symbios-shape 0.4.0

use serde::{Deserialize, Serialize};

use crate::expr::Expr;
use crate::model::Material;
use crate::scope::Vec3;

/// A reference to a production rule, optionally carrying call arguments.
///
/// Every successor position in the grammar — bare rule ops, split slots,
/// `Comp` / `Offset` / `Roof` / `Attach` cases, occlusion conditionals — is a
/// `RuleCall`. Arguments are expressions evaluated in the *calling* shape's
/// context at push time; the callee binds the resulting values to its
/// declared parameter names (see `Interpreter::add_rule_def`).
///
/// ```text
/// Spire(4)                       // bare call with one argument
/// Split(Y) { 3: Base | ~1: Tier(depth + 1) }
/// ```
#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
pub struct RuleCall {
    pub name: String,
    /// Call arguments; empty for plain references. Skipped in serde when
    /// empty so pre-0.3 serialized ops round-trip unchanged.
    #[serde(default, skip_serializing_if = "Vec::is_empty")]
    pub args: Vec<Expr>,
}

impl RuleCall {
    /// Plain, argument-less reference.
    pub fn new(name: impl Into<String>) -> Self {
        Self {
            name: name.into(),
            args: Vec::new(),
        }
    }

    /// Reference with call arguments.
    pub fn with_args(name: impl Into<String>, args: Vec<Expr>) -> Self {
        Self {
            name: name.into(),
            args,
        }
    }
}

impl From<&str> for RuleCall {
    fn from(name: &str) -> Self {
        Self::new(name)
    }
}

/// Argument-less calls compare equal to their bare name — keeps assertions
/// and look-ups terse (`assert_eq!(slot.rule, "Floor")`). A call *with*
/// arguments never equals a bare name.
impl PartialEq<&str> for RuleCall {
    fn eq(&self, other: &&str) -> bool {
        self.args.is_empty() && self.name == *other
    }
}

impl PartialEq<str> for RuleCall {
    fn eq(&self, other: &str) -> bool {
        self.args.is_empty() && self.name == other
    }
}

impl From<String> for RuleCall {
    fn from(name: String) -> Self {
        Self::new(name)
    }
}

/// Optional snap-binding attached to a `Split` op.
///
/// When set, after the slot sizes are resolved the interior boundaries are
/// snapped to the nearest registered snap-plane along the split axis carrying
/// the matching `label`, provided the snap-plane lies within `tolerance`
/// world-space units of the resolved boundary. Slots on either side of a
/// snapped boundary stretch / shrink to absorb the offset; total scope
/// length is preserved.
#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
pub struct SnapBinding {
    /// Group label of snap-planes to align to (matches `RegSnap("label")`).
    pub label: String,
    /// Maximum world-space distance between a slot boundary and a snap-plane
    /// for the snap to apply. When `None`, defaults to `5%` of the split-axis
    /// scope length at interpret time.
    pub tolerance: Option<f64>,
}

/// The axis along which a `Split` or `Repeat` operation acts.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
pub enum Axis {
    X,
    Y,
    Z,
}

/// Sizing mode for a single slot within a `Split` operation.
///
/// Mirrors CityEngine CGA syntax:
/// - `Absolute(e)`: a fixed world-unit size.
/// - `Relative(e)`: a fraction of the scope's total dimension (prefix `'` in CGA text).
/// - `Floating(e)`: a weight that shares the remaining space after absolutes are consumed
///   (prefix `~` in CGA text). Multiple floating slots divide the remainder proportionally.
///
/// Sizes are [`Expr`]s evaluated per shape at derivation time; validation
/// (finite, positive) happens on the evaluated value in the interpreter.
#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
pub enum SplitSize {
    Absolute(Expr),
    Relative(Expr),
    Floating(Expr),
}

impl SplitSize {
    /// Convenience constructors for literal sizes (tests, programmatic use).
    pub fn abs(v: f64) -> Self {
        SplitSize::Absolute(Expr::lit(v))
    }
    pub fn rel(v: f64) -> Self {
        SplitSize::Relative(Expr::lit(v))
    }
    pub fn float(v: f64) -> Self {
        SplitSize::Floating(Expr::lit(v))
    }

    /// The size expression, whatever the mode.
    pub fn expr(&self) -> &Expr {
        match self {
            SplitSize::Absolute(e) | SplitSize::Relative(e) | SplitSize::Floating(e) => e,
        }
    }

    /// Mutable access to the size expression (genetics mutation hook).
    pub fn expr_mut(&mut self) -> &mut Expr {
        match self {
            SplitSize::Absolute(e) | SplitSize::Relative(e) | SplitSize::Floating(e) => e,
        }
    }
}

