makeover_tui/

piece.rs

//! The pieces every terminal app draws, drawn once.
//!
//! # Called `widget` until 0.19.0
//!
//! Renamed because `makeover-layout` 0.20.0 took the word for something else,
//! and the two meanings do not sit together. A `Region::Widget` there is
//! host-agnostic: a named assembly of primitives that every renderer draws its
//! own way. What is in this module is the opposite end — renderer-local, the
//! answer to *what a meter looks like in cells*, taking a description plus what
//! only a terminal knows.
//!
//! One word for both would have made the tier unreadable in the crate that
//! implements it. This half moved because the other half is the ecosystem-facing
//! one: a second or third party naming a widget is naming the layout kind, and
//! nothing outside this tree ever needed a word for a drawing routine.
//!
//! `WidgetStyle` went with it and is `PieceStyle`.
//!
//! Arrived in 0.16.0 out of `quasi-tui`, which had written all of them and was
//! the second consumer to do so. A meter, a badge, a control, a figure and a
//! form field are what a screen is made of below the level [`table`](crate::table)
//! works at, and every one of them had been hand-rolled at least twice in this
//! tree before it was lifted.
//!
//! [`activity`] and [`awaiting`] joined them in 0.35.0, out of wiki
//! `loading-and-progress-standard`. They are the one pair here that arrived
//! before their second consumer rather than after it: nothing in the tree drew
//! a wait at all, on any surface, which is why the crate that had the vocabulary
//! for one had never been asked for the drawing.
//!
//! # What these take, and what they leave alone
//!
//! Each takes a `makeover-layout` description, a [`PieceStyle`], and whatever
//! the *host* knows that a description never carries. That last part is the
//! shape worth copying: [`field`] takes what is currently typed in the box as a
//! separate argument, because [`Field`] deliberately does not carry a value and
//! is not going to. `makeover-immediate` reached the same seam from the other
//! side with its `Filling`, and [`Held`] is that seam here.
//!
//! Focus is the other one. Nothing in a description says which control the user
//! is on, so every drawing here takes `focused` as an argument and the caller
//! is what counts. What focus *looks like* is this crate's answer and not the
//! caller's, which is the point of it being here: see
//! [`PieceStyle::focused`].
//!
//! # What they do not do
//!
//! No layout. Each answers rows for a width, or draws into the rect it is
//! given, top-aligned, and never below it. Nothing here measures twice and
//! nothing here places anything relative to anything else, because the moment
//! it did it would be a layout engine with one consumer's flow baked into it.

use makeover_layout::{Act, Awaiting, Field, FieldKind, Figure, Heading, Meter, Token, Tone};
use ratatui::buffer::Buffer;
use ratatui::layout::Rect;
use ratatui::style::{Modifier, Style};
use ratatui::text::{Line, Span};

use crate::text;
use std::time::Duration;

/// The colours and marks the drawings below use.
///
/// [`TableStyle`](crate::table::TableStyle)'s shape, for its reasons: an
/// ungated struct of styles with a [`Default`], plus a
/// [`from_theme`](Self::from_theme) that is what a consumer holding a loaded
/// theme should reach for first. A consumer painting bevels and nothing else
/// should not have to supply text tones it never uses, and gating the whole
/// module on `theme` would make these unreachable to anyone hand-picking
/// colours.
///
/// The default is the one that survives a terminal with no colour at all:
/// modifiers only, no foreground anywhere. That is not a placeholder. A
/// two-colour terminal is the case where a `Style` carrying a foreground is a
/// foreground that will not land, and bold-and-reversed is what is left.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct PieceStyle {
    /// Ordinary content, and what [`Tone::Neutral`] reads as.
    pub content: Style,
    /// Content one step back: a field's label, a quoted run.
    pub secondary: Style,
    /// Content two steps back: a caption, a hint, a meter's reading.
    pub muted: Style,
    /// Something worth knowing and nothing to do about it.
    pub info: Style,
    /// Something finished and it worked.
    pub success: Style,
    /// Something the user should look at.
    pub warning: Style,
    /// Something broken, or about to be destroyed.
    pub danger: Style,
    /// A page title.
    pub page: Style,
    /// A section title.
    pub section: Style,
    /// A subsection title.
    pub subsection: Style,
    /// Text that goes somewhere, and a control's label.
    pub action: Style,
    /// A control filled with the action colour, for the one on a screen that is
    /// the thing to press. A form's submit is the case that has it.
    pub filled: Style,
    /// A surface set back from the one it sits on, by colour and nothing else.
    /// What a code run takes, since every cell is monospace and the thing a
    /// webview says with a typeface cannot be said that way here.
    pub sunken: Style,
    /// What "you are on this one" adds to whatever it lands on.
    ///
    /// Reversed video by default, which is the affordance a cell has left once
    /// colour is spent on tone and bold on weight. A webview says it with an
    /// outline; a terminal has no outline that is not four more cells.
    pub focus: Modifier,
    /// How many cells [`meter`] spends on its bar.
    pub meter_cells: u16,
    /// The filled part of a bar.
    pub meter_full: char,
    /// The empty part of a bar.
    pub meter_empty: char,
    /// What marks a compulsory field, appended to its label.
    ///
    /// A knob for `makeover-immediate`'s reason: it is the one piece of *copy*
    /// here, and copy is not a renderer's call.
    pub required_marker: &'static str,
}

impl Default for PieceStyle {
    /// Modifiers only, no foreground: what survives a terminal with two
    /// colours.
    fn default() -> Self {
        Self {
            content: Style::new(),
            secondary: Style::new(),
            muted: Style::new().add_modifier(Modifier::DIM),
            info: Style::new(),
            success: Style::new(),
            warning: Style::new(),
            danger: Style::new().add_modifier(Modifier::BOLD),
            page: Style::new().add_modifier(Modifier::BOLD),
            section: Style::new().add_modifier(Modifier::BOLD),
            subsection: Style::new(),
            action: Style::new().add_modifier(Modifier::UNDERLINED),
            filled: Style::new().add_modifier(Modifier::REVERSED),
            sunken: Style::new().add_modifier(Modifier::DIM),
            focus: Modifier::REVERSED,
            meter_cells: 10,
            meter_full: '#',
            meter_empty: '-',
            required_marker: "*",
        }
    }
}

