//! 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.
//!
//! # Type is relational too
//!
//! [`Text`] names what a piece of text is — body, note, head — and the size
//! follows, exactly as [`Gap`] names what is being separated. It is the same
//! argument: whether a caption should be 13px or 14px cannot be reviewed,
//! whether a piece of text is a caption can.
//!
//! Type has its own rungs rather than reusing [`Step`], because the spacing
//! scale is eighths of the base to match the HIG's distances and a type ramp
//! wants different fractions. What the two axes share is the base, so the
//! reader's root font size moves the text and the space around it together.
//!
//! Unlike spacing, type does not move with [`Density`]. The reason is in
//! [`Text`], and it is the same one that keeps shells out of the touch preset.
//!
//! # Corners, same move again
//!
//! [`Radius`] names what the corner belongs to. The scale is deliberately the
//! shortest of the three, because rounding carries one bit of meaning —
//! whether the thing is meant to be pressed — and a long radius scale is one
//! nobody can choose from. `Square` is a rung rather than the absence of one,
//! so a container can state that it is square and a reader can tell that from
//! a rule nobody wrote.
//!
//! # 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.
//!
//! ## What Touch is a claim about
//!
//! Touch is a claim about the **contact patch and nothing else**. A fingertip
//! is coarse where a cursor hotspot is a point, and the only consequence of
//! that is mis-tap cost: when two adjacent things do different things, an
//! imprecise contact needs more room between them to land on the intended one.
//!
//! Shells are not tap targets. Panel padding and the outer page margin separate
//! a region from the edge of the screen, and no amount of coarseness in the
//! pointing device makes that separation riskier. So **Touch opens the gaps
//! that separate targets and leaves the shells exactly where Pointer put
//! them**:
//!
//! | Gap | Pointer | Touch | why |
//! |---|---|---|---|
//! | bound | 4 | 4 | not a separation at all |
//! | peer | 6 | 10 | adjacent distinct targets, the whole point |
//! | group | 10 | 12 | holds the peer/group distinction open |
//! | section | 12 | 16 | a deliberate break stays legible as one |
//! | pane | 24 | 24 | a shell is not a target |
//! | page | 32 | 32 | a shell is not a target |
//!
//! A Touch preset must never *tighten* `Pane` or `Page` on the argument that
//! outer margin is screen you do not get. **That is a claim about screen
//! budget, not about the input device.** Opening `Section` to 16 while
//! tightening `Pane` below it makes any Pointer `Pane` at or under 16 an
//! inversion, so a derived preset silently sets a floor under the one quoted
//! from the HIG. A phone is small *and* touch; a tablet and a touchscreen
//! laptop are big and touch. Screen budget is a separate axis and does not
//! belong here.
//!
//! ## The one cross-density rule
//!
//! **Touch never resolves tighter than Pointer**, at any gap. Stated as a
//! deliberate claim rather than inherited, and chosen for its direction: it
//! constrains the *derived* preset by the *quoted* one, never the reverse. A
//! Pointer retune downward moves freely and cannot be blocked by Touch. Only a
//! Pointer move upward can push
//! Touch, and that is the correct direction of authority.
//!
//! # Size class: the axis Density kept being asked to carry
//!
//! [`SizeClass`] answers **how much screen there is**, which is not the same
//! question as what is pointing at it. A phone is small and touch; a tablet and
//! a touchscreen laptop are big and touch; a half-width desktop window is small
//! and pointer. Four real combinations, and one axis cannot name them.
//!
//! This is the home for the claim *outer margin is screen you do not get*. On
//! the input device it lets a derived preset set a floor under a quoted one;
//! here it is correct.
//!
//! Boundaries are **quoted** (Material 3 window size classes: 600 and 840)
//! rather than derived, for the same reason the [`Gap`] values are. This crate
//! carries the boundaries only; what appears or disappears at each is a product
//! decision and belongs to `makeover-touch`.
//!
//! Size class feeds [`Gap::step_at_size`], and only the two shells listen to
//! it: `pane` and `page` come down one step on a compact window. That is where
//! "outer margin is screen you don't get" belongs. It does not belong in the
//! Pointer preset, which would be the same mistake one axis over.
//!
//! # Surfaces, and why a TUI is not a third density
//!
//! [`Surface`] is the fourth 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. Opens the gaps that separate distinct tap targets and leaves
    /// the shells where Pointer put them, because a coarse contact patch
    /// raises mis-tap cost and a panel margin is not something you tap. See
    /// the crate-level "Density presets" section for the derivation.
    Touch,
}

impl Density {
    /// The media condition selecting exactly this density, without the
    /// `@media`.
    ///
    /// A capability question rather than a width or a device, which is the
    /// policy this crate settles for [`density_css`].
    ///
    /// The two are **not** each other's textual negation, and that is the
    /// reason they live in one place. Touch is comma-joined, so it is an OR,
    /// and negating an OR gives an AND with both halves inverted. Deriving one
    /// from the other by eye is how the pair drifts apart, and it drifts
    /// silently: a wrong negation still parses, still minifies, and only shows
    /// up as hover states surviving on a phone.
    ///
    /// Pointer's condition is what a renderer wraps a hover rule in.
    /// `makeover-touch` decides *whether* a hover rule should be gated;
    /// this decides what the gate is spelled as.
    #[must_use]
    pub const fn media_condition(self) -> &'static str {
        match self {
            Self::Pointer => "(hover: hover) and (pointer: fine)",
            Self::Touch => "(hover: none), (pointer: coarse)",
        }
    }
}