/// A single slot in a `Split` operation: a size mode paired with a successor rule call.
#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
pub struct SplitSlot {
    pub size: SplitSize,
    /// The shape rule invoked on the resulting child scope.
    pub rule: RuleCall,
}

/// One entry in a `Split` body: a single slot, or a rhythm group
/// `{ a | b }*` whose slot pattern repeats to fill the space left by the
/// entries outside it.
///
/// ```text
/// Split(X) { 1.2: Corner | { 0.5: Pier | ~1: Win }* | 1.2: Corner }
/// ```
///
/// Constraints (enforced at parse / derivation): at most **one** group per
/// split, no nested groups. Allocation: fixed entries outside the group are
/// placed first; the group tiles `k` whole copies of its nominal width into
/// the remainder; leftover space goes to floating slots outside the group,
/// or — when there are none — the copies stretch uniformly to close the gap
/// exactly. Sizes inside a copy resolve like a mini-split of the copy width.
#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
pub enum SplitEntry {
    Slot(SplitSlot),
    Group(Vec<SplitSlot>),
}

impl SplitEntry {
    /// The single slot, when this entry is not a group.
    pub fn as_slot(&self) -> Option<&SplitSlot> {
        match self {
            SplitEntry::Slot(s) => Some(s),
            SplitEntry::Group(_) => None,
        }
    }
}

impl From<SplitSlot> for SplitEntry {
    fn from(s: SplitSlot) -> Self {
        SplitEntry::Slot(s)
    }
}

/// Face selectors for the `Comp(Faces)` decomposition.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Serialize, Deserialize)]
pub enum FaceSelector {
    Top,
    Bottom,
    Front,
    Back,
    Left,
    Right,
    /// Matches all non-top, non-bottom faces (shorthand for the four sides).
    Side,
    /// Matches all faces not otherwise mapped.
    All,
}

impl FaceSelector {
    pub fn parse(s: &str) -> Option<Self> {
        match s {
            "top" | "Top" => Some(Self::Top),
            "bottom" | "Bottom" => Some(Self::Bottom),
            "front" | "Front" => Some(Self::Front),
            "back" | "Back" => Some(Self::Back),
            "left" | "Left" => Some(Self::Left),
            "right" | "Right" => Some(Self::Right),
            "side" | "Side" => Some(Self::Side),
            "all" | "All" | "_" => Some(Self::All),
            _ => None,
        }
    }
}

/// A single mapping in a `Comp(Faces)` block: a face selector → rule name.
#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
pub struct CompFaceCase {
    pub selector: FaceSelector,
    pub rule: RuleCall,
}

/// Edge-class selectors for the `Comp(Edges)` decomposition.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Serialize, Deserialize)]
pub enum EdgeSelector {
    /// Vertical edges: a volume's four corner posts; a face's left/right rim.
    Vertical,
    /// All horizontal edges (both rings on a volume; top+bottom on a face).
    Horizontal,
    /// The top horizontal ring / edge only.
    Top,
    /// The bottom horizontal ring / edge only.
    Bottom,
    /// Matches all edges not otherwise mapped.
    All,
}

impl EdgeSelector {
    pub fn parse(s: &str) -> Option<Self> {
        match s {
            "vertical" | "Vertical" => Some(Self::Vertical),
            "horizontal" | "Horizontal" => Some(Self::Horizontal),
            "top" | "Top" => Some(Self::Top),
            "bottom" | "Bottom" => Some(Self::Bottom),
            "all" | "All" | "_" => Some(Self::All),
            _ => None,
        }
    }
}

/// A single mapping in a `Comp(Edges)` block: an edge selector → rule call.
#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
pub struct CompEdgeCase {
    pub selector: EdgeSelector,
    pub rule: RuleCall,
}

/// The decomposition target for a `Comp` operation.
#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
pub enum CompTarget {
    /// Decomposes the volume into its six axis-aligned face scopes.
    Faces(Vec<CompFaceCase>),
    /// Decomposes into zero-cross-section edge scopes: 12 for a volume,
    /// 4 for a face. Local X runs along the edge; give the scope thickness
    /// with `Size(scope.x, t, t)` and centre it with `Translate`.
    Edges(Vec<CompEdgeCase>),
}

/// Face selectors for the `Offset` operation.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Serialize, Deserialize)]
pub enum OffsetSelector {
    /// The inset/outset region (the area inside the border).
    Inside,
    /// The surrounding border strips.
    Border,
    /// Matches any selector not otherwise mapped.
    All,
}

impl OffsetSelector {
    pub fn parse(s: &str) -> Option<Self> {
        match s {
            "inside" | "Inside" => Some(Self::Inside),
            "border" | "Border" => Some(Self::Border),
            "all" | "All" | "_" => Some(Self::All),
            _ => None,
        }
    }
}

