makeover-geometry 0.2.0

//! The invariant half of the make-family design system.
//!
//! <!-- wiki: makeover-geometry -->
//!
//! [`makeover`] resolves colour, which varies by theme. This crate carries
//! everything that does not: spacing, radius, border width and the type scale.
//! The split is the same one Balanced Breakfast's theme contract has always
//! drawn — *a theme overrides colour tokens only* — moved out of two app
//! stylesheets so the three consumers stop maintaining three copies of it.
//!
//! # Spacing is relational, not numeric
//!
//! The Mac OS 8 Human Interface Guidelines specify white space by *what two
//! things are being separated*, never by a size name, and define no base grid
//! unit. A control and its satellite pop-up are set 4 pixels apart; peers
//! stacked in a list get 6; a group box's inner margin is 10; separated groups
//! and rows of push buttons get 12.
//!
//! That vocabulary is the primary interface here. [`Gap`] names the
//! relationship and the size follows from it, exactly as `surface-raised`
//! names an intent and the hex follows from it. The raw [`Step`] scale exists
//! underneath for distances a relationship does not describe, but reaching for
//! it is a smell worth a second look.
//!
//! Naming the relationship is what makes the rule reviewable. Whether a gap
//! should be 6px or 8px is unanswerable in isolation; whether two things are
//! peers is not.
//!
//! # Ratios, not pixel counts
//!
//! This is the deliberate departure from the HIG, which is specified in hard
//! device pixels because in 1997 there was one pixel density and one text
//! size. Every [`Step`] here is a [`Ratio`] of a single base unit, so the
//! whole system scales from one knob: `--geometry-base`, `1rem` by default.
//!
//! At the default base the ratios land exactly on the HIG's numbers — `Snug`
//! is three eighths of 16px, which is 6px — so nothing is lost in the
//! translation. What is gained is that the layout tracks the user's text size
//! instead of fighting it, an accessibility setting becomes one value rather
//! than a sweep, and the scale means the same thing at any display density.
//!
//! # Density presets
//!
//! Naming relationships instead of sizes is what makes a density preset
//! possible at all. [`Density`] changes what each relationship resolves to
//! without touching a single call site, because no call site names a size.
//! The mobile and desktop builds of a Tauri app should differ mostly by which
//! preset they emit, not by a parallel set of hand-written mode-scoped rules.
//!
//! The two presets are not one scaled copy of the other, and the asymmetry is
//! the point — it is also why a base scalar alone cannot express this. Touch
//! needs *more* room between things you tap, because a fingertip is coarser
//! than a pointer, and *less* room around the edges, because the screen is
//! small and outer margin is screen you do not get to use. So `Peer` and
//! `Section` open up under [`Density::Touch`] while `Pane` and `Page` tighten.
//! `Bound` never moves: it is the one relationship that says "these are one
//! object", and separating them on touch would say the opposite.
//!
//! # Surfaces, and why a TUI is not a third density
//!
//! [`Surface`] is the third axis and the one that carries this to alloy_tui. A
//! terminal is not a density preset; it is a surface whose smallest
//! representable step is one cell rather than one pixel. Give
//! [`Ratio::quanta`] a quantum and it answers in whole units of it, so the
//! relational vocabulary crosses to a character grid with nothing added.
//!
//! Quantising is not a terminal special case either — a display quantises to
//! the pixel. It is only that rounding 6.0 to the nearest pixel is
//! uninteresting, while rounding three eighths of a cell to the nearest cell
//! decides the layout.
//!
//! On [`Surface::terminal`] the pointer preset resolves to 0, 0, 1, 1, 2, 2
//! cells. `Bound` and `Peer` collapsing to nothing is correct rather than lossy:
//! in a grid that dense, both relationships are expressed by adjacency. A
//! coarse surface genuinely has fewer distinctions available, and the model
//! should say so instead of inventing a gap to keep six names distinct.
//!
//! So the three axes are: [`Gap`] is what is being separated, [`Density`] is
//! who is operating it, [`Surface`] is what it is drawn on.