/// How much screen there is, independent of what is pointing at it.
///
/// The second axis, and the one [`Density`] kept being asked to carry. A phone
/// is small *and* touch; a tablet and a touchscreen laptop are big and touch; a
/// half-width window on a desktop is small and pointer. Those are four real
/// combinations and one axis cannot name them, which is what made the old Touch
/// preset tighten shells it had no business tightening.
///
/// **Boundaries are quoted, not derived**, for the same reason the [`Gap`]
/// values are: a derived boundary is one nobody can check. They are Material 3's
/// window size classes, the best-known three-tier split with published numbers.
/// Apple's size classes are two-tier and expressed as regular/compact per axis,
/// which does not give a middle to aim at.
///
/// Source: <https://m3.material.io/foundations/layout/applying-layout/window-size-classes>
///
/// This enum carries the **boundaries only**. What appears, disappears or
/// reflows at each is a product decision and belongs to `makeover-touch`, not
/// here, with one exception: shells tighten on a compact window, through
/// [`Gap::step_at_size`]. That is a look call, so it is eyeballed rather than
/// derived.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord, Default)]
pub enum SizeClass {
    /// Under 600px. Phones in either orientation, and any window narrowed to
    /// phone width regardless of what is pointing at it.
    Compact,
    /// 600px to 839px. Small tablets, split-screen panes, half-width windows.
    #[default]
    Medium,
    /// 840px and up. Laptops, desktops, tablets in landscape.
    Expanded,
}

impl SizeClass {
    /// Lower bound in CSS pixels, inclusive. [`Self::Compact`] starts at zero.
    #[must_use]
    pub const fn min_px(self) -> u16 {
        match self {
            Self::Compact => 0,
            Self::Medium => 600,
            Self::Expanded => 840,
        }
    }

    /// The media condition selecting exactly this class, without the `@media`.
    ///
    /// Bounded on both sides for the middle class, so the three are mutually
    /// exclusive and a rule cannot land in two of them. `max-width` is one below
    /// the next class's `min_px`, because CSS width ranges are inclusive.
    #[must_use]
    pub fn media_condition(self) -> String {
        match self {
            Self::Compact => format!("(max-width: {}px)", Self::Medium.min_px() - 1),
            Self::Medium => format!(
                "(min-width: {}px) and (max-width: {}px)",
                Self::Medium.min_px(),
                Self::Expanded.min_px() - 1
            ),
            Self::Expanded => format!("(min-width: {}px)", Self::Expanded.min_px()),
        }
    }

    /// The CSS class name an app may hang off this, without the leading dot.
    #[must_use]
    pub const fn token(self) -> &'static str {
        match self {
            Self::Compact => "size-compact",
            Self::Medium => "size-medium",
            Self::Expanded => "size-expanded",
        }
    }

    /// Every class, narrowest first.
    #[must_use]
    pub const fn all() -> [Self; 3] {
        [Self::Compact, Self::Medium, Self::Expanded]
    }

    /// The class a given viewport width falls in.
    #[must_use]
    pub const fn at_width(px: u16) -> Self {
        if px >= Self::Expanded.min_px() {
            Self::Expanded
        } else if px >= Self::Medium.min_px() {
            Self::Medium
        } else {
            Self::Compact
        }
    }
}

/// 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, on a
    /// [`SizeClass::Medium`] or wider window.
    ///
    /// [`Density::Pointer`]'s values are the HIG's own. [`Density::Touch`]
    /// opens the three gaps that separate distinct tap targets and holds the
    /// rest, per the crate-level "Density presets" section.
    ///
    /// Shells tighten on a compact window rather than at touch density. Use
    /// [`Self::step_at_size`] where the window width is known; this is the
    /// wider-window answer and the one every existing caller already meant.
    #[must_use]
    pub const fn step_at(self, density: Density) -> Step {
        self.step_at_size(density, SizeClass::Medium)
    }