impl PieceStyle {
    /// The house widgets, from a loaded theme.
    ///
    /// The lift this module exists for. `quasi-tui` carried every line of this
    /// as private methods on its own renderer; a second terminal app wanting a
    /// toned control had no way to reach them and would have picked its own
    /// colours for the same five tones.
    #[cfg(feature = "theme")]
    #[must_use]
    pub fn from_theme(theme: &crate::Theme) -> Self {
        Self {
            content: Style::new().fg(theme.content_primary),
            secondary: Style::new().fg(theme.content_secondary),
            muted: Style::new().fg(theme.content_muted),
            info: Style::new().fg(theme.status_info),
            success: Style::new().fg(theme.status_success),
            warning: Style::new().fg(theme.status_warning),
            danger: Style::new().fg(theme.status_danger),
            // Three depths and two of them are bold, which is the whole of what
            // a terminal has: there is no type scale in a grid of one cell
            // size. A page title takes bold and the accent, a section bold, a
            // subsection the secondary colour. That is the emphasis order a
            // webview's type scale says with size, said with the two axes a
            // cell has.
            page: Style::new()
                .fg(theme.action_primary)
                .add_modifier(Modifier::BOLD),
            section: Style::new()
                .fg(theme.content_primary)
                .add_modifier(Modifier::BOLD),
            subsection: Style::new().fg(theme.content_secondary),
            action: Style::new().fg(theme.action_primary),
            filled: Style::new().fg(theme.selection_on).bg(theme.action_primary),
            sunken: Style::new().bg(theme.surface_sunken),
            focus: Modifier::REVERSED,
            meter_cells: 10,
            meter_full: '#',
            meter_empty: '-',
            required_marker: "*",
        }
    }

    /// The style a tone reads as.
    ///
    /// [`Tone`] is closed and stays closed, so this is total and needs no
    /// fallback arm.
    #[must_use]
    pub const fn tone(&self, tone: Tone) -> Style {
        match tone {
            Tone::Neutral => self.content,
            Tone::Info => self.info,
            Tone::Success => self.success,
            Tone::Warning => self.warning,
            Tone::Danger => self.danger,
        }
    }

    /// The style a heading reads as.
    #[must_use]
    pub const fn heading(&self, level: Heading) -> Style {
        match level {
            Heading::Page => self.page,
            Heading::Section => self.section,
            Heading::Subsection => self.subsection,
        }
    }

    /// `style`, plus the mark that says the user is on this one.
    ///
    /// Takes the flag rather than being called behind an `if`, because every
    /// caller has a bool in hand and the branch is the part that gets forgotten.
    #[must_use]
    pub fn focused(&self, focused: bool, style: Style) -> Style {
        if focused {
            style.add_modifier(self.focus)
        } else {
            style
        }
    }
}

/// What a field currently holds, which a description never carries.
///
/// The terminal counterpart of `makeover_immediate::Filling`, and the same seam:
/// there the widget writes through a `&mut` as the value is edited, and here the
/// caller keeps an edit buffer and lends it out for the draw. Neither is
/// something [`Field`] could carry without becoming a form model.
///
/// An enum rather than a bag of options, for `Filling`'s reason: a checkbox
/// holding a string is unsayable here, where a struct would let it be said and
/// then have to cope.
#[derive(Debug, Default, Clone, Copy, PartialEq, Eq)]
pub enum Held<'a> {
    /// Nothing typed and nothing chosen. The control draws empty.
    #[default]
    Absent,
    /// What is in the box, or the `value` of the chosen [`Choice`].
    ///
    /// [`Choice`]: makeover_layout::Choice
    Text(&'a str),
    /// A checkbox, on or off.
    On(bool),
    /// Both ends of a [`FieldKind::Interval`], lower first.
    ///
    /// Two values rather than one string with a separator, which is
    /// [`makeover_layout::Field::upper_name`]'s reason one level down: an
    /// interval is submitted under two names, so it is held as two values, and
    /// a delimiter this crate owned could appear inside either of them.
    ///
    /// Either end may be empty while the other stands. An open end is an
    /// answer -- "over 120 BPM" -- rather than a half-filled box.
    ///
    /// Added 0.33.0 with makeover-layout 0.34.0.
    Between {
        /// What the lower box holds now.
        lower: &'a str,
        /// What the upper box holds now.
        upper: &'a str,
    },
}

impl<'a> Held<'a> {
    /// What is typed, as a string. A checkbox has no text and answers empty.
    #[must_use]
    pub const fn text(self) -> &'a str {
        match self {
            Self::Text(text) | Self::Between { lower: text, .. } => text,
            Self::Absent | Self::On(_) => "",
        }
    }

    /// The upper end, for the one variant that has one.
    #[must_use]
    pub const fn upper(self) -> &'a str {
        match self {
            Self::Between { upper, .. } => upper,
            Self::Absent | Self::Text(_) | Self::On(_) => "",
        }
    }

    /// Whether a checkbox is ticked.
    #[must_use]
    pub const fn on(self) -> bool {
        matches!(self, Self::On(true))
    }
}

/// What a host can see about a wait that is running.
///
/// Neither half is derivable from a description, which is why both are here and
/// not on [`Awaiting`]. That type says how big the payload is; how much of it
/// has landed is a fact about a transfer in flight, and only whoever is running
/// the transfer knows it.
///
/// The same shape `makeover-immediate` carries, deliberately: a wait is one
/// reading on every surface and the two renderers should not disagree about
/// what a host owes them.
#[derive(Debug, Clone, Copy, Default, PartialEq, Eq)]
pub struct Progress {
    /// How much has arrived, in whatever unit the description counted.
    pub delivered: Option<u64>,
    /// How long the wait has lasted so far.
    ///
    /// The one time value a wait may show. See [`awaiting`] for the three it
    /// may not.
    pub elapsed: Option<Duration>,
}

/// The activity mark: one cell, lit or dark.
///
/// Rule 2 of wiki `loading-and-progress-standard`, and the surface the metaphor
/// came from. A hard-disk light is one cell that blinks, and a terminal draws
/// that with no metaphor in the way — where a webview needs a keyframe and egui
/// needs a repaint schedule, this is a character.
///
/// The two glyphs are [`PieceStyle::meter_full`] and
/// [`PieceStyle::meter_empty`], not a third pair. A bar's filled cell and a lit
/// mark are the same statement in the same alphabet, and a terminal that had to
/// render two vocabularies of "on" would be saying there are two kinds of on.
///
/// **Dark, not absent.** A mark that is drawn half the time is a hole in the
/// line, and the line reflows around it or the reader loses where to look. It
/// occupies its cell either way.
///
/// `lit` is the caller's: this module holds no clock. [`crate::activity_lit`]
/// is the one place the phase is worked out from the cadence, so a caller
/// should reach for that rather than dividing by 500 itself.
#[must_use]
pub fn activity(style: &PieceStyle, lit: bool) -> Span<'static> {
    if lit {
        Span::styled(style.meter_full.to_string(), style.action)
    } else {
        Span::styled(style.meter_empty.to_string(), style.muted)
    }
}

