//! The renderer-agnostic half of the make-family design system.
//!
//! <!-- wiki: makeover-layout -->
//!
//! `makeover` answers *what colour*, and varies by theme. `makeover-geometry`
//! answers *how much space*, and varies by density and surface. This crate
//! answers *what the thing is*, and varies by nothing.
//!
//! # The deferral rule
//!
//! A description names intents and relationships, never values. Say
//! [`Fill::Raised`], never `#D9DDF4`. Say `Gap::Peer`, never `6px`. What is
//! left once colour and spacing are deferred is **composition**: which edges
//! are lit, what inverts on press, what nests in what.
//!
//! The constraint that shapes all of it: a renderer that can only paint
//! rectangles has to be able to express the result. egui has no
//! `box-shadow: inset` and one stroke per widget with no per-side control; a
//! terminal has box-drawing characters and one cell of resolution, and cannot
//! draw a two-tone lit edge at all. A description that assumes per-side edges
//! is a CSS description wearing a neutral name. So this crate names the
//! *intent* — this region is a well — and each renderer chooses an expression
//! it can actually produce, including dropping half of one.
//!
//! # Scope
//!
//! - **Depth.** [`Bevel`], [`Edge`], [`Fill`], [`Depth`]: the bevel and the
//!   surfaces it shapes. Fill and bevel are named together, so a raised bevel
//!   over a recessed fill is unrepresentable.
//! - **Components.** [`Token`] (badge against chip), [`Notice`] (toast against
//!   banner), [`RowPart`], [`CellPart`], [`Heading`], [`Selector`],
//!   [`Readiness`], [`Awaiting`], [`Meter`], [`Figure`], [`Track`],
//!   and [`Tone`], the one intent family they share.
//! - **Schemas.** [`Field`] for forms, [`Column`] for lists and tables,
//!   [`Facet`] for the dimensions a set is narrowed by.
//! - **Structure.** [`Region`] for the parts of a screen, [`Arrangement`] for
//!   how a screen is put together, [`Showing`] for how many of a region's
//!   children are visible at once.
//!
//! # What a member is admitted on
//!
//! A member is added when an app needs a fact the vocabulary cannot state, and
//! refused when what it wants is presentation it should be asking a renderer
//! for. Three tests, all of which have to pass:
//!
//! - **Generic against bespoke.** Is this furniture any app would have, or is
//!   it this app's own? A rule that withholds a word until a second app has
//!   duplicated the code guarantees the duplication. What the app owns
//!   keeps [`Region::Handover`] and [`Region::Ceded`].
//! - **Every host has an honest answer.** A member no renderer can express
//!   without borrowing one host's idiom is not a description.
//! - **It can be laid out before it is filled.** See *First paint is final
//!   paint* below.
//!
//! Count the members a thing needs before refusing it. A refusal is only worth
//! as much as the measurement under it.
//!
//! # What this crate cannot say
//!
//! - **An address.** What a control calls, and where a button goes. [`Act`]
//!   names the act and holds no destination.
//! - **A current value.** A webview reads it out of the DOM and an
//!   immediate-mode renderer holds a `&mut` to the app's own field. A
//!   description carrying it would be a form model.
//! - **What has focus.** See below.
//! - **A duration or a clock.** No estimate of time remaining, no autosave
//!   interval, no animation length.
//! - **A colour, a size or a position.** The deferral rule.
//!
//! # Reach, focus and the focus ring
//!
//! Three terms, and no others, for what sits outside the description.
//! **Reach** is which things can take focus and in what order; a browser reads
//! it off the document, a TUI derives it from draw order, egui from its own id
//! stack. **Focus** is which reached thing has the keyboard right now: the
//! renderer's, live, never described and never round-tripped through a
//! description. The **focus ring** is the visible cue; the token (`focus-ring`,
//! derived by `makeover` from the action colour) is the one shared artifact and
//! the drawing is the renderer's. Retired as names for any of this: "focus
//! stroke", "focus cue", "wants focus". "Caret" is a different thing — the text
//! cursor inside a field — and keeps its name.
//!
//! # The three tones, and what a colour claims
//!
//! One rule for how colour says whether a thing can be
//! used. Every renderer answers to it, and it is stated here because the
//! description is what names the intents.
//!
//! | the thing | intent |
//! |-----------|--------|
//! | active, emphasised, the thing itself | `content` |
//! | inactive but usable: it still answers a press | `content-secondary` |
//! | inert: disabled, or not a control at all | `content-muted` |
//!
//! `content-muted` is the one with a claim in it. [`State::Disabled`] resolves
//! to it, so a live control wearing it is telling the user it will not answer,
//! and being wrong about that is worse than being quiet, because the user's
//! response is to stop trying. A sortable column heading that was never sorted,
//! and every unchosen option in a radio group, read as dead lists if they wear
//! it. What is legitimately muted is a caption, a hint, a placeholder, a meter's reading, an axis label: text that
//! was never going to answer anything.
//!
//! The three are one ramp and not three colours. `makeover`'s `Emphasis` derives
//! the quieter two from the ink, so "one step back" means the same distance in
//! every theme and a renderer cannot land between them by picking its own.
//!
//! # First paint is final paint
//!
//! Nothing may change size or position after it is
//! first drawn, and nothing may stand in for content that has not arrived yet.
//! Both halves are absolute.
//!
//! It is stated here, rather than left to each renderer, because a renderer can
//! only reserve space the description gave it enough to size. A member whose
//! size depends on its content therefore owes whatever makes it sizeable while
