mlt-core 0.19.2

//! In-memory model types specific to tag `0x02` (v2) layers.

use num_enum::TryFromPrimitive;

use crate::codecs::morton::{deinterleave_u64, interleave_u32};
use crate::codecs::presence_coding::PresenceCoding;
use crate::codecs::varint::parse_varint;
use crate::decoder::{ColumnStorage, GeometryType, ParsedMValue, ParsedNested, StreamMeta};
use crate::{
    Analyze as _, DecodeState, Layer01, Lazy, LazyParsed, MValueColumn, MltError, MltRefResult,
    MltResult, Nested, Parsed,
};

/// A tag `0x02` layer: the shared [`Layer01`] core plus the columns only v2 has.
///
/// v1 is a subset of v2, so a later version that diverges further gets its own type
/// here rather than more optional fields on the shared one.
#[cfg(feature = "unstable-v2")]
#[derive(Debug, Clone)]
pub struct Layer02<'a, S: DecodeState = Lazy> {
    pub(crate) layer: Layer01<'a, S>,
    /// Nested columns, each a tree over the features it marks present.
    pub(crate) nested: Vec<Nested<'a, S>>,
    /// Vertex-scoped columns, each running over every vertex of every feature
    /// it marks present.
    pub(crate) m_values: Vec<MValueColumn<'a, S>>,
    /// The layer layout byte as parsed. Decoding resolves all three of its fields
    /// away, so it is kept for tools reporting on how a tile is stored.
    pub(crate) layout: LayerLayout,
}

#[cfg(feature = "unstable-v2")]
pub type ParsedLayer02<'a> = Layer02<'a, Parsed>;

#[cfg(feature = "unstable-v2")]
impl<'a, S: DecodeState> Layer02<'a, S> {
    /// The parts this layer shares with every other version.
    #[must_use]
    pub fn layer(&self) -> &Layer01<'a, S> {
        &self.layer
    }

    /// Drop the v2-only columns, keeping the shared core.
    #[must_use]
    pub fn into_layer(self) -> Layer01<'a, S> {
        self.layer
    }

    /// The layer layout byte this layer was written with.
    #[must_use]
    pub fn layout(&self) -> LayerLayout {
        self.layout
    }
}

#[cfg(feature = "unstable-v2")]
impl Layer02<'_, Lazy> {
    /// Walk every stream in this layer, including the v2-only nested and m-value columns.
    pub fn for_each_stream(&self, cb: &mut dyn FnMut(StreamMeta)) {
        self.layer.for_each_stream(cb);
        for nested in &self.nested {
            if let LazyParsed::Raw(raw) = nested {
                raw.for_each_stream(cb);
            }
        }
        for mvalue in &self.m_values {
            if let LazyParsed::Raw(raw) = mvalue {
                raw.for_each_stream(cb);
            }
        }
    }

    /// Walk every column's storage descriptor, including v2-only string m-values and nested string leaves.
    pub fn for_each_column_storage(&self, cb: &mut dyn FnMut(ColumnStorage)) {
        self.layer.for_each_column_storage(cb);
        for mvalue in &self.m_values {
            if let LazyParsed::Raw(raw) = mvalue
                && let Some(storage) = raw.storage()
            {
                cb(storage);
            }
        }
        for nested in &self.nested {
            if let LazyParsed::Raw(raw) = nested {
                raw.for_each_string_storage(cb);
            }
        }
    }
}

#[cfg(feature = "unstable-v2")]
impl ParsedLayer02<'_> {
    #[must_use]
    pub fn m_values(&self) -> &[ParsedMValue<'_>] {
        &self.m_values
    }

    #[must_use]
    pub fn nested(&self) -> &[ParsedNested<'_>] {
        &self.nested
    }
}

/// Data type of a v2 property column, the low nibble of the column type byte.
///
/// Unlike v1's [`super::ColumnType`] there are no `Opt` variants - nullability
/// lives in the high nibble, see [`Presence02`] - and geometry is not a column
/// (the layer's geometry section precedes the counted columns).
/// A string column spends one code, its layout living in the extension bits of
/// its leading stream's encoding byte, see
/// [`StrLayout`](super::stream::header02::StrLayout).
#[derive(Debug, Clone, Copy, PartialEq, Eq, TryFromPrimitive)]
#[repr(u8)]
pub(crate) enum DataType02 {
    Id = 0x00,
    LongId = 0x01,
    Bool = 0x02,
    I8 = 0x03,
    U8 = 0x04,
    I32 = 0x05,
    U32 = 0x06,
    I64 = 0x07,
    U64 = 0x08,
    F32 = 0x09,
    F64 = 0x0A,
    Str = 0x0B,
    Struct = 0x0C,
    List = 0x0D,
    Map = 0x0E,
}

impl DataType02 {
    /// Whether the column definition includes a name field.
    /// ID columns use implicit naming, same as v1.
    #[must_use]
    pub(crate) fn has_name(self) -> bool {
        !matches!(self, Self::Id | Self::LongId)
    }
}

/// The interior nodes, the three data types that hold other nodes rather than values.
///
/// Split from [`DataType02`] so a nested column's root, which one of these must be,
/// cannot be a scalar.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub(crate) enum Interior02 {
    Struct,
    List,
    Map,
}

impl Interior02 {
    /// Whether a node of this kind ends the implied counts, which a lengths stream does.
    #[must_use]
    pub(crate) fn has_lengths(self) -> bool {
        matches!(self, Self::List | Self::Map)
    }
}

impl From<Interior02> for DataType02 {
    fn from(interior: Interior02) -> Self {
        match interior {
            Interior02::Struct => Self::Struct,
            Interior02::List => Self::List,
            Interior02::Map => Self::Map,
        }
    }
}

/// Where a nested node's presence lives, the high nibble of its node type byte.
///
/// A node below a nested column's root cannot use the layer's shared bitfields,
/// which run over features, so only two nibbles are assigned.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub(crate) enum NodePresence {
    /// Every value the parent hands this node is present and nothing is stored.
    AllPresent,
    /// A `Bool`-family presence stream follows the node type byte and the field name.
    Stream,
}

impl NodePresence {
    /// Nibble of [`Self::AllPresent`], already shifted into place.
    const ALL_PRESENT: u8 = 0b0000_0000;

    /// Nibble of [`Self::Stream`], already shifted into place.
    const STREAM: u8 = 0b0001_0000;

    /// Nibble bit that says this node's children are shape-coded, already shifted into place.
    ///
    /// Nibble values `0b0010` and `0b0011` are reserved, so this sits above them.
    pub(crate) const SHAPES: u8 = 0b0100_0000;

    /// Read a masked nibble, or [`None`] for one this version has no meaning for.
    ///
    /// The shapes bit is a separate axis and is masked off by the caller.
    #[must_use]
    pub(crate) fn parse(nibble: u8) -> Option<Self> {
        match nibble & !Self::SHAPES {
            Self::ALL_PRESENT => Some(Self::AllPresent),
            Self::STREAM => Some(Self::Stream),
            _ => None,
        }
    }