    /// The step this relationship resolves to at a given density and window
    /// size class.
    ///
    /// Only the two shells move, and only on [`SizeClass::Compact`]: `pane`
    /// 24 to 16, `page` 32 to 24, one step down each. The four gaps between
    /// controls do not, because how much room a window has says nothing about
    /// how far apart two tap targets should be.
    ///
    /// **Which axis owns this** (Max). A tighter window edge is a claim about
    /// screen budget, and smuggling screen budget into the density axis is the
    /// bug [`SizeClass`] exists to prevent. So the claim goes here and the
    /// quoted Pointer values hold. Both compact values sit on the eighths
    /// scale, so no `Step` at 7/8 = 14 is needed.
    #[must_use]
    pub const fn step_at_size(self, density: Density, size: SizeClass) -> Step {
        match self {
            // Binding is not a separation, so it does not open up on touch
            // either: separating these would say they are two objects.
            Self::Bound => Step::Tight,

            // The three that carry mis-tap cost. Peer is the one that matters
            // most (stacked rows, adjacent chips) and moves furthest; Group
            // and Section follow only far enough to stay distinct from it.
            Self::Peer => match density {
                Density::Pointer => Step::Snug,
                Density::Touch => Step::Roomy,
            },
            Self::Group => match density {
                Density::Pointer => Step::Roomy,
                Density::Touch => Step::Wide,
            },
            Self::Section => match density {
                Density::Pointer => Step::Wide,
                Density::Touch => Step::Loose,
            },

            // Shells. Not tap targets, so the contact patch has no opinion.
            // Screen budget is the axis that does, and it is this one.
            Self::Pane => match size {
                SizeClass::Compact => Step::Loose,
                SizeClass::Medium | SizeClass::Expanded => Step::Broad,
            },
            Self::Page => match size {
                SizeClass::Compact => Step::Broad,
                SizeClass::Medium | SizeClass::Expanded => Step::Vast,
            },
        }
    }

    /// 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, at a given density and window
    /// size class.
    #[must_use]
    pub const fn px_at_size(self, density: Density, size: SizeClass) -> u16 {
        self.step_at_size(density, size).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,
        ]
    }
}

/// What a piece of text is, from which its size follows.
///
/// The type axis, and the same move [`Gap`] makes on the spacing axis: name
/// the role and let the size follow, so the choice is reviewable. Whether a
/// caption should be 13px or 14px is unanswerable in isolation; whether a
/// piece of text is a caption is not.
///
/// # Why the ratios are their own ramp
///
/// Type does not reuse [`Step`]. The spacing scale is built in eighths of the
/// base because that is what the HIG's distances land on, and a type ramp
/// needs different rungs — 7/8 and 9/8 sit either side of body copy and have
/// no spacing meaning at all, while `Hair` and `Tight` are far below any
/// legible size. Sharing the enum would have meant widening it for rungs
/// spacing never asks for.
///
/// What is shared is the thing that matters: every rung here is a [`Ratio`]
/// of `--geometry-base`, so text tracks the user's chosen root size exactly
/// as spacing does, and one knob still moves the whole design.
///
/// # Why the floor is 3/4
///
/// Twelve pixels at the default base, and nothing below it. Sizes under that
/// are a legibility problem rather than a tier, and a scale that offers one
/// is a scale that invites it. Text that needs to recede should recede by
/// colour or weight, which cost no legibility.
///
/// # Why type does not shift on touch
///
/// [`Density`] is a claim about the contact patch and nothing else, and text
/// is not a tap target. The reader's own root font size is already the knob
/// for how large text should be, and it already moves this whole ramp. So the
/// type axis is density-invariant, and a phone gets the same tiers a desktop
/// does.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum Text {
    /// Three quarters of the base. Timestamps, badges, legal lines.
    Fine,
    /// Seven eighths of the base. Secondary text: metadata, table cells,
    /// captions, form help.
    Note,
    /// The base itself. Running copy, and the size everything else is read
    /// against.
    Body,
    /// Nine eighths of the base. Emphasised copy: intros, card titles.
    Lead,
    /// Five quarters of the base. The third heading level.
    Subhead,
    /// One and a half times the base. Section headings, the second level.
    Head,
    /// Twice the base. The page's own title, the first level.
    Title,
    /// Two and a half times the base. Display copy, above the document
    /// hierarchy rather than at the top of it.
    Display,
    /// Three times the base. One per page at most: a landing hero.
    Hero,
}

impl Text {
    /// This role's size as a fraction of the base unit.
    #[must_use]
    pub const fn ratio(self) -> Ratio {
        let (numerator, denominator) = match self {
            Self::Fine => (3, 4),
            Self::Note => (7, 8),
            Self::Body => (1, 1),
            Self::Lead => (9, 8),
            Self::Subhead => (5, 4),
            Self::Head => (3, 2),
            Self::Title => (2, 1),
            Self::Display => (5, 2),
            Self::Hero => (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::Fine => "text-fine",
            Self::Note => "text-note",
            Self::Body => "text-body",
            Self::Lead => "text-lead",
            Self::Subhead => "text-subhead",
            Self::Head => "text-head",
            Self::Title => "text-title",
            Self::Display => "text-display",
            Self::Hero => "text-hero",
        }
    }