/// A wait as one line, drawn from what is actually known about it.
///
/// [`Awaiting::is_determinate`] is the first branch and there is a second the
/// description cannot answer: whether anything is watching the transfer. A bar
/// wants a total and a numerator both, so a described amount with no
/// [`Progress::delivered`] beside it draws the mark and the size it is waiting
/// on, rather than an empty trough implying somebody is counting.
///
/// So three drawings for three states, which is the point:
///
/// ```text
/// unmeasured                       #            a blinking cell
/// measured, nothing watching       # 41943040   the cell, and how much there is
/// measured and observed            ####------ 17825792/41943040  4s
/// ```
///
/// **What the bar may not do**, from rule 1 of the standard and from
/// [`Awaiting`]'s own docs: what is done over what there is, plus the time it
/// has taken. Never a remaining time, an arrival time, or a rate extrapolated
/// forward. A prediction is wrong the moment the transfer stalls, and being
/// confidently wrong is worse than being honestly indeterminate.
///
/// The numbers are raw. The unit is the app's — bytes for an upload, rows for
/// an import — and a renderer that formatted one as a file size would be
/// dressing up a quantity it was deliberately not told about.
#[must_use]
pub fn awaiting(
    style: &PieceStyle,
    awaiting: Awaiting,
    progress: Progress,
    lit: bool,
) -> Line<'static> {
    let Some(total) = awaiting.amount else {
        return Line::from(vec![activity(style, lit)]);
    };
    let Some(done) = progress.delivered else {
        return Line::from(vec![
            activity(style, lit),
            Span::styled(format!(" {total}"), style.muted),
        ]);
    };
    let cells = u32::from(style.meter_cells);
    // In cells rather than in floating point, the way `meter` does it: a
    // terminal's bar has ten states and rounding through an f64 to reach one of
    // ten is arithmetic nobody needs. Saturating rather than wrapping, because
    // a transfer that over-delivers is a real case and a panicking bar is not
    // the way to report it.
    let filled = u32::try_from(
        done.saturating_mul(u64::from(cells))
            .checked_div(total)
            .unwrap_or(0),
    )
    .unwrap_or(cells)
    .min(cells);
    let bar = format!(
        "{}{}",
        style.meter_full.to_string().repeat(filled as usize),
        style
            .meter_empty
            .to_string()
            .repeat((cells - filled) as usize)
    );
    let reading = match progress.elapsed {
        Some(elapsed) => format!(" {done}/{total}  {}s", elapsed.as_secs()),
        None => format!(" {done}/{total}"),
    };
    Line::from(vec![
        Span::styled(bar, style.action),
        Span::styled(reading, style.muted),
    ])
}

/// A proportion as one line: the bar, then the reading beside it.
///
/// The reading is built here from the two numbers and the noun rather than
/// taken assembled, which is what [`Meter::label`] carrying the noun alone is
/// for: a terminal at one line and a tooltip want different sentence orders.
#[must_use]
pub fn meter(style: &PieceStyle, meter: &Meter<'_>) -> Line<'static> {
    let cells = u32::from(style.meter_cells);
    let filled = meter
        .done
        .checked_mul(cells)
        .and_then(|reached| reached.checked_div(meter.total))
        .unwrap_or(0)
        .min(cells);
    let bar = format!(
        "{}{}",
        style.meter_full.to_string().repeat(filled as usize),
        style
            .meter_empty
            .to_string()
            .repeat((cells - filled) as usize)
    );
    let reading = match meter.label {
        Some(label) => format!(" {}/{} {label}", meter.done, meter.total),
        None => format!(" {}/{}", meter.done, meter.total),
    };
    Line::from(vec![
        Span::styled(bar, style.tone(meter.tone)),
        Span::styled(reading, style.muted),
    ])
}

/// A badge or a chip as one span.
///
/// Round for a badge, square for a chip. A chip answers a press and a badge does
/// not, and the bracket is the only affordance a cell has left once colour is
/// spent on the tone.
///
/// `latched` is a chip that is switched on, and it reads as reversed. So does
/// focus, which is a collision a terminal cannot avoid: latched is "this filter
/// is on" and focused is "you are here", and there is one spare axis for two
/// facts. Said here rather than resolved by inventing a third look nobody would
/// read.
///
/// A chip's removable half is not drawn. The `x` a webview hangs on a chip is a
/// second control inside one span, and a terminal reaches a control by focusing
/// it; two targets in one cell run is a question for whoever owns the
/// interaction, not for a drawing.
#[must_use]
pub fn token(
    style: &PieceStyle,
    label: &str,
    kind: Token,
    tone: Tone,
    latched: bool,
    focused: bool,
) -> Span<'static> {
    let painted = style.tone(tone);
    let painted = if latched {
        painted.add_modifier(style.focus)
    } else {
        style.focused(focused, painted)
    };
    match kind {
        Token::Badge => Span::styled(format!("({label})"), painted),
        Token::Chip { .. } => Span::styled(format!("[{label}]"), painted),
    }
}

/// A control as one line.
///
/// `< Label > (key)`, and the key only where the description named one. That
/// member is the one place `makeover-layout` anticipated a terminal before there
/// was one, and this is the renderer that reads it.
///
/// A disabled control is drawn muted and is not marked focused, whatever the
/// caller passed: it is present, visible and not answering, so a focus mark on
/// it would be an affordance that lies. Whether it is reachable at all is the
/// caller's count to keep — ask [`Act::disabled`].
#[must_use]
pub fn act(style: &PieceStyle, act: &Act<'_>, focused: bool) -> Line<'static> {
    let painted = if act.disabled() {
        style.muted
    } else {
        style.focused(focused, style.tone(act.tone))
    };
    let label = match act.key {
        Some(key) => format!("< {} > ({key})", act.label),
        None => format!("< {} >", act.label),
    };
    Line::from(Span::styled(label, painted))
}

/// A control filled with the action colour, for the one press a screen is about.
///
/// `[ Label ]` rather than `< Label >`, which is the weight difference a webview
/// carries as a primary-versus-secondary button. A form's submit is the case
/// this exists for.
#[must_use]
pub fn filled_act(style: &PieceStyle, label: &str, focused: bool) -> Line<'static> {
    Line::from(Span::styled(
        format!("[ {label} ]"),
        style.focused(focused, style.filled),
    ))
}

/// The rows [`figure`] wants at `width`.
#[must_use]
pub fn figure_height(figure: &Figure<'_>, width: u16) -> u16 {
    text::height(figure.value, width) + text::height(figure.caption, width)
}

