//! Turning a level's parsed tfrags into something the renderer can draw.
//!
//! Nothing here parses bytes; `gc-formats` has done that. This is the join
//! between the format layer and the renderer: it applies LOD selection and
//! morphing for a given camera, then groups triangles by the texture they
//! sample so each texture is one draw rather than one per triangle.
//!
//! # Coordinates
//!
//! The PS2 world is **Z-up** and 1024 raw units to the world unit. Bevy is
//! Y-up and metres, so `(x, y, z) -> (x, z, -y)`. The sign flip on the third
//! axis is what keeps the level from coming out mirrored: it turns the PS2's
//! right-handed frame into Bevy's left-handed one rather than negating a
//! determinant.
//!
//! # Depth
//!
//! The game measures view-space depth, this uses distance from the camera.
//! They agree for a camera looking down -Z with no roll, which is all a
//! fly-camera needs; anything else should move the projection into
//! `gc-formats::tfrag::draw_mode`'s caller rather than being fudged here.

use std::collections::BTreeMap;

use gc_formats::level_core::{texture_table, LevelCoreHeader};
use gc_formats::texture::{base_image, Texture};
use gc_formats::tfrag::{
    parse_tfrag, tfrag_lod_link, tfrag_triangles, TfragAdGifs, TfragDrawMode, TfragGeometry,
    TfragHeader, TfragsBlock,
};
use gc_formats::tfrag_light::{LightBank, NormalTable};

/// Raw position units per world unit.
pub const UNITS_PER_UNIT: f32 = 1024.0;

/// A level's geometry, decoded and ready to draw.
pub struct LevelGeometry {
    /// Draw batches keyed by level texture index.
    pub batches: BTreeMap<usize, TriangleBatch>,
    /// One decoded image per texture actually referenced.
    pub textures: BTreeMap<usize, Texture>,
    pub patches: Vec<PatchBounds>,
    pub lod_distances: [f32; 3],
    pub lod_thresholds_raw: [i32; 3],
    /// Patches that drew nothing, and why they were skipped.
    pub stats: LevelStats,
}

#[derive(Debug, Default, Clone, Copy)]
pub struct LevelStats {
    pub patches: usize,
    pub drawn: usize,
    pub skipped_unparsed: usize,
    pub triangles: usize,
    pub vertices: usize,
    /// Vertices shaded by `LightTfrags`. Below `vertices` means the level fell
    /// back to the tfrag's stored colours, which is a different picture and
    /// should be visible in the log rather than discovered by eye.
    pub lit_vertices: usize,
}

pub struct PatchBounds {
    pub centre: [f32; 3],
    pub radius: f32,
    pub base_only: bool,
    pub mode: TfragDrawMode,
    pub depth: f32,
}

/// One draw batch: every triangle sampling a single texture.
pub struct TriangleBatch {
    pub positions: Vec<[f32; 3]>,
    pub uvs: Vec<[f32; 2]>,
    pub colors: Vec<[f32; 4]>,
    /// Surface normals, from the patch's own light records.
    pub normals: Vec<[f32; 3]>,
    /// Three vertex indices per triangle.
    pub indices: Vec<u32>,
    /// The ad-gif in force per triangle, parallel to `indices`.
    pub ad_gifs: Vec<TfragAdGifs>,
}

/// Everything a level's tfrags need from the unpacked disc.
pub struct LevelSource<'a> {
    pub core_data: &'a [u8],
    pub core_index: &'a [u8],
    pub gs_ram: &'a [u8],
    /// The level overlay, for the normal table. Optional: without it the level
    /// still builds, just without normals.
    pub overlay: Option<&'a [u8]>,
    /// The level's `gameplay` extent, for the light bank. Optional.
    pub gameplay: Option<&'a [u8]>,
}

/// Where a level's terrain sits: the centre of its patch bounding spheres and
/// a radius that contains them.
///
/// The camera has to start here rather than at the origin. Level geometry is
/// absolutely positioned, not centred, and level 0's terrain spans x 96..434
/// and y -36..472 in world units -- far enough that a camera at the origin
/// puts every patch beyond the LOD switch distance and the whole level draws
/// at its coarsest.
pub fn level_bounds(src: &LevelSource<'_>) -> anyhow::Result<([f32; 3], f32)> {
    let region = |off: usize| -> usize {
        i32::from_le_bytes(src.core_index[off..off + 4].try_into().unwrap()).max(0) as usize
    };
    let start = region(0x08);
    let end = [0x0c_usize, 0x10, 0x14]
        .iter()
        .map(|&o| region(o))
        .filter(|&v| v > 0)
        .min()
        .unwrap_or(src.core_data.len());
    let block = &src.core_data[start..end];
    let header = TfragsBlock::parse(block)?;

