo_color,
                    params,
                });
                vertices.push(SymbolVertex {
                    position: [ax, ay, az],
                    glyph_offset: [x0, y1],
                    tex_coord: [u0, v1],
                    color,
                    halo_color,
                    params,
                });
                vertices.push(SymbolVertex {
                    position: [ax, ay, az],
                    glyph_offset: [x1, y1],
                    tex_coord: [u1, v1],
                    color,
                    halo_color,
                    params,
                });
                indices.extend_from_slice(&[
                    base,
                    base + 2,
                    base + 1,
                    base + 1,
                    base + 2,
                    base + 3,
                ]);

                cursor_x += entry.advance_x * scale;
            }
        }
    }

    if vertices.is_empty() {
        return None;
    }

    let vertex_buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
        label: Some("symbol_batch_vb"),
        contents: bytemuck::cast_slice(&vertices),
        usage: wgpu::BufferUsages::VERTEX,
    });
    let index_buffer = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
        label: Some("symbol_batch_ib"),
        contents: bytemuck::cast_slice(&indices),
        usage: wgpu::BufferUsages::INDEX,
    });

    Some(SymbolBatchEntry {
        vertex_buffer,
        index_buffer,
        index_count: indices.len() as u32,
    })
}

// ---------------------------------------------------------------------------
// Tests
// ---------------------------------------------------------------------------

#[cfg(test)]
mod tests {
    use super::*;
    use crate::gpu::tile_atlas::TileAtlas;
    use rustial_engine::{CameraProjection, DecodedImage};
    use std::sync::Arc;

    fn create_test_device() -> Option<wgpu::Device> {
        let instance = wgpu::Instance::new(&wgpu::InstanceDescriptor {
            backends: wgpu::Backends::all(),
            ..Default::default()
        });

        let adapter = pollster::block_on(instance.request_adapter(&wgpu::RequestAdapterOptions {
            power_preference: wgpu::PowerPreference::LowPower,
            compatible_surface: None,
            force_fallback_adapter: false,
        }))
        .ok()?;

        let (device, _) = pollster::block_on(adapter.request_device(&wgpu::DeviceDescriptor {
            label: Some("batch_test_device"),
            ..Default::default()
        }))
        .ok()?;

        Some(device)
    }

    fn test_image() -> DecodedImage {
        DecodedImage {
            width: 1,
            height: 1,
            data: Arc::new(vec![255, 255, 255, 255]),
        }
    }

    #[test]
    fn visible_tile_texture_region_matches_expected_subrect() {
        let tile = VisibleTile {
            target: TileId::new(3, 4, 2),
            actual: TileId::new(1, 1, 0),
            data: None,
            fade_opacity: 1.0,
        };

        let region = tile.texture_region();
        assert!((region.u_min - 0.0).abs() < 1e-6);
        assert!((region.v_min - 0.5).abs() < 1e-6);
        assert!((region.u_max - 0.25).abs() < 1e-6);
        assert!((region.v_max - 0.75).abs() < 1e-6);
    }

    #[test]
    fn terrain_texture_lookup_falls_back_to_visible_ancestor() {
        let target = TileId::new(4, 8, 4);
        let parent = TileId::new(3, 4, 2);
        let visible = vec![VisibleTile {
            target: parent,
            actual: parent,
            data: Some(rustial_engine::TileData::Raster(
                rustial_engine::DecodedImage {
                    width: 1,
                    height: 1,
                    data: vec![255, 255, 255, 255].into(),
                },
            )),
            fade_opacity: 1.0,
        }];

        assert_eq!(find_terrain_texture_actual(target, &visible), Some(parent));
    }

    #[test]
    fn projected_tile_corners_differ_between_planar_projections() {
        let tile = TileId::new(3, 4, 2);
        let merc = projected_tile_corners(tile, CameraProjection::WebMercator, DVec3::ZERO);
        let eq = projected_tile_corners(tile, CameraProjection::Equirectangular, DVec3::ZERO);

        assert!((merc[0].y - eq[0].y).abs() > 1.0);
    }

    #[test]
    fn exact_full_opacity_tile_uses_opaque_pass() {
        let tile = VisibleTile {
            target: TileId::new(3, 4, 2),
            actual: TileId::new(3, 4, 2),
            data: None,
            fade_opacity: 1.0,
        };

        assert!(!tile_requires_blended_pass(&tile));
    }

    #[test]
    fn fading_tile_uses_translucent_pass() {
        let tile = VisibleTile {
            target: TileId::new(3, 4, 2),
            actual: TileId::new(3, 4, 2),
            data: None,
            fade_opacity: 0.5,
        };

        assert!(tile_requires_blended_pass(&tile));
    }

    #[test]
    fn fallback_tile_uses_translucent_pass() {
        let tile = VisibleTile {
            target: TileId::new(4, 8, 4),
            actual: TileId::new(3, 4, 2),
            data: None,
            fade_opacity: 1.0,
        };

        assert!(tile_requires_blended_pass(&tile));
    }

    #[test]
    fn build_tile_batches_routes_exact_tile_to_opaque_batch() {
        let Some(device) = create_test_device() else {
            eprintln!("Skipping batch routing test: no suitable adapter/device available");
            return;
        };

        let mut atlas = TileAtlas::new();
        let tile_id = TileId::new(0, 0, 0);
        atlas.insert(&device, tile_id, &test_image());

        let visible = vec![VisibleTile {
            target: tile_id,
            actual: tile_id,
            data: None,
            fade_opacity: 1.0,
        }];

        let batches = build_tile_batches(
            &device,
            &visible,
            &atlas,
            DVec3::ZERO,
            CameraProjection::WebMercator,
        );

        assert_eq!(batches.len(), 1);
        assert!(batches[0].opaque.is_some());
        assert!(batches[0].translucent.is_none());
    }

    #[test]
    fn build_tile_batches_routes_fading_crossfade_tiles_to_translucent_batch() {
        let Some(device) = create_test_device() else {
            eprintln!(
                "Skipping translucent batch routing test: no suitable adapter/device available"
            );
            return;
        };

        let mut atlas = TileAtlas::new();
        let parent = TileId::new(0, 0, 0);
        let child = TileId::new(1, 0, 0);
        atlas.insert(&device, parent, &test_image());
        atlas.insert(&device, child, &test_image());

        let visible = vec![
            VisibleTile {
                target: child,
                actual: child,
                data: None,
                fade_opacity: 0.5,
            },
            VisibleTile {
                target: child,
                actual: parent,
                data: None,
                fade_opacity: 0.5,
            },
        ];

        let batches = build_tile_batches(
            &device,
            &visible,
            &atlas,
            DVec3::ZERO,
            CameraProjection::WebMercator,
        );

        assert_eq!(batches.len(), 1);
        assert!(batches[0].opaque.is_none());
        assert!(batches[0].translucent.is_some());
    }
}