    #[must_use]
    fn to_nibble(self) -> u8 {
        match self {
            Self::AllPresent => Self::ALL_PRESENT,
            Self::Stream => Self::STREAM,
        }
    }
}

/// What a nested node holds: a leaf's values, or more nodes.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub(crate) enum NodeKind02 {
    Leaf(ValueType02),
    Struct,
    List,
    Map,
}

impl NodeKind02 {
    /// The interior this kind names, or [`None`] for a leaf.
    #[must_use]
    pub(crate) fn interior(self) -> Option<Interior02> {
        match self {
            Self::Leaf(_) => None,
            Self::Struct => Some(Interior02::Struct),
            Self::List => Some(Interior02::List),
            Self::Map => Some(Interior02::Map),
        }
    }
}

impl From<NodeKind02> for DataType02 {
    fn from(kind: NodeKind02) -> Self {
        match kind {
            NodeKind02::Leaf(values) => values.into(),
            NodeKind02::Struct => Self::Struct,
            NodeKind02::List => Self::List,
            NodeKind02::Map => Self::Map,
        }
    }
}

/// The type byte every node below a nested column's root begins with:
/// [`NodePresence`] in bits 7-4, the data type in bits 3-0.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub(crate) struct NodeType02 {
    pub(crate) presence: NodePresence,
    /// Whether this node's children's structure is coded as one shape id per row.
    pub(crate) shapes: bool,
    pub(crate) data: NodeKind02,
}

impl NodeType02 {
    #[must_use]
    pub(crate) fn new(presence: NodePresence, data: NodeKind02) -> Self {
        Self {
            presence,
            shapes: false,
            data,
        }
    }

    /// The same node type with its children's structure coded as row shapes.
    #[must_use]
    pub(crate) fn shaped(mut self) -> Self {
        self.shapes = true;
        self
    }

    /// Read a wire byte, rejecting the nibbles a node cannot hold.
    pub(crate) fn parse(byte: u8) -> MltResult<Self> {
        let err = || MltError::ParsingColumnType(byte);
        let (nibble, data) = ColumnType02::fields(byte);
        let presence = NodePresence::parse(nibble).ok_or_else(err)?;
        let shapes = nibble & NodePresence::SHAPES != 0;
        let data = DataType02::try_from(data).map_err(|_| err())?;
        let data = match data {
            DataType02::Id | DataType02::LongId => return Err(err()),
            DataType02::Struct => NodeKind02::Struct,
            DataType02::List => NodeKind02::List,
            DataType02::Map => NodeKind02::Map,
            DataType02::Bool => NodeKind02::Leaf(ValueType02::Bool),
            DataType02::I8 => NodeKind02::Leaf(ValueType02::I8),
            DataType02::U8 => NodeKind02::Leaf(ValueType02::U8),
            DataType02::I32 => NodeKind02::Leaf(ValueType02::I32),
            DataType02::U32 => NodeKind02::Leaf(ValueType02::U32),
            DataType02::I64 => NodeKind02::Leaf(ValueType02::I64),
            DataType02::U64 => NodeKind02::Leaf(ValueType02::U64),
            DataType02::F32 => NodeKind02::Leaf(ValueType02::F32),
            DataType02::F64 => NodeKind02::Leaf(ValueType02::F64),
            DataType02::Str => NodeKind02::Leaf(ValueType02::Str),
        };
        Ok(Self {
            presence,
            shapes,
            data,
        })
    }

    #[must_use]
    pub(crate) fn to_byte(self) -> u8 {
        self.presence.to_nibble()
            | if self.shapes { NodePresence::SHAPES } else { 0 }
            | DataType02::from(self.data) as u8
    }
}

/// The type of the values a v2 column holds, the data types that name values.
///
/// Split from [`DataType02`] so a feature id, which is a feature's own rather
/// than one of its values, cannot stand in for a value type.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub(crate) enum ValueType02 {
    Bool,
    I8,
    U8,
    I32,
    U32,
    I64,
    U64,
    F32,
    F64,
    Str,
}

impl From<ValueType02> for DataType02 {
    fn from(values: ValueType02) -> Self {
        match values {
            ValueType02::Bool => Self::Bool,
            ValueType02::I8 => Self::I8,
            ValueType02::U8 => Self::U8,
            ValueType02::I32 => Self::I32,
            ValueType02::U32 => Self::U32,
            ValueType02::I64 => Self::I64,
            ValueType02::U64 => Self::U64,
            ValueType02::F32 => Self::F32,
            ValueType02::F64 => Self::F64,
            ValueType02::Str => Self::Str,
        }
    }
}

/// The width of a v2 id column, which is all its data type says beyond it being one.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub(crate) enum IdWidth02 {
    Id32,
    Id64,
}

impl From<IdWidth02> for DataType02 {
    fn from(width: IdWidth02) -> Self {
        match width {
            IdWidth02::Id32 => Self::Id,
            IdWidth02::Id64 => Self::LongId,
        }
    }
}

/// How a shared dictionary stores its corpus, read from the high nibble of its column type byte.
///
/// A shared-dictionary column has no values of its own, so the nibble that names
/// [`Presence02`] elsewhere names the corpus encoding here.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[repr(u8)]
pub(crate) enum SharedDictKind {
    /// Two streams: the entry lengths, then the entries.
    Plain = 0b0000_0000,
    /// Four streams: the entry lengths, the symbol lengths, the symbol table, then the corpus.
    Fsst = 0b0001_0000,
}

impl SharedDictKind {
    /// Read a masked nibble, or [`None`] for one this version has no meaning for.
    #[must_use]
    pub(crate) fn parse(nibble: u8) -> Option<Self> {
        match nibble {
            0b0000_0000 => Some(Self::Plain),
            0b0001_0000 => Some(Self::Fsst),
            _ => None,
        }
    }
}

/// Where a v2 column's presence bits live and how they are coded, the high nibble
/// of the column type byte.
///
/// Nibble `0` says there are none; `1`-`3` name a [`PresenceCoding`] the column
/// carries itself; `4`-`10` point at one of the layer's shared bitfields; `11`-`15`
/// are reserved.
///
/// The coding rides in the nibble rather than in a byte of its own because the
/// bitmap is still the right answer for most columns, and a tag byte would charge
/// every one of those for the two that are not.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub(crate) enum Presence02 {
    /// Every feature has a value and nothing is stored.
    AllPresent,
    /// The column carries its own presence bits, in this coding, after its name.
    Inline(PresenceCoding),
    /// The column reads the layer's shared presence bitfield at this index, so
    /// columns that are null on the same features store one bitfield between them.
    /// See [`LayerLayout::shared_presence`].
    Shared(u8),
}

impl Presence02 {
    /// Nibble of [`Self::AllPresent`], already shifted into place.
    const ALL_PRESENT: u8 = 0b0000_0000;

    /// Nibble of `Shared(0)`; `Shared(i)` is this plus `i << 4`.
    const SHARED_BASE: u8 = 0b0100_0000;