/// A single mapping in an `Offset` block: a selector → rule name.
#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
pub struct OffsetCase {
    pub selector: OffsetSelector,
    pub rule: RuleCall,
}

/// Roof shape types for the `Roof` operation.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
pub enum RoofType {
    // ── Original types ────────────────────────────────────────────────────────
    /// Four triangular slope panels meeting at a single apex.
    Pyramid,
    /// Single slope panel from front eave to back eave.
    Shed,
    /// Two slope panels meeting at a horizontal ridge; two triangular gable ends.
    Gable,
    /// Four trapezoidal slope panels meeting at a horizontal ridge.
    Hip,

    // ── New types ─────────────────────────────────────────────────────────────
    /// Flat horizontal roof — a single horizontal panel covering the scope top.
    Flat,
    /// Two rectangular slope panels only (Gable without the triangular end panels).
    OpenGable,
    /// Two slope panels + two rectangular (non-tapered) gable-end wall panels.
    BoxGable,
    /// Four panels from a rectangular base meeting at a single apex point (no ridge).
    /// Equivalent to `Pyramid` for square footprints; left/right end panels are triangular.
    PyramidHip,
    /// Two inward-tilting slopes forming a central valley (inverted Gable).
    Butterfly,
    /// Four panels forming two parallel ridges with a central valley between them (M profile).
    MShaped,
    /// Two pitches per slope: steeper lower zone + shallower upper zone (barn roof).
    /// Requires `secondary_pitch` in `RoofConfig`.
    Gambrel,
    /// Gambrel applied to all four sides: 4 steep lower panels + 4 shallow upper panels.
    /// Requires `secondary_pitch` in `RoofConfig`.
    Mansard,
    /// Asymmetric Gable: the ridge is offset toward one end (`ridge_offset` in `RoofConfig`).
    /// Front slope is steeper; back slope is shallower. Gable ends are asymmetric triangles.
    Saltbox,
    /// Gable with clipped hip ends: the upper corners of each gable end are replaced by
    /// small triangular hip panels. Controlled by `tier_height` in `RoofConfig`.
    Jerkinhead,
    /// Hip roof with a small gable rising from the ridge centre.
    /// Controlled by `tier_height` (fraction of slope from base where the gable starts).
    DutchGable,
}

impl RoofType {
    pub fn parse(s: &str) -> Option<Self> {
        match s {
            "pyramid" | "Pyramid" => Some(Self::Pyramid),
            "shed" | "Shed" => Some(Self::Shed),
            "gable" | "Gable" => Some(Self::Gable),
            "hip" | "Hip" => Some(Self::Hip),
            "flat" | "Flat" => Some(Self::Flat),
            "openGable" | "OpenGable" => Some(Self::OpenGable),
            "boxGable" | "BoxGable" => Some(Self::BoxGable),
            "pyramidHip" | "PyramidHip" => Some(Self::PyramidHip),
            "butterfly" | "Butterfly" => Some(Self::Butterfly),
            "mShaped" | "MShaped" => Some(Self::MShaped),
            "gambrel" | "Gambrel" => Some(Self::Gambrel),
            "mansard" | "Mansard" => Some(Self::Mansard),
            "saltbox" | "Saltbox" => Some(Self::Saltbox),
            "jerkinhead" | "Jerkinhead" => Some(Self::Jerkinhead),
            "dutchGable" | "DutchGable" => Some(Self::DutchGable),
            _ => None,
        }
    }
}

/// Face selectors for the `Roof` operation.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Serialize, Deserialize)]
pub enum RoofFaceSelector {
    /// The main sloped panel(s) — front/back in most roof types.
    Slope,
    /// The triangular vertical end panels of a Gable or Saltbox roof.
    GableEnd,
    /// The steeper, lower zone of a Gambrel or Mansard roof.
    LowerSlope,
    /// The shallower, upper zone of a Gambrel or Mansard roof.
    UpperSlope,
    /// The small triangular hip panels at the clipped ends of a Jerkinhead roof.
    HipEnd,
    /// The inward-facing slopes of a Butterfly or MShaped valley.
    ValleySlope,
    /// The outer slopes of an MShaped roof (facing away from the valley).
    OuterSlope,
    /// The inner slopes of an MShaped roof (facing toward the valley).
    InnerSlope,
    /// The vertical back wall of a `Shed` roof — the raised face under the
    /// high eave (the glazed "northlight" of a sawtooth factory profile).
    Back,
    /// Vertical fascia bands hanging below the eaves.
    /// Generated when [`RoofConfig::fascia_depth`] is `> 0`. One panel per perimeter
    /// slope eave; supported for all roof types whose slope panels share a horizontal
    /// eave at the perimeter (Gable, Hip, Pyramid, Shed, Saltbox, Jerkinhead, DutchGable,
    /// Gambrel, Mansard, MShaped, BoxGable, OpenGable, PyramidHip). `Flat` and `Butterfly`
    /// have no perimeter eave at the slope-panel level and produce no fascia panels.
    Fascia,
    /// Matches any selector not otherwise mapped.
    All,
}

