use crate::{Depth, Fill, Intent, Priority};

// Names this module's prose links to, resolved for rustdoc.
#[allow(unused_imports)]
use crate::{Awaiting, Meter, Region};

/// What a region is saying, when it is saying something.
///
/// The one intent family shared by badges, notices and nothing else. Kept
/// separate from [`Fill`] because a surface is where a thing sits and a tone is
/// what it means, and the three apps agree on the four statuses:
/// `info_banner` / `warning_banner` in audiofiles, `.toast-info` /
/// `.toast-success` / `.toast-error` in goingson, `.toast.success` /
/// `.toast.error` in Balanced Breakfast.
///
/// The per-tag palette (`category-one` through `category-six`) is deliberately
/// not here. Which colour a *particular* tag takes is app domain, and both
/// webview apps already carry it as a `data-color` attribute.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum Tone {
    /// No status.
    ///
    /// Ordinary content, at full weight. It does not also mean muted: a
    /// badge reads quiet because [`Token::Badge`] answers no click, which is
    /// the renderer's knowledge and not this axis's. A renderer wanting a
    /// muted badge reaches for [`Token::interactive`] itself rather than
    /// expecting `Neutral` to have muted it.
    Neutral,
    /// Something worth knowing and nothing to do about it.
    Info,
    /// Something finished and it worked.
    Success,
    /// Something the user should look at before continuing.
    Warning,
    /// Something broken, or something about to be destroyed.
    Danger,
}

impl Intent for Tone {
    fn token(self) -> &'static str {
        match self {
            // Neutral has no status token of its own, so it takes the plain
            // content intent. It used to answer `content-muted`, which read
            // "no status" as "de-emphasised" and muted every figure value in
            // the webview. Muting is a renderer's call about a particular
            // token, not something the status axis knows.
            Self::Neutral => "content",
            Self::Info => "info",
            Self::Success => "success",
            Self::Warning => "warning",
            Self::Danger => "danger",
        }
    }
}

/// A small labelled thing that sits inside something else.
///
/// Two members, because the three apps drew three taxonomies and only one line
/// runs through all of them: does it answer a click. audiofiles has
/// `classification_badge` (a label) against `tag_chip`, `tag_chip_removable`
/// and `selectable_tag` (all of which do). Balanced Breakfast has `.tag` and
/// `.badge` against `.tag-chip`. goingson is the one that has to move: its
/// `.tag` and `.badge` are a single CSS rule, so every call site has to be read
/// to decide which of the two it always was.
///
/// The evidence that a chip is a real concept rather than a badge with a
/// cursor: audiofiles inverts its bevel on press and Balanced Breakfast latches
/// `.tag-chip.active` with the inset bevel. Two independent arrivals at "a chip
/// holds itself down", which is exactly what [`Depth::pressed`] already says.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum Token {
    /// Non-interactive status or count. Answers no click.
    Badge,
    /// An interactive or removable token. Answers a click, and latches if it
    /// stands for a filter that is either on or off.
    Chip {
        /// Whether it carries its own remove affordance.
        removable: bool,
    },
}

impl Token {
    /// Whether this answers a click.
    ///
    /// The whole difference between the two members, and the reason a renderer
    /// with no hover (a touch surface, a terminal) can still tell them apart.
    #[must_use]
    pub const fn interactive(self) -> bool {
        matches!(self, Self::Chip { .. })
    }

    /// How it sits, given whether it is currently latched down.
    ///
    /// A badge is flat: it is a label, and giving it an edge would say it can
    /// be pressed. A chip is raised, and inset while latched.
    #[must_use]
    pub const fn depth(self, latched: bool) -> Depth {
        match self {
            Self::Badge => Depth::Flat,
            Self::Chip { .. } if latched => Depth::Well,
            Self::Chip { .. } => Depth::Raised,
        }
    }
}