    /// Read a masked presence nibble against a layer that stores `shared_count`
    /// shared bitfields.
    ///
    /// Returns [`None`] for a reference past the last bitfield the layer declared,
    /// which is unreadable.
    #[must_use]
    pub(crate) fn parse(nibble: u8, shared_count: u8) -> Option<Self> {
        match nibble {
            Self::ALL_PRESENT => Some(Self::AllPresent),
            _ if nibble < Self::SHARED_BASE => {
                PresenceCoding::from_byte(nibble >> 4).map(Self::Inline)
            }
            _ => {
                let index = (nibble - Self::SHARED_BASE) >> 4;
                (index < shared_count).then_some(Self::Shared(index))
            }
        }
    }

    /// Whether some features may be null, whichever bitfield holds the answer.
    #[must_use]
    pub(crate) fn is_optional(self) -> bool {
        !matches!(self, Self::AllPresent)
    }

    #[must_use]
    fn to_nibble(self) -> u8 {
        match self {
            Self::AllPresent => Self::ALL_PRESENT,
            Self::Inline(coding) => (coding as u8) << 4,
            Self::Shared(index) => {
                debug_assert!(index < LayerLayout::MAX_SHARED_PRESENCE);
                Self::SHARED_BASE + (index << 4)
            }
        }
    }
}

/// The v2 column type byte: [`Presence02`] in bits 7-4, [`DataType02`] in bits 3-0.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub(crate) struct ColumnType02 {
    pub(crate) presence: Presence02,
    pub(crate) data: DataType02,
}

impl ColumnType02 {
    /// Mask of the byte holding the [`Presence02`].
    pub(crate) const PRESENCE_MASK: u8 = 0b1111_0000;

    /// Mask of the byte holding the [`DataType02`].
    pub(crate) const DATA_TYPE_MASK: u8 = 0b0000_1111;

    #[must_use]
    pub(crate) fn new(presence: Presence02, data: DataType02) -> Self {
        Self { presence, data }
    }

    /// Split a wire byte into its two masked fields, without validating either.
    /// Values stay in place, so they compare directly against the enum variants.
    #[must_use]
    pub(crate) fn fields(byte: u8) -> (u8, u8) {
        (byte & Self::PRESENCE_MASK, byte & Self::DATA_TYPE_MASK)
    }

    /// Split a wire byte into its two fields, rejecting reserved bit patterns.
    ///
    /// `shared_count` is [`LayerLayout::shared_presence`] of the enclosing layer,
    /// so a column pointing past the last shared bitfield is rejected here rather
    /// than resolved to a missing one later.
    pub(crate) fn parse(byte: u8, shared_count: u8) -> MltResult<Self> {
        let err = || MltError::ParsingColumnType(byte);
        let (presence, data) = Self::fields(byte);
        let data = DataType02::try_from(data).map_err(|_| err())?;
        let presence = Presence02::parse(presence, shared_count).ok_or_else(err)?;
        Ok(Self { presence, data })
    }

    /// Read this column type as whichever of the two columns its data type names.
    #[must_use]
    pub(crate) fn split(self) -> ColumnKind02 {
        let values = |values| {
            ColumnKind02::Values(ValuesColumn02 {
                presence: self.presence,
                values,
            })
        };
        match self.data {
            DataType02::Id => ColumnKind02::Id(IdWidth02::Id32),
            DataType02::LongId => ColumnKind02::Id(IdWidth02::Id64),
            DataType02::Bool => values(ValueType02::Bool),
            DataType02::I8 => values(ValueType02::I8),
            DataType02::U8 => values(ValueType02::U8),
            DataType02::I32 => values(ValueType02::I32),
            DataType02::U32 => values(ValueType02::U32),
            DataType02::I64 => values(ValueType02::I64),
            DataType02::U64 => values(ValueType02::U64),
            DataType02::F32 => values(ValueType02::F32),
            DataType02::F64 => values(ValueType02::F64),
            DataType02::Str => values(ValueType02::Str),
            DataType02::Struct => ColumnKind02::Nested(NestedColumn02 {
                presence: self.presence,
                root: Interior02::Struct,
            }),
            DataType02::List => ColumnKind02::Nested(NestedColumn02 {
                presence: self.presence,
                root: Interior02::List,
            }),
            DataType02::Map => ColumnKind02::Nested(NestedColumn02 {
                presence: self.presence,
                root: Interior02::Map,
            }),
        }
    }

    #[must_use]
    pub(crate) fn to_byte(self) -> u8 {
        self.presence.to_nibble() | self.data as u8
    }
}

/// What a v2 column type byte names: a feature's id, or one column of its values.
///
/// The two read different fields after the byte - an id column has no name of its
/// own - so nothing but this split can name what follows.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub(crate) enum ColumnKind02 {
    Id(IdWidth02),
    Values(ValuesColumn02),
    /// The root of a nested column, whose body is a tree of nodes rather than a stream set.
    Nested(NestedColumn02),
}

/// A nested column's root: where its presence bitfield lives, and which interior it is.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub(crate) struct NestedColumn02 {
    pub(crate) presence: Presence02,
    pub(crate) root: Interior02,
}

/// A v2 column of values: where its presence bitfield lives, and what its values are.
///
/// Holding one is proof the column is not an id, so the streams and containers a
/// column of values reads never have to consider one.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub(crate) struct ValuesColumn02 {
    pub(crate) presence: Presence02,
    pub(crate) values: ValueType02,
}

impl ValuesColumn02 {
    /// Read the type byte of an [m-value column](super::root02), which holds neither
    /// a feature id nor a shared dictionary.
    ///
    /// An id belongs to a feature rather than a vertex, and a shared dictionary
    /// introduces counted columns, which the m-value section does not hold.
    pub(crate) fn parse_m_value(byte: u8, shared_count: u8) -> MltResult<Self> {
        match ColumnType02::parse(byte, shared_count)?.split() {
            ColumnKind02::Values(column) => Ok(column),
            ColumnKind02::Id(_) | ColumnKind02::Nested(_) => Err(MltError::ParsingColumnType(byte)),
        }
    }
}

impl From<ValuesColumn02> for ColumnType02 {
    fn from(column: ValuesColumn02) -> Self {
        Self::new(column.presence, column.values.into())
    }
}

/// A v2 column type byte, read as whichever of the two column shapes its data type nibble names.
///
/// The two shapes read the high nibble differently, so nothing but this split can name it.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub(crate) enum Column02 {
    /// A column of values: [`Presence02`] in the high nibble, [`DataType02`] in the low.
    Values(ColumnType02),
    /// A shared dictionary and the columns indexing it: [`SharedDictKind`] in the high nibble.
    SharedDict(SharedDictKind),
}

impl Column02 {
    /// Data type nibble of [`Self::SharedDict`], the one value no [`DataType02`] takes.
    pub(crate) const SHARED_DICT: u8 = 0x0F;

    /// Read a wire byte in the terms of the shape its data type nibble names.
    ///
    /// `shared_count` is [`LayerLayout::shared_presence`] of the enclosing layer.
    pub(crate) fn parse(byte: u8, shared_count: u8) -> MltResult<Self> {
        let err = || MltError::ParsingColumnType(byte);
        let (high, data) = ColumnType02::fields(byte);
        if data == Self::SHARED_DICT {
            return SharedDictKind::parse(high)
                .map(Self::SharedDict)
                .ok_or_else(err);
        }
        ColumnType02::parse(byte, shared_count).map(Self::Values)
    }
}