impl RoofFaceSelector {
    pub fn parse(s: &str) -> Option<Self> {
        match s {
            "slope" | "Slope" => Some(Self::Slope),
            "gable" | "GableEnd" | "gableEnd" => Some(Self::GableEnd),
            "lowerSlope" | "LowerSlope" => Some(Self::LowerSlope),
            "upperSlope" | "UpperSlope" => Some(Self::UpperSlope),
            "hipEnd" | "HipEnd" => Some(Self::HipEnd),
            "valleySlope" | "ValleySlope" => Some(Self::ValleySlope),
            "outerSlope" | "OuterSlope" => Some(Self::OuterSlope),
            "innerSlope" | "InnerSlope" => Some(Self::InnerSlope),
            "back" | "Back" => Some(Self::Back),
            "fascia" | "Fascia" => Some(Self::Fascia),
            "all" | "All" | "_" => Some(Self::All),
            _ => None,
        }
    }
}

/// A single mapping in a `Roof` block: a face selector → rule name.
#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
pub struct RoofCase {
    pub selector: RoofFaceSelector,
    pub rule: RuleCall,
}

/// Rich parametric configuration for the `Roof` operation.
///
/// All angular values are in degrees. Lengths are in world units.
/// Optional fields default as described; see each field doc.
#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
pub struct RoofConfig {
    pub roof_type: RoofType,
    /// Primary pitch angle in degrees from horizontal. Must be in (0°, 90°).
    pub pitch: f64,
    /// Secondary pitch angle in degrees. Used by `Gambrel` (upper zone) and `Mansard`.
    /// If `None` when required, defaults to `pitch / 2`.
    pub secondary_pitch: Option<f64>,
    /// Extra overhang beyond the scope footprint on each side. Default `0.0`.
    pub overhang: f64,
    /// Ridge offset for `Saltbox`: fraction [0, 1] of the scope depth (Z) where the
    /// ridge is positioned from the front. Default `0.5` (symmetric / centred ridge).
    pub ridge_offset: f64,
    /// Thickness of the roof fascia edge in world units. Default `0.0` (flat panels).
    pub fascia_depth: f64,
    /// Normalised tier parameter for `Gambrel`, `Mansard`, `Jerkinhead`, and
    /// `DutchGable`. For the pitch-break types (`Gambrel`, `Mansard`,
    /// `DutchGable`) it is the height at which the break occurs: `0.5` means
    /// the break is at half the eave-to-ridge distance. For `Jerkinhead` it
    /// is the **fraction of the half-depth that is clipped**: `0.25` clips a
    /// quarter of each gable end into a hip-let, and larger values clip
    /// more. `None` uses a type-specific default.
    pub tier_height: Option<f64>,
}

impl RoofConfig {
    /// Creates a minimal config for deterministic types (Pyramid, Shed, Gable, Hip, Flat, …).
    pub fn new(roof_type: RoofType, pitch: f64) -> Self {
        Self {
            roof_type,
            pitch,
            secondary_pitch: None,
            overhang: 0.0,
            ridge_offset: 0.5,
            fascia_depth: 0.0,
            tier_height: None,
        }
    }

    /// Returns the secondary pitch, defaulting to `pitch / 2` if unset.
    pub fn secondary_pitch_or_default(&self) -> f64 {
        self.secondary_pitch.unwrap_or(self.pitch / 2.0)
    }

    /// Returns the tier height, defaulting to `default` if unset.
    pub fn tier_height_or(&self, default: f64) -> f64 {
        self.tier_height.unwrap_or(default)
    }
}