#![forbid(unsafe_code)]

use std::fmt::Write as _;

/// The default base unit in CSS pixels, at a 16px root font size.
pub const DEFAULT_BASE_PX: u16 = 16;

/// The CSS custom property every ratio scales from.
pub const BASE_TOKEN: &str = "geometry-base";

/// A fraction of the base unit.
///
/// Rational rather than floating point so the scale is exact, comparable and
/// usable in a `const`. At the default base every ratio below divides evenly.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub struct Ratio {
    /// Top of the fraction.
    pub numerator: u16,
    /// Bottom of the fraction. Never zero for any ratio this crate defines.
    pub denominator: u16,
}

impl Ratio {
    /// Resolve against a base measured in whole pixels, rounding to nearest.
    ///
    /// Integer maths throughout, and exact for every [`Step`] at
    /// [`DEFAULT_BASE_PX`] because the scale is eighths. This is
    /// [`Self::quanta`] with a quantum of one pixel, kept separate only so the
    /// common case stays `const`.
    #[must_use]
    pub const fn px_at(self, base_px: u16) -> u16 {
        let (n, d) = (self.numerator as u32, self.denominator as u32);
        let scaled = base_px as u32 * n;
        // Round half away from zero without leaving integer arithmetic.
        ((scaled * 2 + d) / (d * 2)) as u16
    }

    /// Resolve against an arbitrary base, keeping the fraction.
    ///
    /// The exact value, before any surface gets a say. Prefer
    /// [`Surface::resolve`] unless you specifically want the unsnapped number.
    #[must_use]
    pub fn scale(self, base: f32) -> f32 {
        base * f32::from(self.numerator) / f32::from(self.denominator)
    }

    /// How many whole quanta this ratio is worth on a surface whose smallest
    /// representable step is `quantum`.
    ///
    /// The generalisation of "round to a pixel". A display quantises to one
    /// pixel and the answer is usually uninteresting; a terminal quantises to
    /// one cell and the answer is the whole design. Rounds to nearest, and
    /// does not floor at one: a gap that lands below half a quantum should
    /// collapse to nothing, because on that surface it *is* nothing.
    ///
    /// A `quantum` that is zero, negative or not finite yields `0` rather than
    /// panicking or returning infinity — a surface with no smallest step is a
    /// caller error, not a layout to guess at.
    #[must_use]
    pub fn quanta(self, base: f32, quantum: f32) -> u32 {
        if !quantum.is_finite() || quantum <= 0.0 || !base.is_finite() {
            return 0;
        }
        let exact = self.scale(base) / quantum;
        if exact <= 0.0 {
            0
        } else {
            // `as` saturates at the integer bound, so a wild base cannot wrap.
            exact.round() as u32
        }
    }

    /// Resolve against a base and snap to a whole number of `quantum`.
    ///
    /// The value [`Self::quanta`] counts, back in the surface's own units.
    /// Guards the degenerate quantum in its own right rather than leaning on
    /// [`Self::quanta`]: a count of zero times a non-finite quantum is NaN,
    /// not zero.
    #[must_use]
    pub fn quantize(self, base: f32, quantum: f32) -> f32 {
        if !quantum.is_finite() || quantum <= 0.0 || !base.is_finite() {
            return 0.0;
        }
        self.quanta(base, quantum) as f32 * quantum
    }

    /// The CSS value, as an expression over [`BASE_TOKEN`].
    ///
    /// A whole multiple of the base emits without a division, and 1:1 emits
    /// the bare `var()`, because `calc(var(--geometry-base) * 1 / 1)` is noise.
    #[must_use]
    pub fn css(self) -> String {
        match (self.numerator, self.denominator) {
            (n, d) if n == d => format!("var(--{BASE_TOKEN})"),
            (n, 1) => format!("calc(var(--{BASE_TOKEN}) * {n})"),
            (n, d) => format!("calc(var(--{BASE_TOKEN}) * {n} / {d})"),
        }
    }
}