/// v2 geometry section layout, the low nibble of the layer layout byte.
///
/// Selects which geometry streams are present and in what fixed order,
/// replacing v1's `stream_count` varint and per-stream `stream_type` bytes.
#[derive(Debug, Clone, Copy, PartialEq, Eq, TryFromPrimitive, strum::IntoStaticStr)]
#[repr(u8)]
pub enum GeoLayout {
    /// `Types`, `Vertices`
    Points = 0x00,
    /// `Types`, `VertexData` (dict), `VertexOffsets`
    PointsDict = 0x01,
    /// `Types`, `GeoLengths`, `Vertices`
    MultiPoints = 0x02,
    /// `Types`, `GeoLengths`, `VertexData` (dict), `VertexOffsets`
    MultiPointsDict = 0x03,
    /// `Types`, `PartLengths`, `Vertices`
    Lines = 0x04,
    /// `Types`, `PartLengths`, `VertexData` (dict), `VertexOffsets`
    LinesDict = 0x05,
    /// `Types`, `GeoLengths`, `PartLengths`, `Vertices`
    MultiLines = 0x06,
    /// `Types`, `GeoLengths`, `PartLengths`, `VertexData` (dict), `VertexOffsets`
    MultiLinesDict = 0x07,
    /// `Types`, `PartLengths`, `RingLengths`, `Vertices`
    Polygons = 0x08,
    /// `Types`, `PartLengths`, `RingLengths`, `VertexData` (dict), `VertexOffsets`
    PolygonsDict = 0x09,
    /// `Types`, `GeoLengths`, `PartLengths`, `RingLengths`, `Vertices`
    MultiPolygons = 0x0A,
    /// `Types`, `GeoLengths`, `PartLengths`, `RingLengths`, `VertexData` (dict), `VertexOffsets`
    MultiPolygonsDict = 0x0B,
    /// `Types`, `TriLengths`, `IndexBuffer`, `Vertices`
    TessPolygons = 0x0C,
    /// `Types`, `GeoLengths`, `PartLengths`, `RingLengths`, `TriLengths`, `IndexBuffer`, `Vertices`
    TessPolygonsWithOutlines = 0x0D,
}

/// How a v2 geometry section stores its vertices, which [`GeoLayout`] names alongside the topology.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub(crate) enum VertexStorage {
    /// One entry per vertex, in a single stream.
    Plain,
    /// The distinct vertices once, then one index per vertex into them.
    Dict,
}

/// The nesting of a v2 geometry section below the geometry level, carrying the streams that name it.
///
/// A ring level with no part level above it is not a shape the geometry model can
/// produce, so it is not a shape this type can hold.
#[derive(Debug, Clone, PartialEq, Eq)]
pub(crate) enum Topology {
    /// No nesting: a geometry addresses its vertices directly.
    Flat,
    /// One part per geometry, each addressing its vertices directly.
    Parts(Vec<u32>),
    /// One part per geometry, each addressing a run of rings.
    PartsAndRings { parts: Vec<u32>, rings: Vec<u32> },
}

impl Topology {
    /// The part and ring length streams, in wire order.
    pub(crate) fn streams(&self) -> (Option<&[u32]>, Option<&[u32]>) {
        match self {
            Self::Flat => (None, None),
            Self::Parts(parts) => (Some(parts), None),
            Self::PartsAndRings { parts, rings } => (Some(parts), Some(rings)),
        }
    }

    /// Widen to the full part-and-ring nesting, padding a level this layer never used.
    #[must_use]
    pub(crate) fn with_rings(self) -> Self {
        match self {
            Self::Flat => Self::PartsAndRings {
                parts: Vec::new(),
                rings: Vec::new(),
            },
            Self::Parts(parts) => Self::PartsAndRings {
                parts,
                rings: Vec::new(),
            },
            rings @ Self::PartsAndRings { .. } => rings,
        }
    }
}

impl GeoLayout {
    /// Layout for a stream set with plain or dictionary vertices.
    ///
    /// `has_geo` is independent of the nesting below it: any [`Topology`] can be
    /// addressed either per feature or per sub-geometry.
    #[must_use]
    pub(crate) fn from_topology(
        topology: &Topology,
        has_geo: bool,
        vertices: VertexStorage,
    ) -> Self {
        use VertexStorage as V;
        match (topology, has_geo, vertices) {
            (Topology::Flat, false, V::Plain) => Self::Points,
            (Topology::Flat, false, V::Dict) => Self::PointsDict,
            (Topology::Flat, true, V::Plain) => Self::MultiPoints,
            (Topology::Flat, true, V::Dict) => Self::MultiPointsDict,
            (Topology::Parts(_), false, V::Plain) => Self::Lines,
            (Topology::Parts(_), false, V::Dict) => Self::LinesDict,
            (Topology::Parts(_), true, V::Plain) => Self::MultiLines,
            (Topology::Parts(_), true, V::Dict) => Self::MultiLinesDict,
            (Topology::PartsAndRings { .. }, false, V::Plain) => Self::Polygons,
            (Topology::PartsAndRings { .. }, false, V::Dict) => Self::PolygonsDict,
            (Topology::PartsAndRings { .. }, true, V::Plain) => Self::MultiPolygons,
            (Topology::PartsAndRings { .. }, true, V::Dict) => Self::MultiPolygonsDict,
        }
    }

    /// Layout for a tessellated stream set, which keeps its vertices plain.
    ///
    /// Tessellation pairs with the full outline topology or with none of it, so a
    /// layer holding only part of it pads the streams it is missing with empty ones.
    #[must_use]
    pub(crate) fn tessellated(has_outlines: bool) -> Self {
        if has_outlines {
            Self::TessPolygonsWithOutlines
        } else {
            Self::TessPolygons
        }
    }

    #[must_use]
    pub(crate) fn has_geo_lengths(self) -> bool {
        matches!(
            self,
            Self::MultiPoints
                | Self::MultiPointsDict
                | Self::MultiLines
                | Self::MultiLinesDict
                | Self::MultiPolygons
                | Self::MultiPolygonsDict
                | Self::TessPolygonsWithOutlines
        )
    }

    #[must_use]
    pub(crate) fn has_part_lengths(self) -> bool {
        matches!(
            self,
            Self::Lines
                | Self::LinesDict
                | Self::MultiLines
                | Self::MultiLinesDict
                | Self::Polygons
                | Self::PolygonsDict
                | Self::MultiPolygons
                | Self::MultiPolygonsDict
                | Self::TessPolygonsWithOutlines
        )
    }

    #[must_use]
    pub(crate) fn has_ring_lengths(self) -> bool {
        matches!(
            self,
            Self::Polygons
                | Self::PolygonsDict
                | Self::MultiPolygons
                | Self::MultiPolygonsDict
                | Self::TessPolygonsWithOutlines
        )
    }