/// Expression-valued roof parameters as they appear in the grammar.
///
/// The interpreter evaluates every field against the current shape's context
/// and produces a resolved [`RoofConfig`] for the geometry builder. `pitch`
/// and `height` are mutually exclusive ways to set the roof's steepness:
/// when `height` is `Some`, the pitch is derived from it and the scope's
/// half-span at derivation time (CGA `byHeight` parity), letting mixed-width
/// wings share one ridge line.
#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
pub struct RoofSpec {
    pub roof_type: RoofType,
    /// Primary pitch angle in degrees, exclusive range (0°, 90°). Ignored
    /// when `height` is set.
    pub pitch: Expr,
    /// Absolute roof rise in world units (`height=` named arg). Overrides
    /// `pitch` when present.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub height: Option<Expr>,
    /// Secondary pitch in degrees for `Gambrel` / `Mansard`.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub secondary_pitch: Option<Expr>,
    /// Eave overhang beyond the footprint, world units. Default `0`.
    pub overhang: Expr,
    /// Ridge offset fraction for `Saltbox`. Default `0.5`.
    pub ridge_offset: Expr,
    /// Fascia band depth below each perimeter eave. Default `0`.
    pub fascia_depth: Expr,
    /// Pitch-break height fraction for tiered types.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub tier_height: Option<Expr>,
    /// Forces the ridge to run along the given scope axis (`ridge=X` /
    /// `ridge=Z`), overriding the default longest-axis heuristic.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub ridge_axis: Option<Axis>,
}

/// Wraps every numeric field of a resolved config back into literal
/// expressions — the programmatic bridge for builders that think in numbers.
impl From<RoofConfig> for RoofSpec {
    fn from(c: RoofConfig) -> Self {
        Self {
            roof_type: c.roof_type,
            pitch: Expr::lit(c.pitch),
            height: None,
            secondary_pitch: c.secondary_pitch.map(Expr::lit),
            overhang: Expr::lit(c.overhang),
            ridge_offset: Expr::lit(c.ridge_offset),
            fascia_depth: Expr::lit(c.fascia_depth),
            tier_height: c.tier_height.map(Expr::lit),
            ridge_axis: None,
        }
    }
}

impl RoofSpec {
    /// Literal spec with defaults matching `RoofConfig::new` — the
    /// programmatic construction path for tests and builders.
    pub fn new(roof_type: RoofType, pitch: f64) -> Self {
        Self {
            roof_type,
            pitch: Expr::lit(pitch),
            height: None,
            secondary_pitch: None,
            overhang: Expr::lit(0.0),
            ridge_offset: Expr::lit(0.5),
            fascia_depth: Expr::lit(0.0),
            tier_height: None,
            ridge_axis: None,
        }
    }
}

/// Selector for the `Attach` operation.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Serialize, Deserialize)]
pub enum AttachSelector {
    /// The projected scope that sits on (or comes out of) the surface.
    Surface,
    /// Matches any selector not otherwise mapped.
    All,
}

impl AttachSelector {
    pub fn parse(s: &str) -> Option<Self> {
        match s {
            "surface" | "Surface" => Some(Self::Surface),
            "all" | "All" | "_" => Some(Self::All),
            _ => None,
        }
    }
}

/// A single mapping in an `Attach` block: a selector → rule name.
#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
pub struct AttachCase {
    pub selector: AttachSelector,
    pub rule: RuleCall,
}

/// How one variant of a rule is selected during derivation.
#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
pub enum VariantSelector {
    /// Stochastic: relative weight among the rule's weighted variants
    /// (`70% ops | 30% ops`). Weights need not sum to 1.
    Weight(f64),
    /// Guarded: taken when the expression evaluates non-zero, top-down
    /// (`when(scope.x < 4): ops`). A guarded rule's variants are evaluated
    /// in order; the first true guard wins.
    When(Expr),
    /// Fallback for a guarded rule (`else: ops`); must be last. In weighted
    /// rules `else:` is parse-time sugar resolved into a `Weight` of the
    /// remaining probability mass, so it never reaches the interpreter.
    Else,
}

/// One alternative in a rule: how it is selected, and what it does.
#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
pub struct RuleVariant {
    pub selector: VariantSelector,
    pub ops: Vec<ShapeOp>,
}

impl RuleVariant {
    /// Weighted variant — the pre-0.3 shape.
    pub fn weighted(weight: f64, ops: Vec<ShapeOp>) -> Self {
        Self {
            selector: VariantSelector::Weight(weight),
            ops,
        }
    }

    /// The stochastic weight, when this variant is weighted.
    pub fn weight(&self) -> Option<f64> {
        match self.selector {
            VariantSelector::Weight(w) => Some(w),
            _ => None,
        }
    }
}

/// Region selectors for the `ShapeL` / `ShapeU` footprint-carving ops.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Serialize, Deserialize)]
pub enum CarveSelector {
    /// The carved letter shape. `ShapeL` delivers it as TWO rectangular
    /// scopes (front bar + side leg), `ShapeU` as three — OBB purity means a
    /// letter footprint is a set of boxes, never one polygon.
    Shape,
    /// The rectangular remainder cut away from the letter.
    Remainder,
    /// Matches any selector not otherwise mapped.
    All,
}

impl CarveSelector {
    pub fn parse(s: &str) -> Option<Self> {
        match s {
            "shape" | "Shape" => Some(Self::Shape),
            "remainder" | "Remainder" | "rest" | "Rest" => Some(Self::Remainder),
            "all" | "All" | "_" => Some(Self::All),
            _ => None,
        }
    }
}