/// Something the app is telling the user, unprompted.
///
/// Two concepts, not one with a placement. They differ in more than where they
/// sit: a toast is transient, stacked and self-dismissing, and a banner is
/// persistent, in flow, one per region, and dismissed by fixing the condition
/// it reports. Folding them into one member with a placement parameter would
/// make lifetime, stacking and dismissal all placement-dependent, which is the
/// description leaking renderer policy.
///
/// All three apps have banners: `info_banner` and `warning_banner` in
/// audiofiles, five of them in goingson (sync, sync-result, vacation-day,
/// timer-active, past-review), `.update-banner` in Balanced Breakfast. The two
/// webview apps also have toasts. So neither member is speculative, and no app
/// gains a concept it lacks except audiofiles, whose renderer may legitimately
/// decline to draw a toast at all.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum Notice {
    /// Transient, stacked, dismisses itself.
    Toast,
    /// Persistent, in flow, one per region, dismissed by fixing the cause.
    Banner,
}

impl Notice {
    /// Whether it goes away on its own.
    #[must_use]
    pub const fn transient(self) -> bool {
        matches!(self, Self::Toast)
    }

    /// How it sits.
    ///
    /// A toast floats above the page rather than resting on it, which is
    /// [`Fill::Overlay`]'s whole reason to exist. A banner is a card in the
    /// flow. Both are raised, and they are raised off different things.
    #[must_use]
    pub const fn fill(self) -> Fill {
        match self {
            Self::Toast => Fill::Overlay,
            Self::Banner => Fill::Raised,
        }
    }
}

/// The parts of a list row.
///
/// `#[non_exhaustive]`: a renderer carries a wildcard arm, so a new part is not
/// a lockstep event across three renderers.
///
/// # Meta against Tokens
///
/// The line is whether the thing has its own standing. `Meta` is one short
/// trailing fact about the row, written as text: a count, a size, a date.
/// `Tokens` is a set of small labelled things, each of which can be toned and
/// can answer a click. "3 files" is meta. A status badge that is amber, and a
/// tag you can click to filter by, are tokens.
///
/// Keeping them apart is what a single widened slot would have foreclosed. A
/// renderer can right-align one string and cannot usefully do the same to a
/// strip of chips, and a fact that is not clickable should not be drawn as
/// though it were.
/// How much vertical room a part's text may take.
///
/// A row is an inline run and every part in it is a leaf, so a part's text has
/// always been drawn on one line and no description could say otherwise. Two
/// apps say otherwise in their own stylesheets, both to the same number and
/// both with a comment explaining it: Balanced Breakfast clamps a feed row's
/// title to two lines (`.row--article .row-primary`, whose comment reads
/// "overrides .row-primary's single flex line"), and goingson clamps a
/// problem's body to two ("two lines is enough to recognize one, and the full
/// text is in the task once promoted").
///
/// Two named tiers rather than a line count, and the count is what the measured
/// demand argues against. Both sites want exactly one tier past the default,
/// and a number invites a row whose primary is a paragraph, which is a block
/// and has no business in a run. A third tier is a decision, made here, rather
/// than something a call site can reach for.
///
/// What a renderer owes it: `Tight` is what a run already does and needs no
/// answer. `Relaxed` is at most two lines and then truncation, however that
/// renderer truncates -- a webview clamps, a terminal wraps into two rows of
/// cells, an immediate-mode renderer caps the galley. A renderer that cannot
/// give two lines may draw one; what it may not do is grow without bound,
/// because the run is a line and the row's neighbours are relying on that.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Default)]
#[non_exhaustive]
pub enum Flow {
    /// One line. What every part did before this type existed.
    #[default]
    Tight,
    /// Up to two lines, then truncated.
    Relaxed,
}

impl Flow {
    /// How many lines the part may take.
    ///
    /// A number here rather than in the enum, because a renderer needs one and
    /// a call site does not. That asymmetry is the whole argument for the
    /// tiers: the description says how much room the thing deserves and this
    /// says what that costs, so a third tier changes one line rather than every
    /// consumer's arithmetic.
    #[must_use]
    pub const fn lines(self) -> u8 {
        match self {
            Self::Relaxed => 2,
            // Including any tier added later: one line is the safe reading of
            // an unknown flow, since it is what the run guaranteed before flows
            // existed.
            _ => 1,
        }
    }
}