/// A figure: the number, then what it counts under it.
///
/// The tone lands on the value and its change rather than on the caption, which
/// is what [`Figure::tone`] means: the figure is an ordinary fact and it is the
/// movement that reads as good or bad.
pub fn figure(style: &PieceStyle, figure: &Figure<'_>, area: Rect, buf: &mut Buffer) -> u16 {
    let value = match figure.change {
        Some(change) => format!("{} {change}", figure.value),
        None => figure.value.to_owned(),
    };
    let used = text::draw(
        &value,
        style.tone(figure.tone).add_modifier(Modifier::BOLD),
        area,
        buf,
    );
    used + text::draw(figure.caption, style.muted, below(area, used), buf)
}

/// The rows [`field`] wants at `width`.
///
/// A label row, the control's rows, and a row for whatever went wrong. A hidden
/// field is nothing at all, which is the one field kind a terminal and a webview
/// agree on completely.
#[must_use]
pub fn field_height(style: &PieceStyle, field: &Field<'_>, width: u16) -> u16 {
    if !field.kind.visible() {
        return 0;
    }
    let label = text::height(&label_of(style, field), width);
    // A range is one row like every other single control: the bar, its two ends
    // and the reading are one line by construction, and a bar that wrapped
    // would stop being a bar.
    let body = match field.kind {
        // Both multi-line kinds get the same three rows, keyed on the
        // description's own `multiline` rather than on the member: a markdown
        // field falling through to the single-row arm is one line for a value
        // whose whole point is that it has several. What a terminal does *with*
        // the markdown is another question and the answer here is nothing --
        // the source is the text, and drawing it as text is honest.
        kind if kind.multiline() => 3,
        kind if kind.offers_options() => u16::try_from(field.options.len()).unwrap_or(u16::MAX),
        _ => 1,
    };
    let note = note_of(field).map_or(0, |note| text::height(note, width));
    label + body + note
}

/// A question: its label, the box, and its standing help or what is wrong now.
///
/// `held` is what the user has done to it since the screen arrived, which is the
/// argument a description cannot supply. See [`Held`].
///
/// `focused` marks the box rather than the label, because the box is where the
/// typing lands.
pub fn field(
    style: &PieceStyle,
    field: &Field<'_>,
    held: Held<'_>,
    focused: bool,
    area: Rect,
    buf: &mut Buffer,
) -> u16 {
    // A hidden field is data travelling with the form. There is nothing to
    // draw, and whoever submits carries it.
    if !field.kind.visible() || area.width == 0 || area.height == 0 {
        return 0;
    }

    let mut used = text::draw(&label_of(style, field), style.secondary, area, buf);

    let well = style.focused(focused, style.content);
    let placeholder = field.placeholder.unwrap_or_default();

    used += match field.kind {
        FieldKind::Checkbox => text::draw(
            if held.on() { "[x]" } else { "[ ]" },
            well,
            below(area, used),
            buf,
        ),
        // A range's two ends are what the question means, so they are drawn
        // rather than left to a hint. A terminal has the bar already: this is
        // `meter`'s cells with the extent read out at either side of them.
        //
        // An unbounded range has no extent to draw and falls through to the
        // text path, which is `makeover-immediate`'s answer as well and for the
        // same reason: bounds this crate invented are bounds the user would
        // then drag against.
        FieldKind::Range if field.bounded() => {
            let line = range_line(style, field, held.text(), well);
            text::draw_line(&line, below(area, used), buf)
        }
        // One question, so one line. The two ends read left to right with the
        // word between them, which is what a terminal has instead of two boxes
        // side by side: a second row would read as a second question, and that
        // is the reading the kind exists to prevent.
        FieldKind::Interval => {
            let line = interval_line(style, field, held, well);
            text::draw_line(&line, below(area, used), buf)
        }
        kind if kind.offers_options() => {
            let mut rows = 0;
            for choice in field.options {
                let chosen = held.text() == choice.value;
                // An option that cannot be picked yet reads as inert, which is
                // the one place muted is the truth rather than the lie below:
                // it will not answer, and the reason it will not is on the row
                // beside it rather than nowhere.
                let (mark, painted, suffix) = match choice.unavailable {
                    Some(reason) => ("( )", style.muted, format!(": {reason}")),
                    None if chosen => ("(*)", well, String::new()),
                    // An option that is not chosen is still an option: pressing
                    // it chooses it. So it takes the secondary content intent
                    // and not the muted one, which is what disabled looks like
                    // (`State::Disabled` resolves to it). Muted here read as a
                    // list of five where four were greyed out.
                    None => ("( )", style.secondary, String::new()),
                };
                rows += text::draw(
                    &format!("{mark} {}{suffix}", choice.label),
                    painted,
                    below(area, used + rows),
                    buf,
                );
            }
            rows
        }
        // A secret's dots come from the caller's buffer and can come from
        // nowhere else: a password that comes back down the wire is a password
        // in a page and in a proxy log, so a description carries nothing to dot
        // out. This is the one control that would be undrawable without `held`.
        FieldKind::Secret if !held.text().is_empty() => {
            let dots = "*".repeat(held.text().chars().count());
            text::draw(&dots, well, below(area, used), buf).max(1)
        }
        // A file field has no way back on a terminal any more than it has on an
        // HTTP host. The name is drawn and picking one belongs to whoever owns
        // the interaction.
        //
        // makeover-layout 0.31.0 gave the description an accept list and a
        // multiplicity, and neither changes anything drawn here. Both are the
        // picker's business, and the picker is the caller's: this crate draws
        // what was picked. A terminal that grows its own picker reads them off
        // `Field::accept` and `Field::multiple` at that point rather than
        // through a second spelling invented here.
        _ if held.text().is_empty() => {
            empty_well(style, placeholder, well, focused, below(area, used), buf)
        }
        _ => text::draw(&measured(field, held.text()), well, below(area, used), buf),
    };

    // The error wins over the hint, the same order a webview uses: a hint is
    // what to type and an error is what went wrong, and once something has gone
    // wrong that is the sentence worth the row.
    match note_of(field) {
        Some(note) => {
            let painted = if field.error.is_some() {
                style.danger
            } else {
                style.muted
            };
            used + text::draw(note, painted, below(area, used), buf)
        }
        None => used,
    }
}