    /// Every role, smallest first.
    #[must_use]
    pub const fn all() -> [Self; 9] {
        [
            Self::Fine,
            Self::Note,
            Self::Body,
            Self::Lead,
            Self::Subhead,
            Self::Head,
            Self::Title,
            Self::Display,
            Self::Hero,
        ]
    }
}

/// How rounded a corner is, named for what the corner belongs to.
///
/// The third axis to make the same move as [`Gap`] and [`Text`]: name the
/// thing and let the value follow. Whether a corner should be 3px or 4px is
/// unanswerable in isolation, and answering it once per component is how a
/// stylesheet ends up with 2, 3, 4, 6, 8 and 12 all meaning "slightly
/// rounded".
///
/// # Rounding is an affordance
///
/// The scale is deliberately short, because a corner radius carries one bit
/// of meaning: whether the thing is meant to be pressed. [`Self::Square`]
/// exists as a named rung rather than as the absence of a radius so that a
/// container states that it is square, and a reader can tell a deliberate
/// zero from a rule nobody wrote.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum Radius {
    /// No rounding. Containers: cards, panels, dropdowns, page shells.
    Square,
    /// An eighth of the base. The tightest corner still visible: inline code,
    /// small badges, status chips.
    Fine,
    /// A quarter of the base. Controls: buttons, inputs, selects.
    Control,
    /// Half the base. Surfaces that round rather than sit square: media
    /// covers, callout boxes, feature cards.
    Panel,
    /// A circle, whatever the element's size.
    Round,
}

impl Radius {
    /// This corner as a fraction of the base unit.
    ///
    /// [`Self::Round`] has none, and that is not an oversight: 50% is a
    /// proportion of the element's own box rather than of the base, so it
    /// does not scale with `--geometry-base` and cannot be written as a
    /// [`Ratio`]. Use [`Self::css`], which spells every rung.
    #[must_use]
    pub const fn ratio(self) -> Option<Ratio> {
        let (numerator, denominator) = match self {
            Self::Square => (0, 1),
            Self::Fine => (1, 8),
            Self::Control => (1, 4),
            Self::Panel => (1, 2),
            Self::Round => return None,
        };
        Some(Ratio {
            numerator,
            denominator,
        })
    }

    /// Size in CSS pixels at the default base, or `None` for [`Self::Round`].
    #[must_use]
    pub const fn px(self) -> Option<u16> {
        match self.ratio() {
            Some(r) => Some(r.px_at(DEFAULT_BASE_PX)),
            None => None,
        }
    }

    /// The CSS value for this rung.
    ///
    /// `Square` emits a bare `0` rather than a `calc()` that multiplies the
    /// base by nothing, and `Round` emits the percentage.
    #[must_use]
    pub fn css(self) -> String {
        match self {
            Self::Square => "0".to_owned(),
            Self::Round => "50%".to_owned(),
            other => other
                .ratio()
                .expect("every rung but Round has a ratio")
                .css(),
        }
    }

    /// The CSS custom-property name, without the leading `--`.
    #[must_use]
    pub const fn token(self) -> &'static str {
        match self {
            Self::Square => "radius-square",
            Self::Fine => "radius-fine",
            Self::Control => "radius-control",
            Self::Panel => "radius-panel",
            Self::Round => "radius-round",
        }
    }

    /// Every rung, squarest first.
    #[must_use]
    pub const fn all() -> [Self; 5] {
        [
            Self::Square,
            Self::Fine,
            Self::Control,
            Self::Panel,
            Self::Round,
        ]
    }
}

/// 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 type axis as CSS declarations, no selector.
///
/// Takes no [`Density`]: text is not a tap target, so the contact patch has no
/// opinion on it. See [`Text`] for the derivation.
#[must_use]
pub fn text_css_declarations() -> String {
    let mut out = String::new();
    out.push_str("  /* Type. Named for what the text is; the size follows.\n");
    out.push_str("     Ratios of the base, so text tracks the reader's own\n");
    out.push_str("     root size. Density-invariant: text is not a target. */\n");
    for text in Text::all() {
        let _ = writeln!(out, "  --{}: {};", text.token(), text.ratio().css());
    }
    out
}

/// Emit the corner scale as CSS declarations, no selector.
///
/// Takes no [`Density`] for the same reason [`text_css_declarations`] does
/// not: a corner is not a tap target.
#[must_use]
pub fn radius_css_declarations() -> String {
    let mut out = String::new();
    out.push_str("  /* Corners. Rounding says a thing is meant to be pressed,\n");
    out.push_str("     so the scale is short on purpose and square is a rung\n");
    out.push_str("     rather than the absence of one. */\n");
    for radius in Radius::all() {
        let _ = writeln!(out, "  --{}: {};", radius.token(), radius.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
}