/// A single mapping in a `ShapeL` / `ShapeU` block: selector → rule call.
#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
pub struct CarveCase {
    pub selector: CarveSelector,
    pub rule: RuleCall,
}

/// One candidate in a `Fit` op: the minimum extent it needs, and the rule
/// invoked on the whole scope when it is the first that fits.
#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
pub struct FitCandidate {
    pub min_size: Expr,
    pub rule: RuleCall,
}

/// The atomic CGA operations that the interpreter executes.
///
/// Every operation transforms the current `Scope` into zero or more child scopes,
/// each tagged with a rule name that will be recursively evaluated.
#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
pub enum ShapeOp {
    /// Lifts a 2-D footprint (XZ plane) into a 3-D volume by setting the Y size.
    Extrude(Expr),

    /// Pyramidal taper: scales the top face toward the centroid.
    /// `amount` ∈ `[0, 1]`: 0 = no taper, 1 = full pyramid (top collapses to a point).
    Taper(Expr),

    /// Applies an additional rotation to the scope (cumulative with existing rotation).
    /// Components are `(w, x, y, z)` in grammar order; the evaluated quaternion
    /// is normalized at derivation time.
    Rotate([Expr; 4]),

    /// Translates the scope origin in local space.
    Translate([Expr; 3]),

    /// Scales the scope size along each axis (multiplicative).
    Scale([Expr; 3]),

    /// Divides the scope along `axis` into ordered slots.
    ///
    /// When `snap` is `Some`, interior slot boundaries are snapped to the
    /// nearest registered snap-plane along `axis` (see [`SnapBinding`]).
    Split {
        axis: Axis,
        entries: Vec<SplitEntry>,
        snap: Option<SnapBinding>,
    },

    /// Divides the scope along `axis` by *target areas* instead of lengths.
    ///
    /// Slot sizes are read as areas: absolute = square units, relative (`'`)
    /// = fraction of the face area, floating (`~`) = share of the remaining
    /// area. Lengths are recovered by dividing through the cross-axis extent,
    /// so this is only meaningful on the horizontal axes of a footprint-like
    /// scope; `SplitArea(Y)` is rejected.
    ///
    /// Syntax: `SplitArea(X) { 30: Lot | ~1: Rest }`
    SplitArea { axis: Axis, slots: Vec<SplitSlot> },

    /// Size-fallback choice: invokes the first candidate whose minimum
    /// extent fits the scope along `axis`; candidates are tried in order and
    /// the scope vanishes when none fits (use a `0:` catch-all to avoid
    /// that).
    ///
    /// Syntax: `Fit(X) { 2.2: DoorBay | 1.2: WinBay | 0: Wall }`
    Fit {
        axis: Axis,
        candidates: Vec<FitCandidate>,
    },

    /// Tiles the scope along `axis` with child scopes drawn from `tile_sizes`.
    ///
    /// `tile_sizes` is a per-slot pattern that is cycled to fill the axis range.
    /// Tiles are appended greedily (next tile from the cycle is added while it
    /// still fits inside the remaining range), then **all** placed tiles are
    /// scaled by the same factor `total / Σ(placed)` so they fill the scope
    /// exactly with no gap and no overshoot.
    ///
    /// A single-element list `[t]` is the legacy uniform `Repeat(axis, t)`.
    /// A multi-element list `[a, b, c]` produces an `…, a, b, c, a, b, c, …`
    /// cadence where each tile's relative width is preserved.
    Repeat {
        axis: Axis,
        tile_sizes: Vec<Expr>,
        rule: RuleCall,
    },

    /// Decomposes the scope into its geometric components (faces, edges, vertices).
    Comp(CompTarget),

    /// Terminal: replace the scope with the named mesh asset.
    /// This is the "terminal symbol" — produces a `Terminal` node in the output model.
    I(String),

    /// Sets the material on the current work item.
    /// The material is propagated to the final `Terminal`, allowing downstream
    /// renderers to apply textures / shaders and physics consumers to derive
    /// volumetric mass properties without changing the scope.
    ///
    /// Syntax:
    /// - `Mat("Brick")` / `Mat(Brick)` — id-only material; no density.
    /// - `Mat("Brick", 1800)` — id + density in kg/m³; the interpreter computes
    ///   [`crate::model::MassProperties`] for terminals stamped with this material.
    Mat(Material),