/// A bounded number as one line: the low end, the bar, the high end, then what
/// it currently reads.
///
/// The two ends are drawn because they are the question. A threshold of 0.72
/// says nothing without them, which is the whole argument for
/// [`FieldKind::Range`] being a kind rather than a number with bounds, and a
/// terminal is where it would be easiest to quietly drop them and show a figure.
///
/// The bar is [`meter`]'s cells, so a range and a proportion read as the same
/// object in the same app. What differs is the reading beside it: a meter counts
/// something and a range holds a value.
///
/// A value the host cannot read as a number empties the bar and is still shown
/// as itself. That is [`empty_well`]'s position on an unreadable value: the app
/// put it there, and a terminal that silently rounded it to a bound would be
/// reporting a value nobody set.
fn range_line(style: &PieceStyle, field: &Field<'_>, value: &str, well: Style) -> Line<'static> {
    let cells = usize::from(style.meter_cells);
    let ends = field
        .min
        .zip(field.max)
        .and_then(|(min, max)| Some((min.parse::<f64>().ok()?, max.parse::<f64>().ok()?)));
    let filled = match (ends, value.parse::<f64>()) {
        (Some((min, max)), Ok(number)) if max > min => {
            // Where the value sits is the curve's answer, not a proportion of
            // the extent (makeover-layout 0.32.0). Under `Curve::Linear` the two
            // are the same number, which is why the bar was right before and is
            // unchanged for every range described so far; under a constant ratio
            // they are not, and a bar drawn linearly would put an envelope's
            // whole useful half inside its first cell.
            #[expect(
                clippy::cast_possible_truncation,
                clippy::cast_sign_loss,
                reason = "`position_of` returns 0..=1, and the cell count came from a u16"
            )]
            let reached = (field.curve.position_of(number, min, max) * cells as f64) as usize;
            reached.min(cells)
        }
        _ => 0,
    };
    let bar = format!(
        "{}{}",
        style.meter_full.to_string().repeat(filled),
        style.meter_empty.to_string().repeat(cells - filled)
    );
    Line::from(vec![
        Span::styled(format!("{} ", field.min.unwrap_or_default()), style.muted),
        Span::styled(bar, well),
        Span::styled(format!(" {}", field.max.unwrap_or_default()), style.muted),
        Span::styled(format!(" {}", measured(field, value)), well),
    ])
}

/// An interval as one line: the low end, the word, the high end.
///
/// One line because it is one question. Two rows would read as two questions,
/// which is exactly what [`FieldKind::Interval`] exists to stop the description
/// saying, and a terminal has no side-by-side boxes to fall back on.
///
/// # An open end draws the bound it falls back to
///
/// Muted, because it is where the axis ends rather than a value anybody set.
/// With no bound to fall back on there is nothing honest to draw and the end
/// stays blank: a terminal inventing a number here would report a filter the
/// user never applied, which is [`range_line`]'s position on an unreadable
/// value.
///
/// # The word, not a dash
///
/// A dash between two numbers is a minus sign to anyone reading a signed axis,
/// and half the measured axes are signed -- audiofiles filters loudness in
/// dBFS. `to` costs two cells and cannot be misread.
fn interval_line(
    style: &PieceStyle,
    field: &Field<'_>,
    held: Held<'_>,
    well: Style,
) -> Line<'static> {
    let end = |value: &str, fallback: Option<&str>| match (value.is_empty(), fallback) {
        (false, _) => Span::styled(measured(field, value), well),
        (true, Some(bound)) => Span::styled(measured(field, bound), style.muted),
        (true, None) => Span::styled(String::new(), style.muted),
    };
    Line::from(vec![
        end(held.text(), field.min),
        Span::styled(" to ", style.secondary),
        end(held.upper(), field.max),
    ])
}

/// The unit to draw beside this field's value, if there is one to draw.
///
/// Two conditions rather than one: the field has to carry a unit and its kind
/// has to be one that means anything by it. `FieldKind::measurable` is the
/// description answering the second, so this renderer keeps no list of its own
/// of which kinds are quantities.
fn unit_of<'a>(field: &Field<'a>) -> Option<&'a str> {
    field.unit.filter(|_| field.kind.measurable())
}

/// A value with what it is measured in, as one string.
///
/// The unit rides on the value rather than on the label, which is
/// `makeover-layout` 0.33.0's rule and is what a terminal wants anyway: the
/// label is a line above and the number is the line the eye is on.
fn measured(field: &Field<'_>, value: &str) -> String {
    match unit_of(field) {
        Some(unit) => format!("{value} {unit}"),
        None => value.to_owned(),
    }
}

/// The label, marked where the field is compulsory.
fn label_of(style: &PieceStyle, field: &Field<'_>) -> String {
    if field.required {
        format!("{} {}", field.label, style.required_marker)
    } else {
        field.label.to_owned()
    }
}

/// What goes under the box: what is wrong now, or the standing help.
fn note_of<'a>(field: &Field<'a>) -> Option<&'a str> {
    field.error.or(field.hint)
}

/// A box with nothing in it: the ghost text, and the caret when it has focus.
///
/// The caret is not decoration. An empty field under a style is an empty field,
/// so a focused one with no placeholder drew literally nothing and there was no
/// way to tell the box was where the typing would go. A browser has a blinking
/// bar for this and gets it without asking; a terminal has one cell of reversed
/// video, put on the first column, which is where the first character lands.
fn empty_well(
    style: &PieceStyle,
    placeholder: &str,
    well: Style,
    focused: bool,
    area: Rect,
    buf: &mut Buffer,
) -> u16 {
    let used = text::draw(placeholder, style.muted, area, buf).max(1);
    if focused
        && area.height > 0
        && area.width > 0
        && let Some(cell) = buf.cell_mut((area.x, area.y))
    {
        cell.set_style(well);
    }
    used
}

/// What is left of `area` after `used` rows from the top.
fn below(area: Rect, used: u16) -> Rect {
    let used = used.min(area.height);
    Rect {
        x: area.x,
        y: area.y + used,
        width: area.width,
        height: area.height - used,
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use makeover_layout::{Choice, State};

    /// The style the drawings are read against: one distinguishable modifier
    /// per role, so a test can say which style landed without a colour.
    fn style() -> PieceStyle {
        PieceStyle {
            content: Style::new().add_modifier(Modifier::BOLD),
            secondary: Style::new().add_modifier(Modifier::ITALIC),
            muted: Style::new().add_modifier(Modifier::DIM),
            danger: Style::new().add_modifier(Modifier::CROSSED_OUT),
            ..PieceStyle::default()
        }
    }

    fn buffer(width: u16, height: u16) -> Buffer {
        Buffer::empty(Rect::new(0, 0, width, height))
    }

    /// Everything in the buffer, one string per row.
    fn rows(buf: &Buffer) -> Vec<String> {
        (0..buf.area.height)
            .map(|y| {
                (0..buf.area.width)
                    .map(|x| {
                        buf.cell((x, y))
                            .map_or(' ', |c| c.symbol().chars().next().unwrap_or(' '))
                    })
                    .collect::<String>()
                    .trim_end()
                    .to_owned()
            })
            .collect()
    }