    let mut lo = [f32::INFINITY; 3];
    let mut hi = [f32::NEG_INFINITY; 3];
    let mut n = 0usize;
    for i in 0..header.tfrag_count as usize {
        let Ok(th) = header.header(block, i) else {
            continue;
        };
        let c = to_world([th.bsphere[0], th.bsphere[1], th.bsphere[2]]);
        let r = th.bsphere[3] / UNITS_PER_UNIT;
        for k in 0..3 {
            lo[k] = lo[k].min(c[k] - r);
            hi[k] = hi[k].max(c[k] + r);
        }
        n += 1;
    }
    if n == 0 {
        anyhow::bail!("level has no patches");
    }
    let centre = [
        (lo[0] + hi[0]) / 2.0,
        (lo[1] + hi[1]) / 2.0,
        (lo[2] + hi[2]) / 2.0,
    ];
    let radius = (0..3)
        .map(|k| ((hi[k] - lo[k]) / 2.0).powi(2))
        .sum::<f32>()
        .sqrt();
    Ok((centre, radius))
}

/// PS2 world units to Bevy metres, Z-up to Y-up.
pub fn to_world(p: [f32; 3]) -> [f32; 3] {
    [
        p[0] / UNITS_PER_UNIT,
        p[2] / UNITS_PER_UNIT,
        -p[1] / UNITS_PER_UNIT,
    ]
}

/// World position of a vertex-info entry in raw PS2 units, before the axis
/// swap.
///
/// Morphing has to happen in this space: mixing a converted parent with an
/// unconverted child silently scales the blend by 1024.
pub fn raw_position(g: &TfragGeometry, vinfo: usize) -> [f32; 3] {
    let local = g.positions[g.position_index(vinfo)];
    [
        (g.origin[0] + local[0]) as f32,
        (g.origin[1] + local[1]) as f32,
        (g.origin[2] + local[2]) as f32,
    ]
}

/// World position of a vertex-info entry, in Bevy metres.
pub fn world(g: &TfragGeometry, vinfo: usize) -> [f32; 3] {
    to_world(raw_position(g, vinfo))
}

/// Texture coordinate of a vertex-info entry, as 0..1. The on-disc field is
/// signed 1/4096 fixed point.
pub fn uv(g: &TfragGeometry, vinfo: usize) -> [f32; 2] {
    let v = g.vertex_info.get(vinfo).copied().unwrap_or_default();
    [v.s as f32 / 4096.0, v.t as f32 / 4096.0]
}

/// A vertex as the GPU wants it, after LOD morphing and collapse.
pub struct ResolvedVertex {
    pub position: [f32; 3],
    pub uv: [f32; 2],
    /// The patch's stored per-vertex colour.
    ///
    /// Indexed by *position*, not by vertex-info: several vertex-info entries
    /// can share a position, and the colour array is sized `vert_count`, which
    /// is a position count. This is the game's base colour before lighting
    /// multiplies into it, so on its own it is the unlit albedo.
    pub color: [f32; 4],
}

/// The surface normal of a vertex's position, in Bevy space.
///
/// Taken from the patch's per-vertex light record, which stores an azimuth and
/// elevation index into the overlay's sine table rather than an angle. Morphing
/// does not rotate the normal -- the game lerps positions and leaves lighting
/// to its own pass -- so this is the vertex's own normal regardless of blend.
pub fn vertex_normal(g: &TfragGeometry, vinfo: usize, table: Option<&NormalTable>) -> [f32; 3] {
    let Some(table) = table else {
        return [0.0, 1.0, 0.0];
    };
    let i = g.position_index(vinfo);
    let Some(light) = g.lights.get(i) else {
        return [0.0, 1.0, 0.0];
    };
    let n = table.normal(light.azimuth_index(), light.elevation_index());
    // Same Z-up to Y-up swap as positions, or the lighting comes out sideways.
    [n[0], n[2], -n[1]]
}