    /// Calls a named sub-rule on the current scope unchanged, optionally
    /// passing call arguments (`Tier(depth + 1)`).
    /// Used for grammar rule references that don't transform the scope themselves.
    Rule(RuleCall),

    /// Sets the scope size to absolute world-unit values (CGA `s()` parity).
    /// Components must be finite and non-negative; `0` flattens the axis
    /// (face-scope semantics). Essential for sizing the zero-extent scopes
    /// produced by `Scatter` and `Comp(Edges)`.
    ///
    /// Syntax: `Size(2.1, 0.9, 0.12)` — expressions welcome:
    /// `Size(scope.x, 0.3, 0.3)`.
    Size([Expr; 3]),

    /// Re-centres the scope inside the axis-aligned bounds it occupied when
    /// the current rule was entered, along the masked axes. The scope must
    /// have been shrunk (e.g. by `Size`) for this to move anything.
    ///
    /// Syntax: `Center(X)`, `Center(XY)`, `Center(XYZ)` …
    Center { x: bool, y: bool, z: bool },

    /// Mirrors the *pending face profile* horizontally (Triangle peak,
    /// Trapezoid offset, Polygon points). Scope geometry is untouched —
    /// terminals carry rotations, not reflections, so a scope-level mirror
    /// cannot exist in this engine. Apply after the profile is set.
    ///
    /// Syntax: `Mirror(X)` (only X — profiles are 2-D, mirrored across
    /// their vertical centre line).
    Mirror,

    /// Carves an L footprint: a front bar of depth `front` (along local Z
    /// from the scope origin) plus a side leg of width `side` (along local X)
    /// over the remaining depth. The `Shape` selector receives both boxes;
    /// `Remainder` receives the cut-away rectangle.
    ///
    /// Syntax: `ShapeL(4, 3) { Shape: Wing | Remainder: Court }`
    ShapeL {
        front: Expr,
        side: Expr,
        cases: Vec<CarveCase>,
    },

    /// Carves a U footprint: a front bar plus left and right legs; the
    /// remainder is the inner court between the legs behind the bar.
    ///
    /// Syntax: `ShapeU(4, 3, 3) { Shape: Range | Remainder: Court }`
    ShapeU {
        front: Expr,
        left: Expr,
        right: Expr,
        cases: Vec<CarveCase>,
    },

    /// Rotates the scope so that the specified local axis points in the given world direction.
    ///
    /// Applies the shortest-arc rotation from the current world direction of `local_axis`
    /// to `target`. Useful for recovering from accumulated rotations.
    /// Syntax: `Align(Y, Up)`, `Align(Z, Forward)`, etc.
    /// Named targets: `Up`=(0,1,0), `Down`=(0,-1,0), `Right`=(1,0,0), `Left`=(-1,0,0),
    /// `Forward`=(0,0,-1), `Back`=(0,0,1).
    Align { local_axis: Axis, target: Vec3 },

    /// Creates an inset (`distance < 0`) frame on a 2D face scope.
    ///
    /// Produces up to two kinds of child scopes:
    /// - `Inside`: the inset rectangle.
    /// - `Border`: four surrounding strips (bottom, top, left, right), each invoking the same rule.
    ///
    /// Syntax: `Offset(-0.2) { Inside: Glass | Border: Frame }`
    Offset {
        distance: Expr,
        cases: Vec<OffsetCase>,
    },

    /// Generates a roof structure above the current scope using rich parametric configuration.
    ///
    /// Operates on a volume scope. The `config` contains the roof type, primary pitch angle,
    /// optional secondary pitch, overhang, ridge offset, fascia depth, and tier height.
    ///
    /// Syntax examples:
    /// - `Roof(Gable, 30) { Slope: Tiles | GableEnd: Bricks }` — basic Gable
    /// - `Roof(Hip, 30, 0.5) { Slope: Tiles }` — Hip with overhang
    /// - `Roof(Gambrel, 45, 20) { LowerSlope: Shingles | UpperSlope: Tiles }` — Gambrel
    /// - `Roof(Saltbox, 45, offset=0.3) { Slope: Tiles | GableEnd: Bricks }` — Saltbox
    /// - `Roof(DutchGable, 45, tier=0.7) { Slope: Tiles | GableEnd: Bricks }` — Dutch Gable
    Roof {
        spec: RoofSpec,
        cases: Vec<RoofCase>,
    },

    /// Registers all six face planes of the current scope as snap-planes
    /// under the given label. Subsequent `Split(snap="label")` ops can align
    /// their interior boundaries to these planes. Read-only with respect to
    /// the scope (the scope itself passes through unchanged).
    ///
    /// Syntax: `RegSnap("bays")`
    RegSnap(String),