    #[test]
    fn a_bar_fills_in_proportion_and_reads_out_the_two_numbers() {
        let style = style();
        let line = meter(&style, &Meter::new(3, 10).label("subtasks"));
        let drawn: String = line.spans.iter().map(|s| s.content.as_ref()).collect();
        assert_eq!(drawn, "###------- 3/10 subtasks");
        // The noun is optional and the ratio is not, because a bar with no
        // reading is a bar you cannot check.
        let bare = meter(&style, &Meter::new(3, 10));
        let drawn: String = bare.spans.iter().map(|s| s.content.as_ref()).collect();
        assert_eq!(drawn, "###------- 3/10");
    }

    #[test]
    fn an_empty_set_is_an_empty_bar_rather_than_a_divide_by_zero() {
        // `Meter::total` of zero means there is no set, and the checked
        // division is what keeps that from being a panic in a draw.
        let line = meter(&style(), &Meter::new(0, 0));
        let drawn: String = line.spans.iter().map(|s| s.content.as_ref()).collect();
        assert_eq!(drawn, "---------- 0/0");
    }

    #[test]
    fn an_over_run_fills_the_bar_and_still_reports_the_overflow() {
        // The clamp is for drawing only. The reading is what keeps the fact
        // `Meter::percent` destroys.
        let line = meter(&style(), &Meter::new(14, 10));
        let drawn: String = line.spans.iter().map(|s| s.content.as_ref()).collect();
        assert_eq!(drawn, "########## 14/10");
    }

    #[test]
    fn a_badge_is_round_and_a_chip_is_square() {
        // The one affordance a cell has left once colour is spent on the tone,
        // and the whole of how a terminal says "this one answers a press".
        let style = style();
        let badge = token(&style, "draft", Token::Badge, Tone::Neutral, false, false);
        assert_eq!(badge.content.as_ref(), "(draft)");
        let chip = token(
            &style,
            "rust",
            Token::Chip { removable: false },
            Tone::Neutral,
            false,
            false,
        );
        assert_eq!(chip.content.as_ref(), "[rust]");
    }

    #[test]
    fn a_latched_chip_reads_the_same_as_a_focused_one() {
        // The collision a terminal cannot avoid, asserted rather than left to
        // be rediscovered: latched is "this filter is on" and focused is "you
        // are here", and there is one spare axis for two facts.
        let style = style();
        let kind = Token::Chip { removable: false };
        let latched = token(&style, "rust", kind, Tone::Neutral, true, false);
        let focused = token(&style, "rust", kind, Tone::Neutral, false, true);
        assert_eq!(latched.style, focused.style);
        assert!(latched.style.add_modifier.contains(Modifier::REVERSED));
    }

    #[test]
    fn a_control_draws_its_key_only_where_one_was_named() {
        let style = style();
        let line = act(&style, &Act::new("Delete"), false);
        assert_eq!(line.spans[0].content.as_ref(), "< Delete >");
        let line = act(&style, &Act::new("Quit").key("q"), false);
        assert_eq!(line.spans[0].content.as_ref(), "< Quit > (q)");
    }

    #[test]
    fn a_disabled_control_is_never_marked_focused() {
        // Present, visible, and not answering. A focus mark on it would be an
        // affordance that lies, so the flag is overridden rather than trusted.
        let style = style();
        let disabled = Act::new("Save").state(State::Disabled);
        let line = act(&style, &disabled, true);
        assert!(
            !line.spans[0]
                .style
                .add_modifier
                .contains(Modifier::REVERSED)
        );
        assert_eq!(line.spans[0].style, style.muted);
        // The same call on a control the description says nothing about: the
        // mark is this renderer's own focus flag and always was, which is why
        // only `Disabled` can override it.
        let unstated = Act::new("Save");
        let line = act(&style, &unstated, true);
        assert!(
            line.spans[0]
                .style
                .add_modifier
                .contains(Modifier::REVERSED)
        );
    }

    #[test]
    fn a_danger_control_keeps_its_tone_under_focus() {
        // Focus adds a modifier rather than repainting, so the fact that this
        // is the button that destroys something survives being landed on.
        let style = style();
        let line = act(&style, &Act::new("Delete").tone(Tone::Danger), true);
        assert_eq!(
            line.spans[0].style.add_modifier,
            style.danger.add_modifier | Modifier::REVERSED
        );
    }

    #[test]
    fn a_figure_puts_the_number_over_what_it_counts() {
        let style = style();
        let figure_ = Figure::new("42", "open tasks");
        let mut buf = buffer(20, 4);
        let used = figure(&style, &figure_, buf.area, &mut buf);
        assert_eq!(used, 2);
        assert_eq!(rows(&buf)[..2], ["42".to_owned(), "open tasks".to_owned()]);
        assert_eq!(figure_height(&figure_, 20), 2);
    }

    #[test]
    fn a_figures_change_rides_on_the_value_row() {
        // The delta is the toned part and the value is an ordinary fact, so the
        // two share a row rather than the caption growing a second sentence.
        let style = style();
        let figure_ = Figure::new("42", "open tasks")
            .change("+3")
            .tone(Tone::Success);
        let mut buf = buffer(20, 4);
        figure(&style, &figure_, buf.area, &mut buf);
        assert_eq!(rows(&buf)[0], "42 +3");
    }

    #[test]
    fn a_compulsory_field_says_so_in_its_label() {
        let style = style();
        let mut field_ = Field::new(FieldKind::Text, "email", "Email");
        field_.required = true;
        let mut buf = buffer(20, 4);
        field(&style, &field_, Held::Absent, false, buf.area, &mut buf);
        assert_eq!(rows(&buf)[0], "Email *");
    }

    #[test]
    fn a_hidden_field_costs_no_rows_at_all() {
        // The one field kind a terminal and a webview agree on completely.
        let style = style();
        let field_ = Field::new(FieldKind::Hidden, "csrf", "Token");
        let mut buf = buffer(20, 4);
        assert_eq!(
            field(
                &style,
                &field_,
                Held::Text("abc"),
                false,
                buf.area,
                &mut buf
            ),
            0
        );
        assert_eq!(field_height(&style, &field_, 20), 0);
        assert_eq!(rows(&buf)[0], "");
    }

    #[test]
    fn a_secret_is_dotted_from_the_callers_buffer_and_never_from_the_description() {
        // The one control that would be undrawable without `held`: a password
        // that came back down the wire is a password in a page and in a log.
        let style = style();
        let field_ = Field::new(FieldKind::Secret, "password", "Password");
        let mut buf = buffer(20, 4);
        field(
            &style,
            &field_,
            Held::Text("hunter2"),
            false,
            buf.area,
            &mut buf,
        );
        assert_eq!(rows(&buf)[1], "*******");
    }