/// A vertex's colour after the lighting pass, and whether it was lit at all.
///
/// Lighting runs over *positions*, not vertex-info entries: a common entry and
/// a LOD entry can share a position, and the light record is indexed by
/// position, so every entry resolves through the same lookup.
///
/// Takes no distance. The directional pass does not depend on it; the point
/// lights, which are not implemented yet, will.
///
/// Falls back to the tfrag's stored colour when there is no usable light bank.
/// That is a real difference from the game, so the caller reports how many
/// vertices were actually lit rather than quietly showing stored colours as
/// though they were lit.
fn lit_color(
    g: &TfragGeometry,
    vinfo: usize,
    stored: [f32; 4],
    lighting: Option<(&NormalTable, &LightBank)>,
) -> ([f32; 4], bool) {
    let Some((table, bank)) = lighting.filter(|(_, b)| !b.sets.is_empty()) else {
        return (stored, false);
    };
    let i = g.position_index(vinfo);
    let Some(light) = g.lights.get(i) else {
        return ([1.0, 1.0, 1.0, 1.0], true);
    };
    let n = crate::tfrag_light::vertex_normal_bits(
        table,
        light.azimuth_index(),
        light.elevation_index(),
    );
    match crate::tfrag_light::light_vertex(bank, n, light) {
        Some(c) => (
            [
                c[0] as f32 / 255.0,
                c[1] as f32 / 255.0,
                c[2] as f32 / 255.0,
                1.0,
            ],
            true,
        ),
        // The light set this vertex names does not exist. Count it as unlit
        // rather than inventing a colour.
        None => (stored, false),
    }
}

/// The stored colour of a vertex's position, or opaque white if absent.
pub fn vertex_color(g: &TfragGeometry, vinfo: usize) -> [f32; 4] {
    let i = g.position_index(vinfo);
    match g.rgba.get(i) {
        // The disc's 0x80-is-1.0 alpha convention, same as the texture CLUT.
        // `gc_formats::gs::alpha_to_8` states this rule now; kept inline here
        // because this path wants a float, and the rule has only one live site.
        Some(c) => {
            let a = if c.a < 0x80 { c.a as f32 / 128.0 } else { 1.0 };
            [
                c.r as f32 / 255.0,
                c.g as f32 / 255.0,
                c.b as f32 / 255.0,
                a,
            ]
        }
        None => [1.0, 1.0, 1.0, 1.0],
    }
}

/// Resolve one vertex-info entry at a given view depth.
///
/// Two things happen here and they are not the same. **Morphing** blends the
/// vertex toward the midpoint of its two parents across its tier's distance
/// band. **Collapse** replaces the vertex outright -- position *and* UV -- with
/// its first parent once it is past the band's far edge, which is what stops
/// distant terrain from shimmering. Morphing is a blend; collapse is a cut.
pub fn resolve_vertex(
    g: &TfragGeometry,
    vinfo: usize,
    depth: f32,
    lod_distances: [f32; 3],
    collapse: bool,
) -> ResolvedVertex {
    let Some(link) = tfrag_lod_link(g, vinfo) else {
        return ResolvedVertex {
            position: world(g, vinfo),
            uv: uv(g, vinfo),
            color: vertex_color(g, vinfo),
        };
    };
    let limit = link.tier.collapse_distance(lod_distances);
    if collapse && depth >= limit {
        return ResolvedVertex {
            position: world(g, link.parent1_vinfo),
            uv: uv(g, link.parent1_vinfo),
            // Collapse replaces the vertex outright, colour included.
            color: vertex_color(g, link.parent1_vinfo),
        };
    }
    if !link.morphs {
        return ResolvedVertex {
            position: world(g, vinfo),
            uv: uv(g, vinfo),
            color: vertex_color(g, vinfo),
        };
    }
    let u = link.tier.weight(depth, lod_distances);
    if u <= 0.0 {
        return ResolvedVertex {
            position: world(g, vinfo),
            uv: uv(g, vinfo),
            color: vertex_color(g, vinfo),
        };
    }
    let p1 = raw_position(g, link.parent1_vinfo);
    let p2 = g.positions[link.parent2_position];
    let mid = [
        (p1[0] + p2[0] as f32) / 2.0,
        (p1[1] + p2[1] as f32) / 2.0,
        (p1[2] + p2[2] as f32) / 2.0,
    ];
    let here = raw_position(g, vinfo);
    let blended = [
        here[0] + (mid[0] - here[0]) * u,
        here[1] + (mid[1] - here[1]) * u,
        here[2] + (mid[2] - here[2]) * u,
    ];
    // UVs and colours are not morphed: the blend is positional only, and the
    // game keeps the high-detail texture coordinate and colour.
    ResolvedVertex {
        position: to_world(blended),
        uv: uv(g, vinfo),
        color: vertex_color(g, vinfo),
    }
}