    /// Conditionally invokes `rule` on the current scope only when no
    /// already-emitted terminal occludes the scope (true OBB overlap test).
    /// Useful for placing decorative elements that should only appear where
    /// no structural element has already been placed.
    ///
    /// The grammar author is responsible for ordering — terminals derived
    /// before this op participate in the test; later terminals do not.
    ///
    /// Syntax: `IfClear { Window }` / `IfClear("chimneys") { Window }` —
    /// the optional label restricts the test to terminals stamped with that
    /// `Label`.
    IfClear {
        rule: RuleCall,
        #[serde(default, skip_serializing_if = "Option::is_none")]
        label: Option<String>,
    },

    /// Inverse of [`ShapeOp::IfClear`]: invokes `rule` only when the current
    /// scope **is** occluded by an already-emitted terminal.
    ///
    /// Syntax: `IfOccluded { Patch }` / `IfOccluded("roof") { Patch }`.
    IfOccluded {
        rule: RuleCall,
        #[serde(default, skip_serializing_if = "Option::is_none")]
        label: Option<String>,
    },

    /// Graded occlusion: invokes `rule` only when the scope is FULLY inside
    /// a single already-emitted terminal (optionally of one label class).
    ///
    /// Syntax: `IfInside { Core }` / `IfInside("mass") { Core }`
    IfInside {
        rule: RuleCall,
        #[serde(default, skip_serializing_if = "Option::is_none")]
        label: Option<String>,
    },

    /// Graded occlusion: invokes `rule` only when the scope is in surface
    /// contact with a terminal — overlapping at a hair's growth but not at a
    /// hair's shrinkage (optionally restricted to one label class).
    ///
    /// Syntax: `IfTouches { Trim }` / `IfTouches("walls") { Trim }`
    IfTouches {
        rule: RuleCall,
        #[serde(default, skip_serializing_if = "Option::is_none")]
        label: Option<String>,
    },

    /// Coordination key: a weighted choice resolved once per derivation —
    /// every `Pick` with the same key picks the SAME index, wherever it
    /// appears in the tree. The poor man's CGA++ event: all floors agree on
    /// one window variant, front and back facades match.
    ///
    /// The choice is a pure function of `(interpreter seed, key)` — no
    /// derivation-order dependence.
    ///
    /// Syntax: `Pick("winStyle") { 60% WinA | 40% WinB }`
    Pick {
        key: String,
        /// `(weight, successor)` pairs; weights need not sum to 1.
        choices: Vec<(f64, RuleCall)>,
    },

    /// Stamps an occlusion label on subsequent terminals of this branch
    /// (propagates like `Mat`). Labelled terminals form a named class the
    /// occlusion conditionals can filter on.
    ///
    /// Syntax: `Label("chimneys")`
    Label(String),

    /// Scatters `count` zero-size point scopes uniformly over the scope's
    /// top face (`Top`) or through its volume (`Volume`), invoking `rule` on
    /// each. Points are drawn from the shape's RNG stream (seed-stable);
    /// give them extent with `Size(..)`. `count` is capped at 1024.
    ///
    /// Syntax: `Scatter(Top, 12) { Bush }`
    Scatter {
        volume: bool,
        count: Expr,
        rule: RuleCall,
    },

    /// Stamps an explicit polygonal `FaceProfile` on the next terminal in this rule.
    ///
    /// Mirrors how [`ShapeOp::Taper`] sets a profile override: the next `I(...)`
    /// (or implicit terminal) emits a `Terminal` whose `face_profile` is
    /// [`crate::model::FaceProfile::Polygon`] with the provided vertex list.
    /// Vertices are 2-D points in **normalized `[0, 1]²` scope coordinates**
    /// (X: 0 = left edge → 1 = right edge; Y: 0 = bottom → 1 = top of the
    /// face); the renderer triangulates and stretches them across the
    /// scope's extent. They are NOT world units.
    ///
    /// Syntax: `Polygon((0,0), (4,0), (4,2), (2,2), (2,4), (0,4))`
    /// (variadic `(x,y)` list, capped at 256 vertices for parser DoS hardening).
    Polygon(Vec<glam::DVec2>),

    /// Projects a new horizontal scope out of a sloped face for attaching dormers or details.
    ///
    /// `world_axis` defines the "up" direction for the attached scope (usually world Y).
    /// The resulting scope sits on the face's surface with its Y axis aligned to `world_axis`,
    /// inheriting the face's width and height but with depth = 0.
    ///
    /// Syntax: `Attach(Up) { Surface: DormerMass }`
    Attach {
        world_axis: Vec3,
        cases: Vec<AttachCase>,
    },
}