/// How deep a row sits inside a set: a tree, an outline, a threaded list.
///
/// [`RowPart`] below already names what is *in* a row; nothing named where a
/// row sits relative to its siblings, so every consumer that had a hierarchy
/// carried a bare number and every renderer decided for itself what one was
/// worth.
///
/// # Not `Depth`, and the collision is the reason
///
/// [`Depth`] is taken and means something else entirely: surface bevel --
/// `Flat`, `Raised`, `Well`, `Sunken`, `Overlay` -- a fact about a surface
/// rather than a position in a hierarchy. Two meanings under one word in one
/// crate is the collision that costs a reader an hour, and the word this
/// concept wants is the one that would cause it.
///
/// # The magnitude is the renderer's, and that is the precedent
///
/// [`Awaiting`] is the shape: this crate names the fact and declines to name
/// what it is worth. makeover-webview writes the rule as a custom property with
/// a fallback -- the way it writes `margin-inline-start: var(--awaiting-gap,
/// 0.5ch)` -- so a level has one answer per renderer and an app can override
/// it. A terminal spends columns, a browser spends inline space, and neither
/// number belongs in a description.
///
/// # Zero is a real answer
///
/// [`top`](Self::top) is the default and is what a flat list says: every row is
/// at the top level, which is true and is the reading a renderer needs. An
/// `Option` here would make "not nested" and "nested at zero" two spellings of
/// one thing, the same argument `Discovery`'s `indexable` makes about defaults
/// that are meaningful.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Default, PartialOrd, Ord)]
#[non_exhaustive]
pub struct Nesting {
    /// How many levels in, counting from zero.
    ///
    /// `u8` because a hierarchy a reader can follow is not 256 deep, and a
    /// renderer indenting by a level has to multiply it by something -- a wider
    /// integer here is a wider integer in every renderer's arithmetic for a
    /// range nothing will use.
    pub level: u8,
}

impl Nesting {
    /// The top level: not nested. The default, and what a flat list says.
    #[must_use]
    pub const fn top() -> Self {
        Self { level: 0 }
    }

    /// A row this many levels in.
    #[must_use]
    pub const fn at(level: u8) -> Self {
        Self { level }
    }

    /// Whether this row sits under another.
    ///
    /// The question every renderer asks before it spends anything on indenting,
    /// answered once here rather than by a `> 0` in each -- which is
    /// [`Awaiting::is_determinate`]'s reason too.
    #[must_use]
    pub const fn is_nested(self) -> bool {
        self.level > 0
    }
}

#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
#[non_exhaustive]
pub enum RowPart {
    /// The thing itself. What the row is called.
    Primary,
    /// Supporting text under the primary.
    Secondary,
    /// A short trailing fact: a count, a size, a date.
    Meta,
    /// Controls that act on this row.
    Actions,
    /// Small labelled things belonging to the row: badges, chips, tags.
    ///
    /// Each carries its own [`Token`] kind and [`Tone`], so a renderer with no
    /// colour still has the kind to work with, and one with no chips still has
    /// the label. That is the constrained-consumer test this vocabulary exists
    /// to pass, and it is why the tone lives on the token rather than on the
    /// part.
    Tokens,
    /// How much of a set the row's thing has done: a [`Meter`] in the row.
    ///
    /// A row holds no nodes, by the rule that a row part may not carry an
    /// arbitrary node, which is the door through which a description becomes a
    /// templating language. So the part carries the *description of a bar* rather than a node, exactly as
    /// `Tokens` carries tags rather than nodes.
    ///
    /// Without it a row flattens the proportion into [`Meta`](Self::Meta) as
    /// "3/7 subtasks", which keeps both numbers and loses the reading, the same
    /// way a toned status badge read as prose before `Tokens`.
    Proportion,
}

impl RowPart {
    /// What the part is worth when the run does not fit.
    ///
    /// The default only. A part may say otherwise, and a renderer reads the
    /// part rather than the role; this is what a description that has never
    /// heard of [`Priority`] means, which is every description written before
    /// the field existed.
    ///
    /// Deriving it from the role is the thing this vocabulary has otherwise
    /// been moving away from, and it is right here for one reason: the roles
    /// already encode this ranking and every consumer already assumes it.
    /// [`Primary`](Self::Primary) is what the row is called, and
    /// [`Priority::Essential`]'s own doc was written about exactly that --
    /// "without it the row does not identify itself".
    ///
    /// [`Actions`](Self::Actions) is `Essential` and it is the interesting one.
    /// A control is not a fact, so dropping it does not cost the reader a
    /// detail; it costs them the only way to act on the row, and in a terminal
    /// it silently removes something focus had already been claimed for. A
    /// renderer that needs room takes it from what the row *says*, never from
    /// what it *offers*.
    ///
    /// An unknown member reads as [`Priority::Secondary`]: droppable, but not
    /// first, since guessing `Optional` for something this crate has not been
    /// taught would make a new member the first thing to vanish.
    #[must_use]
    pub const fn priority(self) -> Priority {
        match self {
            Self::Primary | Self::Actions => Priority::Essential,
            Self::Meta | Self::Proportion => Priority::Optional,
            _ => Priority::Secondary,
        }
    }