/// Whether a draw mode collapses vertices past their tier's limit rather than
/// only morphing them.
pub fn mode_collapses(mode: TfragDrawMode) -> bool {
    matches!(
        mode,
        TfragDrawMode::Lod1Collapse | TfragDrawMode::Lod0Collapse | TfragDrawMode::Lod2
    )
}

/// Decode every patch in a level's tfrags block, as seen from `camera`.
pub fn build_level(src: &LevelSource<'_>, camera: [f32; 3]) -> anyhow::Result<LevelGeometry> {
    let normals = src.overlay.and_then(|o| NormalTable::from_overlay(o).ok());
    let bank = src.gameplay.and_then(|g| LightBank::parse(g).ok());
    let lighting = normals.as_ref().zip(bank.as_ref());
    let ch = LevelCoreHeader::parse(src.core_index)?;
    let table = texture_table(src.core_index, ch.tfrag_textures)?;

    let region = |off: usize| -> usize {
        i32::from_le_bytes(src.core_index[off..off + 4].try_into().unwrap()).max(0) as usize
    };
    let start = region(0x08);
    let end = [0x0c_usize, 0x10, 0x14]
        .iter()
        .map(|&o| region(o))
        .filter(|&v| v > 0)
        .min()
        .unwrap_or(src.core_data.len());
    if start >= end {
        anyhow::bail!("tfrags region is empty");
    }
    let block = &src.core_data[start..end];
    let header = TfragsBlock::parse(block)?;
    let header_off = header.table_offset as usize;
    let thresholds = header.lod_thresholds_raw();
    let lod_distances = header.lod_distances();

    let textures_base = ch.textures_base.max(0) as usize;
    let texture_block = src.core_data.get(textures_base..).ok_or_else(|| {
        anyhow::anyhow!("textures base {textures_base} is past the end of core data")
    })?;

    let mut stats = LevelStats {
        patches: header.tfrag_count as usize,
        ..Default::default()
    };
    let mut patches = Vec::with_capacity(header.tfrag_count as usize);
    let mut batches: BTreeMap<usize, TriangleBatch> = BTreeMap::new();

    for i in 0..header.tfrag_count as usize {
        let th: TfragHeader = match header.header(block, i) {
            Ok(h) => h,
            Err(_) => {
                stats.skipped_unparsed += 1;
                continue;
            }
        };
        let data_base = header_off.wrapping_add(th.data as usize);
        if data_base >= block.len() {
            stats.skipped_unparsed += 1;
            continue;
        }
        let g = parse_tfrag(&block[data_base..], &th)?;

        let centre = to_world([th.bsphere[0], th.bsphere[1], th.bsphere[2]]);
        let radius = th.bsphere[3] / UNITS_PER_UNIT;
        let depth = distance(camera, centre).max(0.0);
        let mode = g.draw_mode(depth * UNITS_PER_UNIT, thresholds);
        let collapse = mode_collapses(mode);
        let lod = mode.lod();

        let tris = tfrag_triangles(&g, lod)?;
        let limit = g.lod_vinfo_count(lod);
        let mut drawn_here = 0usize;

        for t in tris {
            let vi = [t.a as usize, t.b as usize, t.c as usize];
            if vi.iter().any(|&v| v >= limit) {
                continue;
            }
            let Some(ad) = g.ad_gifs.get(t.ad_gif as usize) else {
                continue;
            };
            let batch = batches
                .entry(ad.texture_index())
                .or_insert_with(|| TriangleBatch {
                    positions: Vec::new(),
                    uvs: Vec::new(),
                    colors: Vec::new(),
                    normals: Vec::new(),
                    indices: Vec::new(),
                    ad_gifs: Vec::new(),
                });
            let base = batch.positions.len() as u32;
            for v in vi {
                let r = resolve_vertex(&g, v, depth, lod_distances, collapse);
                batch.positions.push(r.position);
                batch.uvs.push(r.uv);
                let (colour, was_lit) = lit_color(&g, v, r.color, lighting);
                if was_lit {
                    stats.lit_vertices += 1;
                }
                batch.colors.push(colour);
                batch.normals.push(vertex_normal(&g, v, normals.as_ref()));
            }
            batch.indices.extend_from_slice(&[base, base + 1, base + 2]);
            batch.ad_gifs.push(*ad);
            drawn_here += 1;
        }