/// The cascade layer every stylesheet the make-family generates is wrapped in.
///
/// One name shared by every emitter in the family, so an app writes it once and
/// the design system's output lands in one place it can order against:
///
/// ```css
/// @layer makeover, base, components, responsive;
/// ```
///
/// # Why a layer at all
///
/// The cascade resolves origin and importance, then layer, then specificity,
/// then source order, and **unlayered normal declarations outrank every named
/// layer**. So the moment an app declares any layer of its own, every rule it
/// owns loses to unlayered generated CSS regardless of specificity or of
/// loading last. Emitting into a layer is what stops that, and putting the name
/// here rather than in each app is what stops three apps picking three names.
///
/// # Why this constant lives in the geometry crate
///
/// Not because spacing owns it. This crate is the only one every CSS-emitting
/// crate in the family already depends on, and it is already the crate that
/// spells CSS for the family (`media_condition`, `Step::token`, `Ratio::css`).
/// A second copy in `makeover-webview` is exactly the drift
/// [`Density::media_condition`] exists to prevent, one layer up.
pub const CSS_LAYER: &str = "makeover";

/// Wrap generated CSS in [`CSS_LAYER`].
///
/// Every whole-stylesheet emitter in the family ends with this call. Exposed
/// rather than kept private because an app that assembles its own stylesheet
/// out of this family's pieces has to put it in the same layer: goingson builds
/// `tables.css` in its own `build.rs` from `makeover_webview::list`, and those
/// rules are as generated as the ones in `layout.css`.
#[must_use]
pub fn in_css_layer(css: &str) -> String {
    let mut out = format!("@layer {CSS_LAYER} {{\n");
    for line in css.lines() {
        // Blank lines stay blank; indenting one leaves trailing whitespace.
        if line.is_empty() {
            out.push('\n');
        } else {
            let _ = writeln!(out, "    {line}");
        }
    }
    out.push_str("}\n");
    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.
///
/// The density argument reaches the gaps only. The scale and the type ramp are
/// the same at every density, which is why neither takes one.
#[must_use]
pub fn geometry_css_vars(density: Density) -> String {
    format!(
        ":root {{\n{}\n{}\n{}\n{}}}\n",
        scale_css_declarations(),
        gap_css_declarations(density),
        text_css_declarations(),
        radius_css_declarations()
    )
}

/// 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.
///
/// Never sniff the user agent for this: that asks what device this is as a
/// proxy for a capability the browser already reports.
///
/// Emitted inside [`CSS_LAYER`]. Custom properties follow the
/// ordinary cascade, so unlayered ones outrank layered ones: an app that puts
/// its own `:root` overrides in a named layer while this file stayed unlayered
/// would find the generated tokens beating the overrides meant to replace them.
/// That is the same trap the component sheet had, and it is not visible until
/// the app adopts layers.
#[must_use]
pub fn density_css(explicit_touch: Option<&str>) -> String {
    in_css_layer(&density_declarations(explicit_touch))
}

/// [`density_css`] without the layer wrapper.
fn density_declarations(explicit_touch: Option<&str>) -> String {
    let mut css = geometry_css_vars(Density::Pointer);
    css.push_str("\n/* Touch: targets separate, shells hold. */\n");
    css.push_str("@media ");
    css.push_str(Density::Touch.media_condition());
    css.push_str(" {\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 the compact-window shell override.
///
/// The one place this crate is allowed to ask how wide the window is. Density
/// must never be selected by width — [`density_css`] has a test forbidding a
/// breakpoint from appearing in it at all — because a capability query answers
/// "what is pointing at this" and a breakpoint does not. Size class is the
/// opposite: width is exactly what it means, so it gets its own emitter and
/// its own media query rather than being folded into that file.
///
/// Only the two shells move, and only below [`SizeClass::Medium`]'s boundary:
/// `pane` 24 to 16, `page` 32 to 24. The four gaps between controls are absent
/// from the block, so a narrow window never reasons about tap targets.
///
/// Emitted inside [`CSS_LAYER`] for the same reason [`density_css`] is: an
/// unlayered custom property outranks a layered one, so an app that layers its
/// own overrides would otherwise lose to this.
///
/// ```
/// # use makeover_geometry::size_class_css;
/// let css = size_class_css();
/// assert!(css.contains("--gap-pane"));
/// assert!(!css.contains("--gap-peer"), "a control gap crept into a width query");
/// ```
#[must_use]
pub fn size_class_css() -> String {
    in_css_layer(&size_class_declarations())
}