    /// Whether vertex data is stored as a dictionary + offsets pair.
    #[must_use]
    pub(crate) fn is_dict(self) -> bool {
        matches!(
            self,
            Self::PointsDict
                | Self::MultiPointsDict
                | Self::LinesDict
                | Self::MultiLinesDict
                | Self::PolygonsDict
                | Self::MultiPolygonsDict
        )
    }

    /// Whether the topology gives every feature's vertex count, which an
    /// [m-value column](super::root02) needs to read its values back per feature.
    ///
    /// A point layer holds one vertex per feature, so a vertex-scoped column there
    /// is a property column. A tessellated layer without outlines has no per-feature
    /// vertex count at all.
    #[must_use]
    pub(crate) fn allows_m_values(self) -> bool {
        !matches!(self, Self::Points | Self::PointsDict | Self::TessPolygons)
    }

    /// Whether tessellation streams (`TriLengths`, `IndexBuffer`) are present.
    #[must_use]
    pub(crate) fn is_tess(self) -> bool {
        matches!(self, Self::TessPolygons | Self::TessPolygonsWithOutlines)
    }
}

/// The v2 extent code, the low nibble of the [`LayerHeader02`] byte.
///
/// v2 stores only power-of-two extents, so the nibble holds `log2(extent) - 6`
/// rather than the extent itself: `0x0` is 64 and `0xF` is 2097152.
/// Every code is assigned, so no nibble is rejected.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub(crate) struct Extent02(u8);

impl Extent02 {
    /// Mask of the nibble holding the extent code.
    const MAX_EXPONENT: u8 = 0b0000_1111;
    pub(crate) const EXPONENT_MASK: u8 = Self::MAX_EXPONENT;

    /// Exponent of the smallest extent, which the code is offset by.
    const EXPONENT_OFFSET: u32 = 6;

    /// Code an extent, rejecting one that is not a power of two in `64..=2097152`.
    pub(crate) fn new(extent: u32) -> MltResult<Self> {
        if extent.is_power_of_two()
            && let Some(code) = extent.ilog2().checked_sub(Self::EXPONENT_OFFSET)
            && code <= u32::from(Self::MAX_EXPONENT)
        {
            #[expect(clippy::cast_possible_truncation, reason = "16 < 256")]
            Ok(Self(code as u8))
        } else {
            Err(MltError::UnsupportedExtent02(extent))
        }
    }

    /// Read the code out of a header byte, ignoring the fields above it.
    #[must_use]
    pub(crate) fn from_byte(byte: u8) -> Self {
        Self(byte & Self::EXPONENT_MASK)
    }

    /// The extent itself, always a power of two in `64..=2097152`.
    #[must_use]
    pub(crate) fn get(self) -> u32 {
        1 << (u32::from(self.0) + Self::EXPONENT_OFFSET)
    }

    #[must_use]
    pub(crate) fn to_nibble(self) -> u8 {
        self.0
    }
}

/// The v2 layer header byte: an m-value flag in bit 7, the uniform geometry type
/// in bits 6-4, the [`Extent02`] code in bits 3-0.
///
/// The m-value flag sits here rather than in the [`LayerLayout`] byte.
/// That leaves the layout byte's high nibble whole for the shared presence count.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub(crate) struct LayerHeader02 {
    /// Whether an [m-value section](super::root02) ends the layer body, after the
    /// counted columns.
    pub(crate) m_values: bool,
    /// The type every feature has, when the geometry section writes no types stream.
    ///
    /// Such a types stream is a single run holding nothing this nibble cannot.
    pub(crate) uniform_type: Option<GeometryType>,
    pub(crate) extent: Extent02,
}

impl LayerHeader02 {
    /// Mask of the bit saying an [m-value section](super::root02) ends the body.
    pub(crate) const M_VALUES_MASK: u8 = 0b1000_0000;

    /// Mask of the nibble holding the uniform geometry type.
    pub(crate) const UNIFORM_TYPE_MASK: u8 = 0b0111_0000;

    /// Uniform type code of a layer whose geometry section leads with a types stream.
    const TYPES_STREAM: u8 = 0;

    #[must_use]
    pub(crate) fn new(
        extent: Extent02,
        uniform_type: Option<GeometryType>,
        m_values: bool,
    ) -> Self {
        Self {
            m_values,
            uniform_type,
            extent,
        }
    }

    /// Split a wire byte into its three fields, without validating any of them.
    /// The m-value flag stays in place, the other two are shifted down.
    #[must_use]
    fn fields(byte: u8) -> (u8, u8, u8) {
        (
            byte & Self::M_VALUES_MASK,
            (byte & Self::UNIFORM_TYPE_MASK) >> 4,
            byte & Extent02::EXPONENT_MASK,
        )
    }

    /// Read the uniform type nibble of a wire byte, rejecting the one code no
    /// geometry type takes.
    fn parse_uniform_type(byte: u8) -> MltResult<Option<GeometryType>> {
        let (_, nibble, _) = Self::fields(byte);
        if nibble == Self::TYPES_STREAM {
            return Ok(None);
        }
        GeometryType::try_from(nibble - 1)
            .map(Some)
            .map_err(|_| MltError::ParsingLayerHeader02(byte))
    }

    /// Split a wire byte into its three fields, rejecting reserved bit patterns.
    pub(crate) fn parse(byte: u8) -> MltResult<Self> {
        let (m_values, _, _) = Self::fields(byte);
        Ok(Self {
            m_values: m_values != 0,
            uniform_type: Self::parse_uniform_type(byte)?,
            extent: Extent02::from_byte(byte),
        })
    }

    #[must_use]
    pub(crate) fn to_byte(self) -> u8 {
        let m_values = if self.m_values {
            Self::M_VALUES_MASK
        } else {
            0
        };
        let uniform = self
            .uniform_type
            .map_or(Self::TYPES_STREAM, |t| t as u8 + 1);
        m_values | (uniform << 4) | self.extent.to_nibble()
    }
}

/// The v2 layer layout byte: the shared coding flag in bit 7, the shared presence
/// bitfield count in bits 6-4, [`GeoLayout`] in bits 3-0.
///
/// It sits at the layer root, right after the layer header byte, and every
/// bit of it is spent. Decoding resolves all three fields away, so a layer keeps
/// the parsed byte for tools reporting on how a tile is stored.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct LayerLayout {
    /// How many shared presence bitfields the layer stores, at most 7.
    ///
    /// The bitfields themselves follow this byte immediately, before the geometry
    /// section, in index order. Columns read one through their shared presence
    /// nibble, so a set of columns that are null on the same features pays for one
    /// bitfield rather than one each.
    pub shared_presence: u8,
    /// Whether each shared bitfield leads with the byte naming its presence coding.
    /// Without it every shared bitfield is a plain bitmap.
    pub shared_coded: bool,
    /// Which streams the geometry section holds, and in what order.
    pub geometry: GeoLayout,
}

impl LayerLayout {
    /// Bit of the byte saying the shared bitfields carry their coding.
    pub(crate) const SHARED_CODED_BIT: u8 = 0b1000_0000;

    /// Mask of the byte holding the shared presence column count.
    pub(crate) const SHARED_PRESENCE_MASK: u8 = 0b0111_0000;