        stats.triangles += drawn_here;
        if drawn_here > 0 {
            stats.drawn += 1;
        }
        patches.push(PatchBounds {
            centre,
            radius,
            base_only: th.base_only != 0,
            mode,
            depth,
        });
    }

    let mut textures = BTreeMap::new();
    for &tex in batches.keys() {
        if let Some(e) = table.get(tex) {
            if let Ok(img) = base_image(texture_block, src.gs_ram, e) {
                if let Ok(t) = img.decode() {
                    textures.insert(tex, t);
                }
            }
        }
    }
    for b in batches.values() {
        stats.vertices += b.positions.len();
    }
    // The vertex bounds are the check that matters: positions are i16 offsets
    // from a per-patch origin, so a wrong origin collapses the level to a
    // +-32 unit blob around the world origin while every parser test still
    // passes. Print the real extent to confirm the level is where the patch
    // bounding spheres say it is.
    if std::env::var_os("GC_ENGINE_DEBUG_BOUNDS").is_some() {
        let mut lo = [f32::INFINITY; 3];
        let mut hi = [f32::NEG_INFINITY; 3];
        for b in batches.values() {
            for p in &b.positions {
                for k in 0..3 {
                    lo[k] = lo[k].min(p[k]);
                    hi[k] = hi[k].max(p[k]);
                }
            }
        }
        eprintln!("vertex bounds lo {lo:?} hi {hi:?}");
    }

    Ok(LevelGeometry {
        batches,
        textures,
        patches,
        lod_distances,
        lod_thresholds_raw: thresholds,
        stats,
    })
}

impl LevelGeometry {
    pub fn triangle_count(&self) -> usize {
        self.batches.values().map(|b| b.indices.len() / 3).sum()
    }
}

fn distance(a: [f32; 3], b: [f32; 3]) -> f32 {
    let d = [a[0] - b[0], a[1] - b[1], a[2] - b[2]];
    (d[0] * d[0] + d[1] * d[1] + d[2] * d[2]).sqrt()
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn the_axis_swap_keeps_up_up() {
        // PS2 +Z is up, so it must land on Bevy +Y.
        let w = to_world([0.0, 0.0, 2048.0]);
        assert_eq!(w, [0.0, 2.0, 0.0]);
        // PS2 +X is right, so it stays right.
        assert_eq!(to_world([1024.0, 0.0, 0.0])[0], 1.0);
        // PS2 +Y is forward, which is Bevy -Z.
        assert_eq!(to_world([0.0, 1024.0, 0.0])[2], -1.0);
    }

    #[test]
    fn only_collapse_modes_collapse() {
        assert!(mode_collapses(TfragDrawMode::Lod2));
        assert!(mode_collapses(TfragDrawMode::Lod0Collapse));
        assert!(mode_collapses(TfragDrawMode::Lod1Collapse));
        assert!(!mode_collapses(TfragDrawMode::Lod0));
        assert!(!mode_collapses(TfragDrawMode::Lod0Morph));
        assert!(!mode_collapses(TfragDrawMode::Lod1Morph));
    }

    #[test]
    fn a_common_vertex_does_not_move() {
        let mut g = TfragGeometry {
            origin: [0, 0, 0, 0],
            positions: vec![[1024, 0, 0], [0, 2048, 0]],
            vertex_info: vec![
                gc_formats::tfrag::TfragVertexInfo {
                    s: 0,
                    t: 0,
                    parent: 0,
                    vertex: 0,
                },
                gc_formats::tfrag::TfragVertexInfo {
                    s: 4096,
                    t: 2048,
                    parent: 0,
                    vertex: 2,
                },
            ],
            ..TfragGeometry::default()
        };
        let v = resolve_vertex(&g, 0, 999.0, [192.0, 128.0, 64.0], true);
        assert_eq!(v.position, [1.0, 0.0, 0.0]);
        assert_eq!(v.uv, [0.0, 0.0]);
        // UVs are 1/4096 fixed point.
        assert_eq!(uv(&g, 1), [1.0, 0.5]);
        g.origin = [0, 0, 0, 0];
    }