//! the content is still absent, and that is the second admission test for a new
//! member: not only does it compose something this crate already names, it can
//! be laid out before it is filled.
//!
//! The mechanism is a reservation, and [`Sort`]'s caret is the worked example.
//! The caret is drawn into a box its own width whether or not the column is
//! sorted, so pressing a heading cannot reflow the row it sits in. The box names
//! no magnitude, which is what keeps it out of `makeover-geometry`'s territory.
//! Reserve from what is known; never discover geometry from what has not
//! arrived.
//!
//! The trap is an `Option` that means "not yet". [`Readiness::Pending`] is the
//! honest way to say a region is still waiting. An optional *measurement* is
//! not: a count that shows up later widens the text that prints it and moves
//! everything beside it, which is the reflow this rule exists to forbid. So an
//! `Option` on a measurement means the host will never know it — a property of
//! the query, fixed for the life of the screen — and a renderer sizes for the
//! answer it was handed rather than for the one it hopes is coming.
//!
//! # Any width, one answer
//!
//! The sibling of the rule above. That one is
//! independence from *when*; this one is independence from *how you got here*.
//!
//! A rendering is a pure function of the description and the viewport. The same
//! description at the same width is the same output, whatever widths came
//! before it. No renderer may carry geometry across frames, and none may narrow
//! by counting.
//!
//! The failure this forbids is ordinary enough to be the default everywhere
//! else: a page that hides its sidebar below some width, remembers that it hid
//! it, and does not bring it back the same way. Layout there is a function of
//! `(width, history)`, so dragging a window to 900 wide is a different screen
//! depending on whether you came from 1400 or from 600. Nobody chose that; it
//! is what measuring and remembering produce.
//!
//! The mechanism is [`Width`] for what grows and [`Priority`] for what drops.
//! Both are declared, both are read off the description, and neither needs a
//! measurement. A renderer narrows by raising a cutoff over a total order,
//! never by counting what fits and stopping — `makeover-tui`'s table states
//! that as its own rule and tests it, and `makeover-webview` reaches the same
//! place with `@media` and `display: none`, which is path-independent by
//! construction because CSS has nowhere to keep the previous width.
//!
//! Two things follow for anything new. A member that would need last frame's
//! size to lay out this frame is refused, the same way a member that cannot be
//! sized before it is filled is refused. And a fact about what disappears
//! belongs in the description, because a host that has to infer it can only
//! infer it from a measurement.
//!
//! # Where the description stops
//!
//! A member is added when an app needs a fact the vocabulary cannot state, and
//! refused when what it wants is presentation it should be asking a renderer
//! for. It is not a quota, and the goal is every screen described.
//!
//! A timeline is describable. What it needs and could not previously get is two
//! integers, where a thing starts and how long it lasts, which is [`Track`].
//! Slot heights, gridline colour, how overlapping things stack and which hour
//! scrolls into view stay the renderer's, and `Track` carries none of them.
//!
//! A kanban board is describable, and the member is [`Region::Columns`]. Every
//! card fact is already sayable through `Row`'s parts; what nothing else could
//! say is that the columns are *peers*, since [`Arrangement`] offers only
//! list-detail and sidebar-content and a board described as either is a lie
//! about the screen. Dragging a card between columns does not enter into it: a
//! drop's effect is "set status", a discrete action `Row`'s menu already
//! carries, and the drag itself is affordance.
//!
//! A calendar takes no members. The month grid's primacy in calendar apps is an
//! artifact of paper: paper cannot be queried, so it has to show every day at
//! once as a fallback index, and routes, search and ranking do that job better.
//! Three jobs survive that reasoning, and only one of them needs a grid:
//!
//! 1. **Spans across days**, a stretch of leave, a trip, a sprint. You cannot
//!    see "away the 3rd to the 17th" in a list without diffing dates. This is
//!    [`Track`] with [`Unit::Days`], and [`Track::days`] is it.
//! 2. **Density at a glance**, which weeks were heavy. That is a heatmap, and a
//!    heatmap describes as a list.
//! 3. **Weekday periodicity**, "every other Tuesday", "the 15th is a Saturday".
//!    This is the only job that needs the seven-column wrap, because alignment
//!    is the whole of what makes it visible.
//!
//! Job 3 is the only open question, and nothing in the tree asks for it. A
//! month grid otherwise renders as a [`Table`](crate::Column): seven weekday
//! columns, weeks as rows, blanks for the offset. If a screen wants one,
//! measure the members it needs before adding any.
//!
//! [`Region::Handover`] and [`Region::Ceded`] remain for the genuinely
//! app-owned. The description
//! names the *place* and the app owns the contents, so a screen containing a
//! timeline is still a whole screen and still routable. Without it, the screens
//! that make an app worth using would need a second, undescribed path beside
//! the router, and two paths is how a vocabulary drifts from its app.
//!
//! [`Region::Widget`] sits between that limit and the primitives, and it does
//! not move the limit. A widget is an assembly of members this crate *already*
//! has, under a name a renderer may or may not recognise. Anything that needs a
//! member the vocabulary does not have is still a finding about the vocabulary
//! or still bespoke; naming an assembly buys no new expressive power, which is
//! why it is safe to let the set grow outside this crate.