/// [`size_class_css`] without the layer wrapper.
fn size_class_declarations() -> String {
    // Compact is everything below Medium's lower bound, so the query ends one
    // step under it. Fractional, because a 599.5px viewport is reachable on a
    // fractional-scaling display and an integer bound would drop it into
    // neither class.
    let ceiling = f32::from(SizeClass::Medium.min_px()) - 0.02;
    let mut css = String::new();
    css.push_str("/* Compact window: shells tighten. Outer margin is screen you\n");
    css.push_str("   don't get, which is a claim about the window and not about\n");
    css.push_str("   what is pointing at it, so it lives here and not in the\n");
    css.push_str("   density presets. Targets are untouched. */\n");
    let _ = writeln!(css, "@media (max-width: {ceiling}px) {{");
    css.push_str("    :root {\n");
    for gap in [Gap::Pane, Gap::Page] {
        let _ = writeln!(
            css,
            "        --{}: var(--{});",
            gap.token(),
            gap.step_at_size(Density::Pointer, SizeClass::Compact)
                .token()
        );
    }
    css.push_str("    }\n}\n");
    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 the_spacing_layer_is_emitted_inside_the_family_layer() {
        // Unlayered declarations outrank layered ones, so an app that layers
        // its own :root overrides would lose to an unlayered geometry.css.
        let css = density_css(None);
        assert!(css.starts_with(&format!("@layer {CSS_LAYER} {{\n")));
        assert!(css.trim_end().ends_with('}'));
        // Everything still there, one level in.
        assert!(css.contains("    :root {"));
        assert!(css.contains("--gap-peer"));
    }

    #[test]
    fn the_type_ramp_ascends_and_never_repeats_a_size() {
        // A tier that resolves to the same size as its neighbour is a name
        // with no distinction behind it, which is how a scale grows rungs
        // nobody can choose between.
        let sizes: Vec<u16> = Text::all().iter().map(|t| t.px()).collect();
        assert!(sizes.windows(2).all(|w| w[0] < w[1]), "{sizes:?}");
        assert_eq!(sizes, vec![12, 14, 16, 18, 20, 24, 32, 40, 48]);
    }

    #[test]
    fn the_type_ramp_has_a_legibility_floor() {
        // Nothing under 12px at the default base. Text that should recede
        // recedes by colour or weight, not by shrinking out of legibility.
        assert_eq!(Text::Fine.px(), 12);
        assert!(Text::all().iter().all(|t| t.px() >= 12));
    }

    #[test]
    fn the_corner_scale_is_short_and_ordered() {
        // A radius carries one bit of meaning, whether the thing is meant to
        // be pressed, so a long scale is a scale nobody can choose from.
        let px: Vec<Option<u16>> = Radius::all().iter().map(|r| r.px()).collect();
        assert_eq!(px, vec![Some(0), Some(2), Some(4), Some(8), None]);
    }

    #[test]
    fn square_and_round_are_spelled_not_calculated() {
        // `calc(var(--geometry-base) * 0 / 1)` is a zero nobody can read, and
        // 50% is a proportion of the element rather than of the base.
        assert_eq!(Radius::Square.css(), "0");
        assert_eq!(Radius::Round.css(), "50%");
        assert_eq!(Radius::Round.ratio(), None);
        assert!(Radius::Control.css().contains(BASE_TOKEN));
    }

    #[test]
    fn type_does_not_move_with_density() {
        // Density is a claim about the contact patch, and text is not a
        // target. A --text-* inside the touch override means that argument
        // was lost somewhere.
        let css = density_css(Some(".ui-mode-mobile"));
        let root_end = css.find("@media").expect("a touch block");
        assert!(css[..root_end].contains("--text-body"));
        assert!(!css[root_end..].contains("--text-"), "{}", &css[root_end..]);
    }

    #[test]
    fn every_type_token_scales_from_the_one_base() {
        // A literal rem here would be a size that stops tracking the reader's
        // root font size, which is the whole point of the base.
        for text in Text::all() {
            let css = text.ratio().css();
            assert!(css.contains(BASE_TOKEN), "{}: {css}", text.token());
        }
    }

    #[test]
    fn wrapping_leaves_no_trailing_whitespace_on_blank_lines() {
        // A formatter strips these later and calls it a diff.
        let css = in_css_layer("a {\n\nb\n}\n");
        assert!(!css.lines().any(|l| l != l.trim_end()), "{css:?}");
    }

    #[test]
    fn the_two_density_conditions_are_complements_and_not_negations() {
        let pointer = Density::Pointer.media_condition();
        let touch = Density::Touch.media_condition();

        // Both halves are inverted, feature for feature.
        assert!(pointer.contains("hover: hover") && touch.contains("hover: none"));
        assert!(pointer.contains("pointer: fine") && touch.contains("pointer: coarse"));

        // And the joins are inverted too, which is the part that gets written
        // wrong by hand: touch is an OR, so not-touch is an AND. A pointer
        // condition joined with a comma would match every touchscreen.
        assert!(touch.contains(", "), "touch must be an OR");
        assert!(pointer.contains(" and "), "pointer must be an AND");
        assert!(!pointer.contains(','), "pointer must not be an OR");
    }

    #[test]
    fn the_emitted_touch_block_is_the_condition_and_not_a_second_copy_of_it() {
        // The literal used to be inline here. Nothing may re-spell it.
        let css = density_css(None);
        assert!(css.contains(&format!("@media {}", Density::Touch.media_condition())));
    }