/// What the layout is being drawn on: a base unit, and the smallest step the
/// surface can actually represent.
///
/// Both are in the surface's own units, and the crate never assumes those are
/// pixels. A display measures in pixels and can represent one of them; a
/// terminal measures in cells and cannot represent less than one. That single
/// difference is the whole of the terminal story — a TUI is not a density, it
/// is a surface with a coarse quantum, and the relational vocabulary above
/// crosses over untouched.
///
/// Quantising is not a terminal special case. A display does it too; it is
/// just that rounding 6.0 to the nearest pixel is uninteresting, whereas
/// rounding three eighths of a cell to the nearest cell is a design decision
/// the surface makes for you.
#[derive(Debug, Clone, Copy, PartialEq)]
pub struct Surface {
    /// The base unit, in this surface's units.
    pub base: f32,
    /// The smallest step this surface can represent, in the same units.
    pub quantum: f32,
}

impl Surface {
    /// A display measuring in CSS pixels: a 16px base, one-pixel quantum.
    #[must_use]
    pub fn web() -> Self {
        Self {
            base: f32::from(DEFAULT_BASE_PX),
            quantum: 1.0,
        }
    }

    /// A terminal measuring in cells: a one-cell base, one-cell quantum.
    ///
    /// The coarsest surface in the family, and the one that proves the
    /// vocabulary. `bound` and `peer` collapse to no cells at all, which is
    /// correct — in a grid this dense, "belongs to" and "is a peer of" are
    /// both expressed by adjacency, not by a gap.
    #[must_use]
    pub fn terminal() -> Self {
        Self {
            base: 1.0,
            quantum: 1.0,
        }
    }

    /// Resolve a ratio on this surface, snapped to its quantum.
    #[must_use]
    pub fn resolve(self, ratio: Ratio) -> f32 {
        ratio.quantize(self.base, self.quantum)
    }

    /// How many whole quanta a ratio is worth here.
    ///
    /// What a cell-addressed layout actually wants: the count, not the size.
    #[must_use]
    pub fn quanta(self, ratio: Ratio) -> u32 {
        ratio.quanta(self.base, self.quantum)
    }

    /// Resolve a relationship on this surface at a given density, in quanta.
    ///
    /// The whole model in one call: *what* is being separated, *who* is
    /// operating it, *what* it is drawn on.
    #[must_use]
    pub fn gap(self, gap: Gap, density: Density) -> u32 {
        self.quanta(gap.step_at(density).ratio())
    }
}

/// Which input the layout is being sized for.
///
/// A preset, not a breakpoint, and orthogonal to [`Surface`]: density decides
/// which step a relationship picks, the surface decides how that step lands.
/// Which density applies is the app's call — GoingsOn and Balanced Breakfast
/// already decide it once and hang a `ui-mode-*` class off the result.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Default)]
pub enum Density {
    /// Mouse or trackpad. Resolves to the Mac OS 8 HIG's own proportions.
    #[default]
    Pointer,
    /// Finger. Targets separate, shells tighten.
    Touch,
}

/// A named separation between two things.
///
/// Pick by relationship. The size is a consequence of the name, not the other
/// way round, and callers should never care what it is.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord)]
pub enum Gap {
    /// A control and the thing it belongs to: an edit field and its pop-up, a
    /// checkbox and its label, an icon and the text it labels. Reads as one
    /// object.
    Bound,
    /// Items of the same kind in a list: stacked checkboxes, radio buttons,
    /// rows, chips in a row. Reads as a set.
    Peer,
    /// A container's inner margin, and the distance between sibling groups
    /// side by side. Reads as "inside this box".
    Group,
    /// Separated groups, and rows of actions. The first gap that reads as a
    /// deliberate break rather than as breathing room.
    Section,
    /// Panel padding and content shells. Layout, not controls.
    Pane,
    /// The outermost shell margin. One per screen, usually.
    Page,
}