    /// The content intent the part takes.
    #[must_use]
    pub const fn intent(self) -> &'static str {
        match self {
            Self::Primary => "content",
            Self::Secondary => "content-secondary",
            Self::Meta => "content-muted",
            // Actions carry controls rather than text, so they inherit.
            Self::Actions => "content",
            // So do tokens: each one carries its own tone, and a part-level
            // intent underneath it would fight the token that sits on it.
            Self::Tokens => "content",
            // And so does a proportion, for the same reason: the meter carries
            // the tone, and it is about the ratio rather than about the row.
            Self::Proportion => "content",
        }
    }
}

/// How far down the heading tree a title sits.
///
/// Three, and only the three that are actually headings. The bands those used
/// to be filed with (goingson's `.page-header`, Balanced Breakfast's `.header`
/// and `.detail-header`) are arrangement, not type, and live at
/// [`Region::Band`]. One of them contains no text at all.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum Heading {
    /// Names the whole screen. One per screen.
    Page,
    /// Names a block within the screen.
    Section,
    /// Names a sub-block inside an already-named section.
    Subsection,
}

impl Heading {
    /// Whether a rule follows the heading.
    ///
    /// audiofiles' `section_header` draws a separator and its
    /// `subsection_label` deliberately does not, which is the only thing
    /// distinguishing the two once weight and colour are deferred.
    #[must_use]
    pub const fn separated(self) -> bool {
        matches!(self, Self::Section)
    }
}

/// A control that picks between things.
///
/// Three, because three distinct behaviours are in play and collapsing any two
/// loses something. A segmented control picks a value; a tab picks a pane; a
/// toggle picks nothing and simply holds itself on or off.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum Selector {
    /// Exactly one of N, and the options abut.
    Segmented,
    /// Independent on or off, on its own.
    Toggle,
    /// Navigation between panes. The folder semantic.
    Tabs,
}

impl Selector {
    /// How the chosen option sits.
    ///
    /// Held in for a segmented control and a toggle, which is the same shape
    /// pressing produces and the whole economy of the idiom: one appearance,
    /// two reasons to wear it. A tab is the exception, because the selected
    /// folder tab comes *forward* to join the pane it opens.
    #[must_use]
    pub const fn chosen(self) -> Depth {
        match self {
            Self::Segmented | Self::Toggle => Depth::Well,
            Self::Tabs => Depth::Raised,
        }
    }

    /// How the options that were *not* picked sit.
    ///
    /// Describing only [`Selector::chosen`] left the unchosen option falling
    /// through to [`Depth::Flat`], which says it is level with the strip it
    /// sits in, and no renderer emitted anything for it. That is wrong in both
    /// directions and goingson proved it: its unchosen tabs are recessed by
    /// hand, and being recessed is *why* the chosen one reads as coming
    /// forward. Against a flat strip, a raised chosen tab is a bevel drawn on
    /// the strip's own colour, which is a much weaker folder effect than the
    /// contrast the idiom is named after.
    ///
    /// Each member is the inverse of its chosen state, which is the whole
    /// content of "picked" once colour is deferred:
    ///
    /// - Tabs recede, so the chosen one comes forward.
    /// - A segment and a toggle stand up, so the chosen one is held in.
    #[must_use]
    pub const fn unchosen(self) -> Depth {
        match self {
            Self::Tabs => Depth::Sunken,
            Self::Segmented | Self::Toggle => Depth::Raised,
        }
    }

    /// Whether the options touch.
    ///
    /// The gap is the entire difference between a segmented control and a row
    /// of buttons that happen to sit near each other, which is what audiofiles'
    /// `segmented_control` says in its own comment and why it zeroes the
    /// spacing by hand.
    #[must_use]
    pub const fn abutting(self) -> bool {
        matches!(self, Self::Segmented | Self::Tabs)
    }
}