    #[test]
    fn an_error_takes_the_row_the_hint_would_have_had() {
        // Once something has gone wrong that is the sentence worth the row,
        // which is the order a webview uses too.
        let style = style();
        let mut field_ = Field::new(FieldKind::Text, "email", "Email");
        field_.hint = Some("work address");
        field_.error = Some("not an address");
        let mut buf = buffer(20, 5);
        field(
            &style,
            &field_,
            Held::Text("nope"),
            false,
            buf.area,
            &mut buf,
        );
        assert_eq!(rows(&buf)[2], "not an address");
        assert_eq!(field_height(&style, &field_, 20), 3);
    }

    #[test]
    fn a_focused_empty_box_shows_where_the_typing_will_land() {
        // An empty field under a style is an empty field. Without the caret a
        // focused box with no placeholder drew literally nothing.
        let style = style();
        let field_ = Field::new(FieldKind::Text, "email", "Email");
        let mut buf = buffer(20, 4);
        field(&style, &field_, Held::Absent, true, buf.area, &mut buf);
        let caret = buf.cell((0, 1)).expect("the well's first cell").style();
        assert!(caret.add_modifier.contains(Modifier::REVERSED));
    }

    #[test]
    fn a_choice_field_marks_the_chosen_option_and_costs_a_row_each() {
        let style = style();
        let mut field_ = Field::new(FieldKind::Radio, "size", "Size");
        let options = [Choice::plain("small"), Choice::plain("large")];
        field_.options = &options;
        let mut buf = buffer(20, 5);
        field(
            &style,
            &field_,
            Held::Text("large"),
            false,
            buf.area,
            &mut buf,
        );
        assert_eq!(rows(&buf)[1], "( ) small");
        assert_eq!(rows(&buf)[2], "(*) large");
        assert_eq!(field_height(&style, &field_, 20), 3);
    }

    #[test]
    fn a_range_draws_its_two_ends_and_where_the_value_sits_between_them() {
        let style = style();
        let field_ = Field::range("review", "Review above", "0", "1");
        let mut buf = buffer(40, 3);
        field(
            &style,
            &field_,
            Held::Text("0.5"),
            false,
            buf.area,
            &mut buf,
        );
        // Ten cells by default, half of them filled, with the extent read out
        // at either side: 0.5 means nothing without the 0 and the 1.
        assert_eq!(rows(&buf)[1].trim_end(), "0 #####----- 1 0.5");
        assert_eq!(field_height(&style, &field_, 40), 2);
    }

    #[test]
    fn a_unit_rides_on_the_value_and_not_on_the_label() {
        // The label is a line above; the number is the line the eye is on.
        let style = style();
        let field_ = Field {
            unit: Some("s"),
            ..Field::range("attack", "Attack", "0", "5")
        };
        let mut buf = buffer(40, 3);
        field(
            &style,
            &field_,
            Held::Text("2.5"),
            false,
            buf.area,
            &mut buf,
        );
        assert_eq!(rows(&buf)[0].trim_end(), "Attack");
        assert_eq!(rows(&buf)[1].trim_end(), "0 #####----- 5 2.5 s");
    }

    #[test]
    fn a_typed_number_reads_with_its_unit_too() {
        let style = style();
        let field_ = Field {
            unit: Some("ms"),
            ..Field::new(FieldKind::Number, "fade", "Fade")
        };
        let mut buf = buffer(40, 3);
        field(&style, &field_, Held::Text("50"), false, buf.area, &mut buf);
        assert_eq!(rows(&buf)[1].trim_end(), "50 ms");
    }

    #[test]
    fn a_unit_on_a_kind_that_is_not_a_quantity_is_ignored() {
        // Which kinds are quantities is the description's answer, not a
        // `matches!` kept in this crate.
        let style = style();
        let field_ = Field {
            unit: Some("s"),
            ..Field::new(FieldKind::Text, "name", "Name")
        };
        let mut buf = buffer(40, 3);
        field(
            &style,
            &field_,
            Held::Text("kick"),
            false,
            buf.area,
            &mut buf,
        );
        assert_eq!(rows(&buf)[1].trim_end(), "kick");
    }

    #[test]
    fn an_interval_is_one_line_with_both_ends_on_it() {
        // One question, one line. Two rows would read as two questions, which
        // is the reading the kind exists to prevent.
        let style = style();
        let field_ = Field {
            min: Some("0"),
            max: Some("300"),
            unit: Some("BPM"),
            ..Field::interval("bpm_min", "bpm_max", "BPM range")
        };
        let mut buf = buffer(40, 3);
        field(
            &style,
            &field_,
            Held::Between {
                lower: "90",
                upper: "130",
            },
            false,
            buf.area,
            &mut buf,
        );
        assert_eq!(rows(&buf)[0].trim_end(), "BPM range");
        assert_eq!(rows(&buf)[1].trim_end(), "90 BPM to 130 BPM");
        assert_eq!(rows(&buf)[2].trim_end(), "");
    }

    #[test]
    fn an_open_end_falls_back_to_the_bound_it_means() {
        // "Over 120" is an answer rather than a half-filled box, and where the
        // axis ends is what the empty end stands for.
        let style = style();
        let field_ = Field {
            min: Some("0"),
            max: Some("300"),
            ..Field::interval("bpm_min", "bpm_max", "BPM range")
        };
        let mut buf = buffer(40, 3);
        field(
            &style,
            &field_,
            Held::Between {
                lower: "120",
                upper: "",
            },
            false,
            buf.area,
            &mut buf,
        );
        assert_eq!(rows(&buf)[1].trim_end(), "120 to 300");
    }

    #[test]
    fn an_unbounded_open_end_draws_nothing_rather_than_a_number() {
        // A terminal inventing a bound here would report a filter nobody
        // applied, which is `range_line`'s position on an unreadable value.
        // What is left reads as the sentence it is: up to 130.
        let style = style();
        let field_ = Field::interval("bpm_min", "bpm_max", "BPM range");
        let mut buf = buffer(40, 3);
        field(
            &style,
            &field_,
            Held::Between {
                lower: "",
                upper: "130",
            },
            false,
            buf.area,
            &mut buf,
        );
        assert_eq!(rows(&buf)[1].trim_end(), "to 130");
    }

    #[test]
    fn a_range_holding_something_unreadable_still_shows_it() {
        // The app put the value there. A terminal that quietly rounded it to a
        // bound would be reporting a value nobody set, which is `empty_well`'s
        // position on the same problem.
        let style = style();
        let field_ = Field::range("review", "Review above", "0", "1");
        let mut buf = buffer(40, 3);
        field(
            &style,
            &field_,
            Held::Text("unset"),
            false,
            buf.area,
            &mut buf,
        );
        assert_eq!(rows(&buf)[1].trim_end(), "0 ---------- 1 unset");
    }