/// A raw step on the underlying scale.
///
/// Present because not every distance is a relationship between two controls —
/// an optical nudge inside a badge is not a `Gap`. Prefer [`Gap`] wherever one
/// fits: a step name says how big, a gap name says why.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord)]
pub enum Step {
    /// An eighth of the base. Optical nudges inside small inline elements.
    Hair,
    /// A quarter of the base.
    Tight,
    /// Three eighths of the base.
    Snug,
    /// Half the base.
    Base,
    /// Five eighths of the base.
    Roomy,
    /// Three quarters of the base.
    Wide,
    /// The base itself.
    Loose,
    /// One and a half times the base.
    Broad,
    /// Twice the base.
    Vast,
    /// Three times the base.
    Colossal,
}

impl Step {
    /// This step as a fraction of the base unit.
    #[must_use]
    pub const fn ratio(self) -> Ratio {
        let (numerator, denominator) = match self {
            Self::Hair => (1, 8),
            Self::Tight => (1, 4),
            Self::Snug => (3, 8),
            Self::Base => (1, 2),
            Self::Roomy => (5, 8),
            Self::Wide => (3, 4),
            Self::Loose => (1, 1),
            Self::Broad => (3, 2),
            Self::Vast => (2, 1),
            Self::Colossal => (3, 1),
        };
        Ratio {
            numerator,
            denominator,
        }
    }

    /// Size in CSS pixels at the default base.
    #[must_use]
    pub const fn px(self) -> u16 {
        self.ratio().px_at(DEFAULT_BASE_PX)
    }

    /// The CSS custom-property name, without the leading `--`.
    #[must_use]
    pub const fn token(self) -> &'static str {
        match self {
            Self::Hair => "step-hair",
            Self::Tight => "step-tight",
            Self::Snug => "step-snug",
            Self::Base => "step-base",
            Self::Roomy => "step-roomy",
            Self::Wide => "step-wide",
            Self::Loose => "step-loose",
            Self::Broad => "step-broad",
            Self::Vast => "step-vast",
            Self::Colossal => "step-colossal",
        }
    }

    /// Every step, smallest first.
    #[must_use]
    pub const fn all() -> [Self; 10] {
        [
            Self::Hair,
            Self::Tight,
            Self::Snug,
            Self::Base,
            Self::Roomy,
            Self::Wide,
            Self::Loose,
            Self::Broad,
            Self::Vast,
            Self::Colossal,
        ]
    }
}

impl Gap {
    /// The step this relationship resolves to at a given density.
    #[must_use]
    pub const fn step_at(self, density: Density) -> Step {
        match (self, density) {
            // Binding is not a separation, so it does not open up on touch.
            (Self::Bound, _) => Step::Tight,

            (Self::Peer, Density::Pointer) => Step::Snug,
            (Self::Peer, Density::Touch) => Step::Roomy,

            (Self::Group, Density::Pointer) => Step::Roomy,
            (Self::Group, Density::Touch) => Step::Wide,

            (Self::Section, Density::Pointer) => Step::Wide,
            (Self::Section, Density::Touch) => Step::Loose,

            // Shells tighten on touch: outer margin is screen you don't get.
            (Self::Pane, Density::Pointer) => Step::Broad,
            (Self::Pane, Density::Touch) => Step::Loose,

            (Self::Page, Density::Pointer) => Step::Vast,
            (Self::Page, Density::Touch) => Step::Broad,
        }
    }

    /// The step this relationship resolves to at the default density.
    #[must_use]
    pub const fn step(self) -> Step {
        self.step_at(Density::Pointer)
    }

    /// Size in CSS pixels at the default base, at a given density.
    #[must_use]
    pub const fn px_at(self, density: Density) -> u16 {
        self.step_at(density).px()
    }

    /// Size in CSS pixels at the default base and density.
    #[must_use]
    pub const fn px(self) -> u16 {
        self.step().px()
    }

    /// The CSS custom-property name, without the leading `--`.
    #[must_use]
    pub const fn token(self) -> &'static str {
        match self {
            Self::Bound => "gap-bound",
            Self::Peer => "gap-peer",
            Self::Group => "gap-group",
            Self::Section => "gap-section",
            Self::Pane => "gap-pane",
            Self::Page => "gap-page",
        }
    }

    /// Every relationship, tightest first.
    #[must_use]
    pub const fn all() -> [Self; 6] {
        [
            Self::Bound,
            Self::Peer,
            Self::Group,
            Self::Section,
            Self::Pane,
            Self::Page,
        ]
    }
}