    /// Mask of the byte holding the [`GeoLayout`].
    pub(crate) const GEO_LAYOUT_MASK: u8 = 0b0000_1111;

    /// Largest shared presence column count a layer can use, which is all its
    /// three bits can hold.
    pub(crate) const MAX_SHARED_PRESENCE: u8 = 7;

    #[must_use]
    pub(crate) fn new(geometry: GeoLayout, shared_presence: u8, shared_coded: bool) -> Self {
        debug_assert!(shared_presence <= Self::MAX_SHARED_PRESENCE);
        debug_assert!(shared_presence > 0 || !shared_coded);
        Self {
            shared_presence,
            shared_coded,
            geometry,
        }
    }

    /// Split a wire byte into its three fields, without validating any.
    #[must_use]
    pub(crate) fn fields(byte: u8) -> (bool, u8, u8) {
        (
            byte & Self::SHARED_CODED_BIT != 0,
            (byte & Self::SHARED_PRESENCE_MASK) >> 4,
            byte & Self::GEO_LAYOUT_MASK,
        )
    }

    /// Split a wire byte into its fields, rejecting reserved bit patterns.
    pub(crate) fn parse(byte: u8) -> MltResult<Self> {
        let (shared_coded, shared_presence, geometry) = Self::fields(byte);
        // A coding flag with no bitfield to code has a canonical spelling without it.
        if shared_coded && shared_presence == 0 {
            return Err(MltError::ParsingLayerLayout02(byte));
        }
        let geometry =
            GeoLayout::try_from(geometry).map_err(|_| MltError::ParsingGeoLayout(geometry))?;
        Ok(Self {
            shared_presence,
            shared_coded,
            geometry,
        })
    }

    #[must_use]
    pub(crate) fn to_byte(self) -> u8 {
        debug_assert!(self.shared_presence <= Self::MAX_SHARED_PRESENCE);
        let coded = if self.shared_coded {
            Self::SHARED_CODED_BIT
        } else {
            0
        };
        coded | (self.shared_presence << 4) | self.geometry as u8
    }
}

/// The column counts a v2 layer writes between its geometry section and its columns.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub(crate) struct ColumnCounts {
    /// Counted columns: ids, properties and nested columns.
    pub(crate) columns: u32,
    /// M-value columns, zero when the header byte has no m-value section.
    pub(crate) m_values: u32,
}

impl ColumnCounts {
    /// Parse the counts varint, a Morton code of both counts when the header byte says an m-value section follows.
    pub(crate) fn parse(input: &[u8], m_values: bool) -> MltRefResult<'_, Self> {
        if !m_values {
            let (input, columns) = parse_varint::<u32>(input)?;
            return Ok((
                input,
                Self {
                    columns,
                    m_values: 0,
                },
            ));
        }
        let (input, code) = parse_varint::<u64>(input)?;
        let (columns, m_values) = deinterleave_u64(code);
        if m_values == 0 {
            return Err(MltError::EmptyMValueSection);
        }
        Ok((input, Self { columns, m_values }))
    }

    /// The varint value the counts are written as, the inverse of [`Self::parse`].
    #[must_use]
    pub(crate) fn to_varint(self) -> u64 {
        if self.m_values == 0 {
            u64::from(self.columns)
        } else {
            interleave_u32(self.columns, self.m_values)
        }
    }
}

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

    use super::*;

    /// Shared bitfield count of a layer that declares as many as the byte allows.
    const ALL_SHARED: u8 = LayerLayout::MAX_SHARED_PRESENCE;

    fn parts_and_rings() -> Topology {
        Topology::PartsAndRings {
            parts: vec![1],
            rings: vec![4],
        }
    }

    #[rstest]
    #[case::id(0b0000_0000, Presence02::AllPresent, DataType02::Id)]
    #[case::opt_id(
        0b0001_0000,
        Presence02::Inline(PresenceCoding::Bitmap),
        DataType02::Id
    )]
    #[case::i32(0b0000_0101, Presence02::AllPresent, DataType02::I32)]
    #[case::opt_f64(
        0b0001_1010,
        Presence02::Inline(PresenceCoding::Bitmap),
        DataType02::F64
    )]
    #[case::runs(0b0010_0101, Presence02::Inline(PresenceCoding::Runs), DataType02::I32)]
    #[case::indices(
        0b0011_1010,
        Presence02::Inline(PresenceCoding::Indices),
        DataType02::F64
    )]
    #[case::str(0b0000_1011, Presence02::AllPresent, DataType02::Str)]
    #[case::first_shared(0b0100_0101, Presence02::Shared(0), DataType02::I32)]
    #[case::last_shared(0b1010_1010, Presence02::Shared(6), DataType02::F64)]
    fn column_type_byte_roundtrip(
        #[case] byte: u8,
        #[case] presence: Presence02,
        #[case] data: DataType02,
    ) {
        let typ = ColumnType02::parse(byte, ALL_SHARED).unwrap();
        assert_eq!(typ, ColumnType02::new(presence, data));
        assert_eq!(typ.to_byte(), byte);
    }

    #[rstest]
    #[case::values(
        0b0001_0101,
        Column02::Values(ColumnType02::new(
            Presence02::Inline(PresenceCoding::Bitmap),
            DataType02::I32
        ))
    )]
    #[case::plain_shared_dict(0b0000_1111, Column02::SharedDict(SharedDictKind::Plain))]
    #[case::fsst_shared_dict(0b0001_1111, Column02::SharedDict(SharedDictKind::Fsst))]
    fn column_byte_names_the_shape_it_holds(#[case] byte: u8, #[case] column: Column02) {
        assert_eq!(Column02::parse(byte, ALL_SHARED).unwrap(), column);
    }

    #[rstest]
    #[case::smallest(64, 0x00)]
    #[case::mvt_default(4096, 0x06)]
    #[case::wider_than_a_morton_code(65_536, 0x0A)]
    #[case::largest(2_097_152, 0x0F)]
    fn an_extent_round_trips_through_its_nibble(#[case] extent: u32, #[case] byte: u8) {
        let coded = Extent02::new(extent).unwrap();
        assert_eq!(coded.to_nibble(), byte);
        assert_eq!(coded.get(), extent);
        assert_eq!(Extent02::from_byte(byte), coded);
    }

    #[rstest]
    #[case::zero(0)]
    #[case::one(1)]
    #[case::below_the_smallest(32)]
    #[case::not_a_power_of_two(80)]
    #[case::a_multiple_of_the_smallest(192)]
    #[case::a_power_of_two_plus_a_multiple_of_the_smallest(33_024)]
    #[case::just_over_the_largest(2_097_153)]
    #[case::a_power_of_two_over_the_largest(4_194_304)]
    fn an_extent_outside_the_nibble_is_rejected(#[case] extent: u32) {
        let err = Extent02::new(extent).unwrap_err();
        assert!(matches!(err, MltError::UnsupportedExtent02(e) if e == extent));
    }