    #[test]
    fn an_unbounded_range_is_typed_into_rather_than_dragged() {
        // Bounds this crate invented are bounds the user would then drag
        // against. The text path takes every answer the bar would.
        let style = style();
        let field_ = Field {
            max: Some("1"),
            ..Field::new(FieldKind::Range, "review", "Review above")
        };
        let mut buf = buffer(40, 3);
        field(
            &style,
            &field_,
            Held::Text("0.5"),
            false,
            buf.area,
            &mut buf,
        );
        assert_eq!(rows(&buf)[1].trim_end(), "0.5");
    }

    #[test]
    fn an_unavailable_option_reads_as_inert_and_says_why() {
        // The one place muted is the truth rather than the lie the convention
        // warns about: this option will not answer, and the reason is on the
        // row rather than nowhere.
        let style = style();
        let options = [
            Choice::new("chromatic", "Chromatic"),
            Choice::new("multi", "Multi-sample").unless("Drop a second sample."),
        ];
        let mut field_ = Field::new(FieldKind::Radio, "mode", "Mode");
        field_.options = &options;
        let mut buf = buffer(46, 4);
        field(
            &style,
            &field_,
            Held::Text("chromatic"),
            false,
            buf.area,
            &mut buf,
        );
        assert_eq!(rows(&buf)[1].trim_end(), "(*) Chromatic");
        assert_eq!(
            rows(&buf)[2].trim_end(),
            "( ) Multi-sample: Drop a second sample."
        );
        let muted = buf.cell((0, 2)).expect("the unavailable row").style();
        assert!(muted.add_modifier.contains(Modifier::DIM));
    }

    #[test]
    fn an_unchosen_option_does_not_read_as_disabled() {
        // The three-tone convention: muted is inert, and every option in this
        // list answers a press. Drawn muted, a five-option radio read as one
        // live row and four dead ones.
        let style = style();
        let mut field_ = Field::new(FieldKind::Radio, "size", "Size");
        let options = [Choice::plain("small"), Choice::plain("large")];
        field_.options = &options;
        let mut buf = buffer(20, 5);
        field(
            &style,
            &field_,
            Held::Text("large"),
            false,
            buf.area,
            &mut buf,
        );
        let unchosen = buf.cell((0, 1)).expect("the first option").style();
        assert_eq!(unchosen.add_modifier, style.secondary.add_modifier);
        assert_ne!(unchosen.add_modifier, style.muted.add_modifier);
    }

    #[test]
    fn a_checkbox_reads_a_bool_rather_than_a_submitted_string() {
        // `Held::On` exists so a host's own submission convention -- quasi
        // sends "value" -- stays the host's and never reaches a drawing.
        let style = style();
        let field_ = Field::new(FieldKind::Checkbox, "agree", "Agree");
        let mut buf = buffer(20, 4);
        field(&style, &field_, Held::On(true), false, buf.area, &mut buf);
        assert_eq!(rows(&buf)[1], "[x]");
        let mut buf = buffer(20, 4);
        field(&style, &field_, Held::On(false), false, buf.area, &mut buf);
        assert_eq!(rows(&buf)[1], "[ ]");
    }

    #[test]
    fn a_markdown_field_gets_the_rows_a_textarea_does() {
        // Keyed on `multiline`, so a member added upstream does not silently
        // land on the single-row arm. One row for a value whose whole point is
        // that it has several is the failure this replaced.
        let style = PieceStyle::default();
        let rich = Field::new(FieldKind::Rich, "body", "Body");
        let textarea = Field::new(FieldKind::Textarea, "body", "Body");
        let plain = Field::new(FieldKind::Text, "body", "Body");

        assert_eq!(
            field_height(&style, &rich, 40),
            field_height(&style, &textarea, 40)
        );
        assert!(field_height(&style, &rich, 40) > field_height(&style, &plain, 40));
    }

    #[test]
    fn a_tone_and_a_heading_map_without_a_fallback_arm() {
        // Both source enums are closed, which is what lets these be total. A
        // renderer that had to guess would be picking its own colours again.
        let style = style();
        assert_eq!(style.tone(Tone::Neutral), style.content);
        assert_eq!(style.tone(Tone::Danger), style.danger);
        assert_eq!(style.heading(Heading::Page), style.page);
        assert_eq!(style.heading(Heading::Subsection), style.subsection);
    }

    #[test]
    fn the_default_style_carries_no_colour_at_all() {
        // A two-colour terminal is the case where a foreground will not land,
        // so the default is modifiers only rather than a placeholder palette.
        let style = PieceStyle::default();
        for painted in [style.content, style.danger, style.page, style.action] {
            assert_eq!(painted.fg, None);
            assert_eq!(painted.bg, None);
        }
    }

    #[test]
    fn the_three_states_of_a_wait_are_three_drawings() {
        // The whole done condition of `5db1e0ed`: a measured wait and an
        // unmeasured one stopped being the same line.
        let style = PieceStyle::default();
        let bare = awaiting(&style, Awaiting::unmeasured(), Progress::default(), true);
        let sized = awaiting(&style, Awaiting::of(41_943_040), Progress::default(), true);
        let watched = awaiting(
            &style,
            Awaiting::of(40),
            Progress {
                delivered: Some(20),
                elapsed: Some(Duration::from_secs(4)),
            },
            true,
        );
        let read = |line: &Line<'_>| {
            line.spans
                .iter()
                .map(|s| s.content.to_string())
                .collect::<String>()
        };
        assert_eq!(read(&bare), "#");
        assert_eq!(read(&sized), "# 41943040");
        assert_eq!(read(&watched), "#####----- 20/40  4s");
    }

    #[test]
    fn a_dark_mark_still_occupies_its_cell() {
        // Not absent. A line that reflowed every half second would move the
        // content beside it, and the reader would lose where to look.
        let style = PieceStyle::default();
        assert_eq!(activity(&style, true).content.chars().count(), 1);
        assert_eq!(activity(&style, false).content.chars().count(), 1);
    }

    #[test]
    fn an_over_delivered_wait_clamps_and_does_not_panic() {
        // A transfer can hand over more than the size it announced, and the
        // bar has ten cells whatever happens.
        let style = PieceStyle::default();
        let over = awaiting(
            &style,
            Awaiting::of(4),
            Progress {
                delivered: Some(9),
                elapsed: None,
            },
            true,
        );
        assert!(over.spans[0].content.chars().all(|c| c == '#'));
        assert_eq!(over.spans[0].content.chars().count(), 10);
        // A zero payload is no payload rather than a finished one.
        let empty = awaiting(
            &style,
            Awaiting::of(0),
            Progress {
                delivered: Some(9),
                elapsed: None,
            },
            true,
        );
        assert!(empty.spans[0].content.starts_with('-'));
    }
}