/// Emit the base unit and the raw scale as CSS declarations, no selector.
///
/// Density-invariant: the steps are the vocabulary, and only which step a
/// relationship picks changes between presets.
#[must_use]
pub fn scale_css_declarations() -> String {
    let mut out = String::new();
    let _ = writeln!(
        out,
        "  /* Every size below is a ratio of this. Scale it and the whole\n     \
         layout scales with it, including for a user who has asked for\n     \
         larger text. */\n  --{BASE_TOKEN}: 1rem;\n"
    );
    out.push_str("  /* Raw scale. Prefer a --gap-* below; reach here only when\n");
    out.push_str("     no relationship describes the distance. */\n");
    for step in Step::all() {
        let _ = writeln!(out, "  --{}: {};", step.token(), step.ratio().css());
    }
    out
}

/// Emit the relational layer for one density as CSS declarations, no selector.
///
/// Gaps reference their step rather than repeating a value, so the scale has
/// exactly one definition and a reader can see which relationship maps where.
#[must_use]
pub fn gap_css_declarations(density: Density) -> String {
    let mut out = String::new();
    for gap in Gap::all() {
        let _ = writeln!(
            out,
            "  --{}: var(--{});",
            gap.token(),
            gap.step_at(density).token()
        );
    }
    out
}

/// Emit the whole geometry layer as a `:root { … }` block at one density.
///
/// Mirrors `makeover::intent_css_vars`. Unlike the colour layer this is
/// constant, so a web consumer should bake it in at build time rather than
/// apply it from JS on every load.
#[must_use]
pub fn geometry_css_vars(density: Density) -> String {
    format!(
        ":root {{\n{}\n{}}}\n",
        scale_css_declarations(),
        gap_css_declarations(density)
    )
}

/// The whole spacing layer with the canonical density selection, as CSS.
///
/// **Density is a capability, not a device and not a width.** A narrow window
/// on a desktop still has a pointer in it and a tablet at full width still has
/// a finger, so the touch preset hangs off `(hover: none), (pointer: coarse)`
/// rather than off a breakpoint or a user-agent string. That is the question
/// the platform actually answers, and it is the one [`Density`] is asking.
///
/// `explicit_touch` names a selector an app sets when the *user* has chosen.
/// It is emitted last and therefore wins at equal specificity, because
/// detection is a default rather than a verdict: a touchscreen laptop and
/// someone who simply wants roomier targets are both real, and neither is
/// visible to a media query.
///
/// Settles a policy three consumers previously answered three ways. GoingsOn
/// sniffed the user agent behind a mode class, Balanced Breakfast used
/// `(hover: none)` alone, and audiofiles had no switch at all; the first of
/// those asked what device this is as a proxy for a capability already
/// reported.
#[must_use]
pub fn density_css(explicit_touch: Option<&str>) -> String {
    let mut css = geometry_css_vars(Density::Pointer);
    css.push_str("\n@media (hover: none), (pointer: coarse) {\n");
    for line in gap_css_overrides(":root", Density::Touch).lines() {
        css.push_str("    ");
        css.push_str(line);
        css.push('\n');
    }
    css.push_str("}\n");
    if let Some(selector) = explicit_touch {
        css.push_str("\n/* An explicit user choice, last so it wins over detection. */\n");
        css.push_str(&gap_css_overrides(selector, Density::Touch));
    }
    css
}

/// Emit a density preset as a scoped override block.
///
/// Only the relational layer is emitted: the scale and the base do not change
/// between presets, so an app ships [`geometry_css_vars`] at its default
/// density and one of these per mode class it supports.
///
/// ```
/// # use makeover_geometry::{Density, gap_css_overrides};
/// let css = gap_css_overrides(".ui-mode-mobile", Density::Touch);
/// assert!(css.starts_with(".ui-mode-mobile {\n"));
/// ```
#[must_use]
pub fn gap_css_overrides(selector: &str, density: Density) -> String {
    format!("{selector} {{\n{}}}\n", gap_css_declarations(density))
}

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

    #[test]
    fn density_is_selected_by_capability_not_by_width_or_agent() {
        let css = density_css(None);
        assert!(css.contains("@media (hover: none), (pointer: coarse)"));
        // The three things density must never be selected by.
        assert!(!css.contains("max-width"), "a breakpoint crept in");
        assert!(!css.contains("min-width"), "a breakpoint crept in");
        assert!(!css.contains("ui-mode"), "a device mode crept in");
    }