    #[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_holds_the_shells() {
        // The derivation, asserted so that changing it has to come here and say
        // so. Touch is a claim about the contact patch: the gaps between
        // distinct tap targets open, and the gaps that are not tap targets do
        // not move.
        for gap in [Gap::Peer, Gap::Group, Gap::Section] {
            assert!(
                gap.px_at(Density::Touch) > gap.px_at(Density::Pointer),
                "{gap:?} separates tap targets and must open on touch"
            );
        }
        for gap in [Gap::Bound, Gap::Pane, Gap::Page] {
            assert_eq!(
                gap.px_at(Density::Touch),
                gap.px_at(Density::Pointer),
                "{gap:?} is not a tap target and must not move with the input device"
            );
        }
    }

    #[test]
    fn touch_never_resolves_tighter_than_pointer() {
        // The one cross-density rule, and its direction is the point. The
        // preset thrown out on 2026-07-29 tightened Pane and Page on touch,
        // which combined with an opened Section to make any Pointer Pane at or
        // below 16 an inversion: a derived preset set a floor under the one
        // quoted from the HIG, blocking the retune to pane 14 / page 16.
        //
        // Constraining Touch by Pointer instead cannot do that. A Pointer
        // retune downward moves freely; only a Pointer move upward pushes
        // Touch, which is the correct direction of authority.
        for gap in Gap::all() {
            assert!(
                gap.px_at(Density::Touch) >= gap.px_at(Density::Pointer),
                "{gap:?}: Touch resolved tighter than Pointer"
            );
        }
    }

    #[test]
    fn the_pointer_retune_is_not_blocked_by_touch() {
        // Guards the specific regression above rather than trusting the general
        // rule to imply it. Touch's own ordering must hold using Touch values
        // only, so that a Pointer Pane at or below Touch's Section is legal.
        assert!(
            Gap::Section.px_at(Density::Touch) <= Gap::Pane.px_at(Density::Touch),
            "Touch inverted internally, which is what set the old floor"
        );
        // The retune this guarded is WITHDRAWN (Max, 2026-08-09): shells
        // tighten by size class, not by moving the quoted Pointer values, so
        // Pointer pane holds at 24 and never goes under Touch's Section.
        //
        // The assertion stays anyway. It is not about the retune; it is about
        // the direction of authority, and the day a derived preset can set a
        // floor under a quoted one is the day this crate has the 2026-07-29
        // bug back regardless of what anybody wanted to retune.
        assert!(Gap::Section.px_at(Density::Touch) >= 16);
    }

    #[test]
    fn shells_tighten_on_a_compact_window_and_nothing_else_does() {
        // The whole of the 2026-08-09 ruling, in one test. Shells come down
        // one step on a compact window; the four gaps that separate controls
        // do not move, because how much room a window has says nothing about
        // how far apart two tap targets belong.
        for density in [Density::Pointer, Density::Touch] {
            assert_eq!(Gap::Pane.px_at_size(density, SizeClass::Compact), 16);
            assert_eq!(Gap::Page.px_at_size(density, SizeClass::Compact), 24);

            for class in [SizeClass::Medium, SizeClass::Expanded] {
                assert_eq!(Gap::Pane.px_at_size(density, class), 24);
                assert_eq!(Gap::Page.px_at_size(density, class), 32);
            }

            for gap in [Gap::Bound, Gap::Peer, Gap::Group, Gap::Section] {
                for class in SizeClass::all() {
                    assert_eq!(
                        gap.px_at_size(density, class),
                        gap.px_at(density),
                        "{gap:?} moved on {class:?}, and only shells may"
                    );
                }
            }
        }
    }

    #[test]
    fn the_widths_never_needed_a_step_at_seven_eighths() {
        // The retune wanted pane 14, which is 7/8 of the base and off an
        // eighths scale that runs 2/4/6/8/10/12/16/24/32/48. It would have
        // needed a new public Step variant, and naming one is a cost paid
        // forever. Putting the claim on the size-class axis lands both compact
        // values on steps that already exist.
        for gap in [Gap::Pane, Gap::Page] {
            for density in [Density::Pointer, Density::Touch] {
                let step = gap.step_at_size(density, SizeClass::Compact);
                assert!(
                    Step::all().contains(&step),
                    "{gap:?} compact resolved off the scale"
                );
            }
        }
    }

    #[test]
    fn no_size_class_inverts_the_ordering() {
        // Collapse is allowed, inversion is not — the same rule the surface
        // quantum test applies, now across the third axis. Touch on a compact
        // window is the tight one: Section opens to 16 and Pane comes down to
        // 16, so they meet. Meeting is fine. Crossing is not.
        for density in [Density::Pointer, Density::Touch] {
            for class in SizeClass::all() {
                let v: Vec<u16> = Gap::all()
                    .iter()
                    .map(|g| g.px_at_size(density, class))
                    .collect();
                let mut sorted = v.clone();
                sorted.sort_unstable();
                assert_eq!(v, sorted, "{density:?} {class:?} inverted: {v:?}");
            }
        }
    }