    #[rstest]
    #[case::extent_only(0x06, false, None, 4096)]
    #[case::m_values(0x86, true, None, 4096)]
    #[case::uniform_points(0x16, false, Some(GeometryType::Point), 4096)]
    #[case::uniform_multi_polygons(0x60, false, Some(GeometryType::MultiPolygon), 64)]
    #[case::m_values_over_a_uniform_type(0xCF, true, Some(GeometryType::MultiPoint), 2_097_152)]
    fn layer_header_byte_roundtrip(
        #[case] byte: u8,
        #[case] m_values: bool,
        #[case] uniform_type: Option<GeometryType>,
        #[case] extent: u32,
    ) {
        let header = LayerHeader02::parse(byte).unwrap();
        assert_eq!(
            header,
            LayerHeader02::new(Extent02::new(extent).unwrap(), uniform_type, m_values)
        );
        assert_eq!(header.extent.get(), extent);
        assert_eq!(header.to_byte(), byte);
    }

    #[rstest]
    #[case::reserved_uniform_type(0x70)]
    #[case::reserved_uniform_type_over_an_extent(0x7F)]
    #[case::reserved_uniform_type_with_m_values(0xF6)]
    fn layer_header_byte_rejects_the_reserved_uniform_type(#[case] byte: u8) {
        let err = LayerHeader02::parse(byte).unwrap_err();
        assert!(matches!(err, MltError::ParsingLayerHeader02(b) if b == byte));
    }

    #[rstest]
    #[case::reserved_shared_dict_corpus(0b0010_1111, ALL_SHARED)]
    #[case::shared_ref_past_declared_count_over_a_nested_root(0b1001_1100, 4)]
    fn column_byte_rejects_unassigned(#[case] byte: u8, #[case] shared_count: u8) {
        let err = Column02::parse(byte, shared_count).unwrap_err();
        assert!(matches!(err, MltError::ParsingColumnType(b) if b == byte));
    }

    #[rstest]
    #[case::shared_dict_is_not_a_data_type(0b0000_1111, ALL_SHARED)]
    #[case::shared_ref_past_declared_count_over_a_nested_root(0b1001_1100, 4)]
    #[case::shared_ref_past_declared_count(0b1001_0101, 3)]
    #[case::shared_ref_one_past_the_last_bitfield(0b1011_0101, ALL_SHARED)]
    #[case::reserved_shared_nibble(0b1111_0101, ALL_SHARED)]
    #[case::shared_ref_without_shared_columns(0b0100_0101, 0)]
    #[case::shared_ref_past_declared_count(0b0101_0101, 1)]
    #[case::unassigned_inline_coding(0b0101_0101, 0)]
    fn column_type_byte_rejects_unassigned(#[case] byte: u8, #[case] shared_count: u8) {
        let err = ColumnType02::parse(byte, shared_count).unwrap_err();
        assert!(matches!(err, MltError::ParsingColumnType(b) if b == byte));
    }

    #[rstest]
    #[case::points(Topology::Flat, false, VertexStorage::Plain, GeoLayout::Points)]
    #[case::points_dict(Topology::Flat, false, VertexStorage::Dict, GeoLayout::PointsDict)]
    #[case::multi_points(Topology::Flat, true, VertexStorage::Plain, GeoLayout::MultiPoints)]
    #[case::lines_dict(Topology::Parts(vec![2]), false, VertexStorage::Dict, GeoLayout::LinesDict)]
    #[case::multi_lines(Topology::Parts(vec![2]), true, VertexStorage::Plain, GeoLayout::MultiLines)]
    #[case::polygons(parts_and_rings(), false, VertexStorage::Plain, GeoLayout::Polygons)]
    #[case::multi_polygons_dict(
        parts_and_rings(),
        true,
        VertexStorage::Dict,
        GeoLayout::MultiPolygonsDict
    )]
    fn a_stream_set_picks_its_layout(
        #[case] topology: Topology,
        #[case] has_geo: bool,
        #[case] vertices: VertexStorage,
        #[case] expected: GeoLayout,
    ) {
        let layout = GeoLayout::from_topology(&topology, has_geo, vertices);
        let (parts, rings) = topology.streams();
        assert_eq!(layout, expected);
        assert_eq!(layout.has_geo_lengths(), has_geo);
        assert_eq!(layout.has_part_lengths(), parts.is_some());
        assert_eq!(layout.has_ring_lengths(), rings.is_some());
        assert_eq!(layout.is_dict(), vertices == VertexStorage::Dict);
        assert!(!layout.is_tess());
    }

    #[rstest]
    #[case::tess(false, GeoLayout::TessPolygons)]
    #[case::tess_with_outlines(true, GeoLayout::TessPolygonsWithOutlines)]
    fn a_tessellated_stream_set_picks_its_layout(
        #[case] has_outlines: bool,
        #[case] expected: GeoLayout,
    ) {
        let layout = GeoLayout::tessellated(has_outlines);
        assert_eq!(layout, expected);
        assert!(layout.is_tess());
        assert!(!layout.is_dict());
        assert_eq!(layout.has_geo_lengths(), has_outlines);
        assert_eq!(layout.has_part_lengths(), has_outlines);
        assert_eq!(layout.has_ring_lengths(), has_outlines);
    }

    #[rstest]
    #[case::flat(Topology::Flat)]
    #[case::parts(Topology::Parts(vec![2]))]
    #[case::parts_and_rings(parts_and_rings())]
    fn a_topology_widened_to_rings_keeps_both_streams(#[case] topology: Topology) {
        let widened = topology.with_rings();
        let (parts, rings) = widened.streams();
        assert!(parts.is_some() && rings.is_some());
    }

    #[rstest]
    #[case::points(0b0000_0000, 0, false, GeoLayout::Points)]
    #[case::multi_polygons(0b0000_1010, 0, false, GeoLayout::MultiPolygons)]
    #[case::one_shared_presence(0b0001_0100, 1, false, GeoLayout::Lines)]
    #[case::max_shared_presence(0b0111_0000, ALL_SHARED, false, GeoLayout::Points)]
    #[case::one_coded_shared_presence(0b1001_0100, 1, true, GeoLayout::Lines)]
    #[case::max_coded_shared_presence(0b1111_0110, ALL_SHARED, true, GeoLayout::MultiLines)]
    fn layer_layout_byte_roundtrip(
        #[case] byte: u8,
        #[case] shared_presence: u8,
        #[case] shared_coded: bool,
        #[case] geometry: GeoLayout,
    ) {
        let layout = LayerLayout::parse(byte).unwrap();
        assert_eq!(
            layout,
            LayerLayout::new(geometry, shared_presence, shared_coded)
        );
        assert_eq!(layout.to_byte(), byte);
    }

    #[rstest]
    #[case::no_bitfield(0b1000_0000)]
    #[case::no_bitfield_over_a_geo_layout(0b1000_1010)]
    fn layer_layout_byte_rejects_a_coding_flag_with_nothing_to_code(#[case] byte: u8) {
        let err = LayerLayout::parse(byte).unwrap_err();
        assert!(matches!(err, MltError::ParsingLayerLayout02(b) if b == byte));
    }