    #[test]
    fn an_explicit_choice_is_emitted_after_the_detection() {
        let css = density_css(Some(".ui-mode-mobile"));
        let media = css.find("@media").expect("media query");
        let explicit = css.find(".ui-mode-mobile").expect("explicit selector");
        // Equal specificity, so order is the whole mechanism: the user's
        // choice has to come last or detection quietly overrides it.
        assert!(explicit > media, "the explicit selector must come last");
    }

    #[test]
    fn without_an_explicit_selector_there_are_exactly_two_presets() {
        assert_eq!(density_css(None).matches("--gap-peer").count(), 2);
    }

    #[test]
    fn the_hig_relationships_land_on_the_hig_values() {
        // Mac OS 8 HIG, Control Layout Guidelines. The ratios are ours, but at
        // the default base they must resolve to the numbers the HIG specifies,
        // or the departure has cost us the thing it was translating.
        assert_eq!(Gap::Bound.px(), 4);
        assert_eq!(Gap::Peer.px(), 6);
        assert_eq!(Gap::Group.px(), 10);
        assert_eq!(Gap::Section.px(), 12);
    }

    #[test]
    fn every_ratio_divides_the_default_base_exactly() {
        for step in Step::all() {
            let r = step.ratio();
            assert_eq!(
                u32::from(DEFAULT_BASE_PX) * u32::from(r.numerator) % u32::from(r.denominator),
                0,
                "{step:?} is fractional at the default base"
            );
        }
    }

    #[test]
    fn ratios_scale_linearly() {
        for step in Step::all() {
            assert_eq!(
                step.ratio().px_at(DEFAULT_BASE_PX * 2),
                step.px() * 2,
                "{step:?} does not double with the base"
            );
        }
    }

    #[test]
    fn steps_ascend_and_never_repeat() {
        let px: Vec<u16> = Step::all().iter().map(|s| s.px()).collect();
        let mut sorted = px.clone();
        sorted.sort_unstable();
        sorted.dedup();
        assert_eq!(px, sorted, "steps must be strictly ascending");
    }

    #[test]
    fn gaps_ascend_with_their_relationships_at_every_density() {
        for density in [Density::Pointer, Density::Touch] {
            let px: Vec<u16> = Gap::all().iter().map(|g| g.px_at(density)).collect();
            let mut sorted = px.clone();
            sorted.sort_unstable();
            assert_eq!(px, sorted, "{density:?}: a looser relationship is tighter");
        }
    }

    #[test]
    fn touch_separates_targets_and_tightens_shells() {
        // The asymmetry is the whole reason a base scalar would not do.
        for gap in [Gap::Peer, Gap::Section] {
            assert!(
                gap.px_at(Density::Touch) > gap.px_at(Density::Pointer),
                "{gap:?} must open up for a fingertip"
            );
        }
        for gap in [Gap::Pane, Gap::Page] {
            assert!(
                gap.px_at(Density::Touch) < gap.px_at(Density::Pointer),
                "{gap:?} must tighten on a small screen"
            );
        }
        assert_eq!(
            Gap::Bound.px_at(Density::Touch),
            Gap::Bound.px_at(Density::Pointer),
            "bound things stay bound"
        );
    }

    #[test]
    fn a_terminal_resolves_the_vocabulary_to_whole_cells() {
        let t = Surface::terminal();
        let cells: Vec<u32> = Gap::all()
            .iter()
            .map(|g| t.gap(*g, Density::Pointer))
            .collect();
        // bound, peer | group, section | pane, page
        assert_eq!(cells, vec![0, 0, 1, 1, 2, 2]);
    }

    #[test]
    fn collapsing_is_allowed_but_inverting_is_not() {
        // A coarse surface has fewer distinctions, so neighbouring gaps may
        // land on the same quantum. What must never happen is a looser
        // relationship coming out tighter than a closer one.
        for quantum in [0.5_f32, 1.0, 2.0, 3.0, 7.0] {
            for density in [Density::Pointer, Density::Touch] {
                let s = Surface {
                    base: 16.0,
                    quantum,
                };
                let v: Vec<u32> = Gap::all().iter().map(|g| s.gap(*g, density)).collect();
                let mut sorted = v.clone();
                sorted.sort_unstable();
                assert_eq!(v, sorted, "quantum {quantum} {density:?} inverted: {v:?}");
            }
        }
    }