#![forbid(unsafe_code)]

/// A colour intent this crate refers to but never resolves.
///
/// The string is the token name `makeover` publishes, so a renderer can look
/// it up without this crate knowing what colour came back.
pub trait Intent {
    /// The `makeover` intent token this resolves against.
    fn token(self) -> &'static str;
}

mod act;
mod choice;
mod column;
mod component;
mod depth;
mod facet;
mod field;
mod figure;
mod readiness;
mod region;
mod text;
mod theme_choice;
mod track;

// The crate's entire API is these names at the root. Named rather than glob so
// a member added to a module and left out here fails at the first call site
// outside the crate instead of quietly leaving the root API.
pub use act::Act;
pub use choice::{Candidate, Choice, Curve};
pub use column::{CellPart, Column, Fallback, Priority, Sort, Width};
pub use component::{Flow, Heading, Nesting, Notice, RowPart, Selector, Token, Tone};
pub use depth::{Bevel, Depth, Edge, Fill, State};
pub use facet::{Facet, FacetValue, Selecting, Standing};
pub use field::{Accepted, DATE_FORMAT, DATETIME_FORMAT, Family, Field, FieldKind};
pub use figure::{Bar, Chart, Extent, Figure, Fit, Meter};
pub use readiness::{Awaiting, Loading, Readiness};
pub use region::{Arrangement, Measure, Paging, Region, Share, Showing, Window};
pub use text::{Change, Syntax};
pub use theme_choice::{Contrast, ThemeChoice, ThemeVariant};
pub use track::{Placement, Span, Track, Unit};

#[cfg(test)]
mod tests;