    #[rstest]
    #[case::bool(0b0000_0010, ValueType02::Bool)]
    #[case::opt_i32(0b0001_0101, ValueType02::I32)]
    #[case::shared_f64(0b0100_1010, ValueType02::F64)]
    #[case::str(0b0000_1011, ValueType02::Str)]
    fn m_value_type_byte_roundtrip(#[case] byte: u8, #[case] values: ValueType02) {
        let column = ValuesColumn02::parse_m_value(byte, ALL_SHARED).unwrap();
        assert_eq!(column.values, values);
        assert_eq!(ColumnType02::from(column).to_byte(), byte);
    }

    #[rstest]
    #[case::id(0b0000_0000)]
    #[case::opt_id(0b0001_0000)]
    #[case::long_id(0b0000_0001)]
    #[case::shared_dict(0b0000_1111)]
    #[case::struct_root(0b0000_1100)]
    #[case::list_root(0b0000_1101)]
    #[case::map_root(0b0000_1110)]
    fn m_value_type_byte_rejects_what_a_vertex_cannot_hold(#[case] byte: u8) {
        let err = ValuesColumn02::parse_m_value(byte, ALL_SHARED).unwrap_err();
        assert!(matches!(err, MltError::ParsingColumnType(b) if b == byte));
    }

    #[test]
    fn an_m_value_column_cannot_name_a_bitfield_the_layer_lacks() {
        let byte = 0b1001_0101;
        let err = ValuesColumn02::parse_m_value(byte, 4).unwrap_err();
        assert!(matches!(err, MltError::ParsingColumnType(b) if b == byte));
    }

    #[rstest]
    #[case::points(GeoLayout::Points, false)]
    #[case::points_dict(GeoLayout::PointsDict, false)]
    #[case::multi_points(GeoLayout::MultiPoints, true)]
    #[case::lines(GeoLayout::Lines, true)]
    #[case::multi_polygons_dict(GeoLayout::MultiPolygonsDict, true)]
    #[case::tess_polygons(GeoLayout::TessPolygons, false)]
    #[case::tess_polygons_with_outlines(GeoLayout::TessPolygonsWithOutlines, true)]
    fn only_a_layout_with_per_feature_vertex_counts_takes_m_values(
        #[case] layout: GeoLayout,
        #[case] allowed: bool,
    ) {
        assert_eq!(layout.allows_m_values(), allowed);
    }

    #[rstest]
    #[case::i32_leaf(
        0b0000_0101,
        NodePresence::AllPresent,
        NodeKind02::Leaf(ValueType02::I32)
    )]
    #[case::optional_str_leaf(
        0b0001_1011,
        NodePresence::Stream,
        NodeKind02::Leaf(ValueType02::Str)
    )]
    #[case::struct_node(0b0000_1100, NodePresence::AllPresent, NodeKind02::Struct)]
    #[case::optional_list_node(0b0001_1101, NodePresence::Stream, NodeKind02::List)]
    #[case::map_node(0b0000_1110, NodePresence::AllPresent, NodeKind02::Map)]
    fn node_type_byte_roundtrip(
        #[case] byte: u8,
        #[case] presence: NodePresence,
        #[case] data: NodeKind02,
    ) {
        let typ = NodeType02::parse(byte).unwrap();
        assert_eq!(typ, NodeType02::new(presence, data));
        assert_eq!(typ.to_byte(), byte);
    }

    #[rstest]
    #[case::shaped_struct(0b0100_1100, NodePresence::AllPresent, NodeKind02::Struct)]
    #[case::shaped_map_over_nulls(0b0101_1110, NodePresence::Stream, NodeKind02::Map)]
    fn the_shapes_bit_reads_beside_a_nodes_own_presence(
        #[case] byte: u8,
        #[case] presence: NodePresence,
        #[case] data: NodeKind02,
    ) {
        let typ = NodeType02::parse(byte).unwrap();
        assert_eq!(typ, NodeType02::new(presence, data).shaped());
        assert!(typ.shapes);
        assert_eq!(typ.to_byte(), byte);
    }

    #[rstest]
    #[case::id_is_a_features_own(0b0000_0000)]
    #[case::long_id_is_a_features_own(0b0000_0001)]
    #[case::shared_dict_introduces_columns(0b0000_1111)]
    #[case::reserved_node_presence(0b0010_0101)]
    #[case::reserved_node_presence_top(0b1111_0101)]
    fn node_type_byte_rejects_what_a_node_cannot_hold(#[case] byte: u8) {
        let err = NodeType02::parse(byte).unwrap_err();
        assert!(matches!(err, MltError::ParsingColumnType(b) if b == byte));
    }

    #[rstest]
    #[case::struct_root(0b0000_1100, Interior02::Struct)]
    #[case::list_root(0b0001_1101, Interior02::List)]
    #[case::map_root(0b0010_1110, Interior02::Map)]
    fn a_nested_root_reads_as_the_interior_its_nibble_names(
        #[case] byte: u8,
        #[case] root: Interior02,
    ) {
        let typ = ColumnType02::parse(byte, ALL_SHARED).unwrap();
        let ColumnKind02::Nested(column) = typ.split() else {
            panic!("expected a nested root")
        };
        assert_eq!(column.root, root);
        assert_eq!(typ.to_byte(), byte);
    }

    fn varint(value: u64) -> Vec<u8> {
        let mut buf = [0u8; 10];
        let written = integer_encoding::VarInt::encode_var(value, &mut buf);
        buf[..written].to_vec()
    }

    #[rstest]
    #[case::no_m_values(false, 0, 0, &[0x00])]
    #[case::three_columns(false, 3, 0, &[0x03])]
    #[case::one_m_value(true, 0, 1, &[0x02])]
    #[case::three_columns_two_m_values(true, 3, 2, &[0b0000_1101])]
    #[case::one_byte_limits(true, 15, 7, &[0x7F])]
    #[case::sixteen_columns_spill_over(true, 16, 1, &[0x82, 0x02])]
    #[case::eight_m_values_spill_over(true, 0, 8, &[0x80, 0x01])]
    fn column_counts_roundtrip(
        #[case] m_values: bool,
        #[case] columns: u32,
        #[case] m_value_columns: u32,
        #[case] bytes: &[u8],
    ) {
        let counts = ColumnCounts {
            columns,
            m_values: m_value_columns,
        };
        assert_eq!(varint(counts.to_varint()), bytes);
        assert_eq!(
            ColumnCounts::parse(bytes, m_values).unwrap(),
            (&[][..], counts)
        );
    }

    #[rstest]
    #[case::no_columns(&[0x00])]
    #[case::columns_only(&[0x05])]
    fn column_counts_reject_a_flagged_section_without_m_values(#[case] bytes: &[u8]) {
        let err = ColumnCounts::parse(bytes, true).unwrap_err();
        assert!(matches!(err, MltError::EmptyMValueSection));
    }

    #[rstest]
    #[case::unassigned_geo_layout(0b0000_1110)]
    #[case::unassigned_geo_layout_with_shared(0b0010_1111)]
    fn layer_layout_byte_rejects_unassigned_geo_layout(#[case] byte: u8) {
        let err = LayerLayout::parse(byte).unwrap_err();
        assert!(
            matches!(err, MltError::ParsingGeoLayout(b) if b == byte & LayerLayout::GEO_LAYOUT_MASK)
        );
    }
}