    #[test]
    fn morphing_blends_toward_the_parent_midpoint() {
        // One common vertex at the origin, one LOD-0 vertex far away whose two
        // parents are both the common one, so the midpoint is the common one.
        let mut g = TfragGeometry {
            positions: vec![[0, 0, 0], [4096, 0, 0]],
            vertex_info: vec![
                gc_formats::tfrag::TfragVertexInfo {
                    s: 0,
                    t: 0,
                    parent: 0,
                    vertex: 0,
                },
                gc_formats::tfrag::TfragVertexInfo {
                    s: 0,
                    t: 0,
                    parent: 0,
                    vertex: 2,
                },
            ],
            vinfo_tier: [1, 0, 1],
            vu_positions_lod_0_count: 1,
            parent_indices_lod0: vec![0],
            ..TfragGeometry::default()
        };
        g.origin = [0, 0, 0, 0];
        // Fully morphed: the vertex sits on the midpoint of its two parents.
        let full = resolve_vertex(&g, 1, 200.0, [192.0, 128.0, 64.0], false);
        assert!(
            (full.position[0] - 0.0).abs() < 1e-4,
            "got {:?}",
            full.position
        );
        // Not morphed: it stays where it is.
        let none = resolve_vertex(&g, 1, 0.0, [192.0, 128.0, 64.0], false);
        assert!(
            (none.position[0] - 4.0).abs() < 1e-4,
            "got {:?}",
            none.position
        );
        // Collapsed: replaced outright by the first parent.
        let cut = resolve_vertex(&g, 1, 200.0, [192.0, 128.0, 64.0], true);
        assert!(
            (cut.position[0] - 0.0).abs() < 1e-4,
            "got {:?}",
            cut.position
        );
    }
}

#[cfg(test)]
mod normal_tests {
    use super::*;
    use gc_formats::tfrag::TfragLight;
    use gc_formats::tfrag_light::NormalTable;

    fn table() -> NormalTable {
        // 256 entries of (cos, sin) at i * pi / 128, as the overlay holds them.
        let mut cos_sin = Vec::with_capacity(256);
        for i in 0..256usize {
            let a = i as f64 * std::f64::consts::PI / 128.0;
            cos_sin.push([(a.cos() as f32).to_bits(), (a.sin() as f32).to_bits()]);
        }
        NormalTable { cos_sin }
    }

    fn geometry_with_light(azimuth: i16, elevation: i16) -> TfragGeometry {
        TfragGeometry {
            positions: vec![[0, 0, 0]],
            vertex_info: vec![gc_formats::tfrag::TfragVertexInfo {
                s: 0,
                t: 0,
                parent: 0,
                vertex: 0,
            }],
            lights: vec![TfragLight {
                unknown_0: 0,
                intensity: 0,
                azimuth: azimuth as i8,
                elevation: elevation as i8,
                color: 0,
                select: 0,
            }],
            ..TfragGeometry::default()
        }
    }

    #[test]
    fn the_light_record_indexes_the_table() {
        // Azimuth 0, elevation 0 is -X in the table, which becomes -X in Bevy.
        let g = geometry_with_light(0, 0);
        let n = vertex_normal(&g, 0, Some(&table()));
        assert!((n[0] + 1.0).abs() < 1e-5, "{n:?}");
        assert!(n[1].abs() < 1e-5 && n[2].abs() < 1e-5, "{n:?}");
    }

    #[test]
    fn normals_get_the_same_axis_swap_as_positions() {
        // Elevation 64 is pi/2, so the table gives a normal along Z, negated.
        let g = geometry_with_light(0, 64);
        let n = vertex_normal(&g, 0, Some(&table()));
        // PS2 (0, 0, -1) must land on Bevy -Y.
        assert!(n[0].abs() < 1e-5 && n[2].abs() < 1e-5, "{n:?}");
        assert!((n[1] + 1.0).abs() < 1e-5, "{n:?}");
    }

    #[test]
    fn an_unsigned_index_is_used_even_when_the_field_reads_negative() {
        // 200 as an i8 is -56, but it indexes table entry 200, not 56.
        let g = geometry_with_light(-56, 0);
        let from_field = vertex_normal(&g, 0, Some(&table()));
        let mut g2 = g.clone();
        g2.lights[0].azimuth = -56;
        assert_eq!(from_field, vertex_normal(&g2, 0, Some(&table())));
    }

    #[test]
    fn missing_normals_fall_back_rather_than_panicking() {
        let g = geometry_with_light(0, 0);
        assert_eq!(vertex_normal(&g, 0, None), [0.0, 1.0, 0.0]);
        let empty = TfragGeometry::default();
        assert_eq!(vertex_normal(&empty, 7, Some(&table())), [0.0, 1.0, 0.0]);
    }
}