    #[test]
    fn step_at_is_the_wider_window_answer() {
        // Every caller that predates the size-class axis meant the wide
        // window, so the old entry point has to keep resolving to it or a
        // consumer tightens silently on a bump it did not read about.
        for gap in Gap::all() {
            for density in [Density::Pointer, Density::Touch] {
                assert_eq!(
                    gap.step_at(density),
                    gap.step_at_size(density, SizeClass::Medium)
                );
                assert_eq!(
                    gap.step_at(density),
                    gap.step_at_size(density, SizeClass::Expanded)
                );
            }
        }
    }

    #[test]
    fn the_width_query_lives_outside_the_density_file() {
        // density_is_selected_by_capability_not_by_width_or_agent forbids a
        // breakpoint in density_css. This is the other half of that rule: the
        // width query has to exist somewhere, and somewhere is here.
        let css = size_class_css();
        assert!(css.contains("max-width"));
        assert!(css.contains("--gap-pane"));
        assert!(css.contains("--gap-page"));
        // Targets never appear in a width query.
        for token in ["--gap-bound", "--gap-peer", "--gap-group", "--gap-section"] {
            assert!(!css.contains(token), "{token} crept into a width query");
        }
        // And it stays out of the density file.
        assert!(!density_css(None).contains("max-width"));
    }

    #[test]
    fn size_classes_partition_every_width_exactly_once() {
        // Mutually exclusive and exhaustive, or a rule lands in two classes and
        // whichever is emitted last silently wins. Checked at every width up to
        // well past the top boundary rather than at the boundaries alone.
        for px in 0..=4000u16 {
            let hits: Vec<SizeClass> = SizeClass::all()
                .into_iter()
                .filter(|c| {
                    let lo = c.min_px();
                    let hi = match c {
                        SizeClass::Compact => SizeClass::Medium.min_px() - 1,
                        SizeClass::Medium => SizeClass::Expanded.min_px() - 1,
                        SizeClass::Expanded => u16::MAX,
                    };
                    px >= lo && px <= hi
                })
                .collect();
            assert_eq!(hits.len(), 1, "{px}px matched {hits:?}");
            assert_eq!(hits[0], SizeClass::at_width(px), "{px}px disagrees");
        }
    }

    #[test]
    fn the_quoted_boundaries_are_the_ones_material_publishes() {
        // Quoted, not derived. Changing these means departing from the source,
        // which is a decision to record rather than a value to nudge.
        assert_eq!(SizeClass::Compact.min_px(), 0);
        assert_eq!(SizeClass::Medium.min_px(), 600);
        assert_eq!(SizeClass::Expanded.min_px(), 840);
    }

    #[test]
    fn the_media_conditions_do_not_overlap_at_the_boundary() {
        // The off-by-one that makes CSS width ranges overlap: max-width is
        // inclusive, so it must be one below the next class's min-width.
        assert_eq!(
            SizeClass::Compact.media_condition(),
            "(max-width: 599px)",
            "Compact must stop one pixel below Medium"
        );
        assert_eq!(
            SizeClass::Medium.media_condition(),
            "(min-width: 600px) and (max-width: 839px)"
        );
        assert_eq!(SizeClass::Expanded.media_condition(), "(min-width: 840px)");
    }

    #[test]
    fn the_gap_scale_is_reachable_without_naming_a_size_class() {
        // This was size_class_does_not_reach_the_gap_scale, a tripwire holding
        // the axis unwired: "asserted so that wiring it in has to come here and
        // say so". Saying so, 2026-08-09 — it is wired, Max ruled it, and the
        // reasoning is in the docs on Gap::step_at_size.
        //
        // What survives is the half that was always the real assertion: the
        // whole spacing layer stays reachable without naming a size class. A
        // caller that has no idea how wide the window is still gets an answer,
        // and it is the wide-window one. Only step_at_size asks.
        let _ = geometry_css_vars(Density::Pointer);
        let _ = Gap::Page.px_at(Density::Touch);
        assert_eq!(SizeClass::all().len(), 3);

        // The tripwire's other half, kept as a real check now that there is
        // something to check: no size class may reach the four control gaps.
        // That is the line whose crossing would be the 2026-07-29 bug again.
        for gap in [Gap::Bound, Gap::Peer, Gap::Group, Gap::Section] {
            for class in SizeClass::all() {
                for density in [Density::Pointer, Density::Touch] {
                    assert_eq!(
                        gap.step_at_size(density, class),
                        gap.step_at(density),
                        "{gap:?} moved on {class:?}: screen budget reached a target gap"
                    );
                }
            }
        }
    }

    #[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()));
        names.extend(Text::all().iter().map(|t| t.token()));
        names.extend(Radius::all().iter().map(|r| r.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);
        // Which step Peer lands on is the preset's business, asserted in
        // touch_separates_targets_and_holds_the_shells. This says only that the
        // gap is emitted and references a step.
        assert!(css.contains("--gap-peer: var(--step-"));
        // 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:?}"
        );
    }
}