    #[test]
    fn the_web_surface_agrees_with_the_pixel_helper() {
        let w = Surface::web();
        for step in Step::all() {
            assert_eq!(
                w.resolve(step.ratio()) as u16,
                step.px(),
                "{step:?} disagrees between surface and px_at"
            );
        }
    }

    #[test]
    fn quantising_is_monotonic_in_the_ratio() {
        let (base, quantum) = (16.0, 1.0);
        let mut previous = 0;
        for step in Step::all() {
            let q = step.ratio().quanta(base, quantum);
            assert!(q >= previous, "{step:?} went backwards");
            previous = q;
        }
    }

    #[test]
    fn a_degenerate_quantum_yields_nothing_rather_than_panicking() {
        let r = Step::Loose.ratio();
        for bad in [0.0_f32, -1.0, f32::NAN] {
            assert_eq!(r.quanta(16.0, bad), 0);
            assert!(r.quantize(16.0, bad).abs() < f32::EPSILON);
        }
        assert_eq!(r.quanta(f32::INFINITY, 1.0), 0);
    }

    #[test]
    fn px_at_rounds_rather_than_truncating() {
        // Eighths divide 16 exactly, so the rounding only shows on a base
        // that does not: 3/8 of 15 is 5.625, which is 6px, not 5.
        assert_eq!(Step::Snug.ratio().px_at(15), 6);
        assert_eq!(Step::Snug.ratio().px_at(DEFAULT_BASE_PX), 6);
    }

    #[test]
    fn tokens_are_unique() {
        let mut names: Vec<&str> = Step::all().iter().map(|s| s.token()).collect();
        names.extend(Gap::all().iter().map(|g| g.token()));
        let count = names.len();
        names.sort_unstable();
        names.dedup();
        assert_eq!(names.len(), count, "token names collide");
    }

    #[test]
    fn css_is_expressed_over_the_base_never_in_pixels() {
        let css = geometry_css_vars(Density::Pointer);
        assert!(css.starts_with(":root {\n"));
        assert!(css.trim_end().ends_with('}'));
        assert!(css.contains("--geometry-base: 1rem;"));
        for step in Step::all() {
            let line = format!("--{}: {}", step.token(), step.ratio().css());
            assert!(css.contains(&line), "missing or wrong: {line}");
        }
        // A hard pixel count anywhere in the scale defeats the point.
        let scale = scale_css_declarations();
        assert!(
            !scale.contains("px;"),
            "the scale must not emit pixel literals:\n{scale}"
        );
    }

    #[test]
    fn ratio_css_drops_redundant_arithmetic() {
        assert_eq!(Step::Loose.ratio().css(), "var(--geometry-base)");
        assert_eq!(Step::Vast.ratio().css(), "calc(var(--geometry-base) * 2)");
        assert_eq!(
            Step::Snug.ratio().css(),
            "calc(var(--geometry-base) * 3 / 8)"
        );
    }

    #[test]
    fn gaps_reference_steps_rather_than_repeating_values() {
        let css = geometry_css_vars(Density::Pointer);
        assert!(css.contains("--gap-peer: var(--step-snug);"));
        assert!(!css.contains("--gap-peer: calc"));
    }

    #[test]
    fn a_density_override_emits_only_the_relational_layer() {
        let css = gap_css_overrides(".ui-mode-mobile", Density::Touch);
        assert!(css.contains("--gap-peer: var(--step-roomy);"));
        // Referencing a step is the point; re-declaring one would fork the
        // scale, so the check is on declarations, not on mentions.
        let declared: Vec<&str> = css
            .lines()
            .filter_map(|l| l.trim().strip_prefix("--"))
            .filter_map(|l| l.split(':').next())
            .collect();
        assert!(
            declared.iter().all(|t| t.starts_with("gap-")),
            "only the relational layer may be overridden, got {declared:?}"
        );
    }
}