//! Column layout and row structure for tables.
//!
//! `makeover-webview`'s `list` module in the shape a terminal allows. It owns
//! the same four things: which columns exist, how wide they are, which ones
//! survive a narrow viewport, and what each part of a cell is. It does not own
//! what goes in a cell, for the reason that module states: a cell holds whatever
//! the app builds, and a description expressive enough to emit a task row's five
//! nested spans is a templating language wearing a description's name.
//!
//! # What ratatui already answers
//!
//! Most of the drawing. [`ratatui::widgets::Table`] lays tracks out from
//! [`Constraint`]s, draws a header, highlights a selected row and scrolls
//! through [`TableState`](ratatui::widgets::TableState). So this is a mapping
//! layer over it rather than a second table implementation, and it hands back a
//! `Table` instead of painting one: selection and scroll belong to the app's
//! state, and a function that painted would have to take that state to give it
//! back.
//!
//! Two things ratatui does not answer, and they are what this module is:
//!
//! - **Content measurement.** There is no track that sizes to what is in it, so
//!   [`Width::Content`] is measured here from the cells and the heading.
//! - **Narrowing.** A terminal window is resized far more often than a browser
//!   one, and [`Priority`] is how a column earns its place. See below.
//!
//! # Why positions are the bug
//!
//! Carried from the webview renderer verbatim, because the mistake is not a CSS
//! mistake. goingson hides its mobile columns with `nth-child(n+5)` against a
//! seven-column table; insert a column left of the cut and the wrong one
//! disappears, silently, because nothing in the rule knows what column five
//! *is*. A renderer narrows by raising a cutoff and never by counting, which is
//! the whole reason [`Priority`] exists. `a_column_inserted_left_of_the_cut_does_not_change_what_drops`
//! is that bug as a test.
//!
//! # What it costs when nothing fits
//!
//! [`Priority::Essential`] never drops, so a window narrower than the essential
//! columns leaves them overflowing rather than emptying the table. That is
//! deliberate: a row that cannot identify itself is not a narrower row, it is a
//! different one, and ratatui truncates a cell it cannot fit. Truncated and
//! present beats absent.

use makeover_layout::{CellPart, Column, Priority, Sort, Width};
use ratatui::layout::Constraint;
use ratatui::style::{Modifier, Style};
use ratatui::text::Line;
use ratatui::widgets::{Cell as TrackCell, Row, Table};

/// The cutoffs, weakest first.
///
/// [`Priority`] is `#[non_exhaustive]` and a tier added upstream has to be added
/// here in its place in the sequence, or a table will never narrow to it. Grep
/// this when adopting a new `makeover-layout`, the way
/// `makeover-webview`'s `part_class` asks to be grepped. The cost of missing one
/// is a column that drops later than it should, which is visible, rather than a
/// build that stops.
const CUTOFFS: [Priority; 3] = [Priority::Optional, Priority::Secondary, Priority::Essential];

/// The lengths the description deferred, in cells.
///
/// [`Width`] says `Content`, `Fixed` or `Fill` and carries no magnitude, because
/// a magnitude is an answer for one renderer and the description is read by
/// three. `makeover-webview`'s `Sizing` is this same type holding CSS lengths;
/// this one holds terminal cells, and both are looked up by column name for the
/// same reason: an app's columns are not all one size.
#[derive(Debug, Clone, Copy, Default)]
pub struct Sizing<'a> {
    /// `(column name, cells)`. The track for a [`Width::Fixed`] column and the
    /// floor for a [`Width::Fill`] one.
    pub lengths: &'a [(&'a str, u16)],
    /// Used for a column with no entry above.
    pub fallback: u16,
}

impl Sizing<'_> {
    /// The length for a named column.
    fn length_for(&self, name: &str) -> u16 {
        self.lengths
            .iter()
            .find(|(column, _)| *column == name)
            .map_or(self.fallback, |(_, length)| *length)
    }
}

/// One cell of a row.
///
/// The contents are a ratatui [`Line`] rather than a string, which is this
/// crate's version of the webview `Cell` holding markup: the app owns what goes
/// in the cell, spans and all, and says which column it belongs to by name.
#[derive(Debug, Clone)]
pub struct Cell<'a> {
    /// Which column this fills, by name.
    pub column: &'a str,
    /// What the cell holds, when the whole cell is one thing.
    ///
    /// `None` for a cell mixing parts. A cell holding a value *and* a strip of
    /// tokens *and* a control is three parts in one cell, and a terminal cell
    /// has one style to give, so the app styles the spans itself. This field is
    /// for the single-part case, which is the common one.
    pub part: Option<CellPart>,
    /// The contents.
    pub content: Line<'a>,
}

impl<'a> Cell<'a> {
    /// A cell with no cell part.
    #[must_use]
    pub fn new(column: &'a str, content: impl Into<Line<'a>>) -> Self {
        Self {
            column,
            part: None,
            content: content.into(),
        }
    }

    /// The same cell, saying which part it is.
    #[must_use]
    pub fn part(mut self, part: CellPart) -> Self {
        self.part = Some(part);
        self
    }
}

/// The tones and metrics a table draws with.
///
/// Apart from [`Palette`] rather than added to it, and the split is the one
/// `makeover-immediate` draws between its palette and its `FieldStyle`:
/// [`Palette`] answers what a *surface* is, which is what
/// [`frame`](crate::frame) needs, and a table is the first thing in this crate
/// that draws text. Folding text tones into [`Palette`] would make every
/// consumer that only paints a bevel supply six colours it never uses.
///
/// [`from_theme`](Self::from_theme) is the answer for anyone with a loaded
/// theme, and is what a consumer should reach for first.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct TableStyle {
    /// The heading row.
    pub header: Style,
    /// The heading of the column the table is ordered by.
    pub sorted: Style,
    /// The heading of a column that offers to reorder and is not doing it now.
    ///
    /// The middle of three tones (wiki `three-tone-convention`): it answers a
    /// press, so it is neither the emphasised thing nor the inert one. A
    /// heading that took [`header`](Self::header) here would be indistinguishable
    /// from a column that cannot be reordered at all, which is the state this
    /// separates it from.
    pub sortable: Style,
    /// A cell that is text.
    pub value: Style,
    /// A cell holding badges or chips. They carry their own tone, so this is
    /// what sits under one rather than what paints it.
    pub tokens: Style,
    /// A cell holding controls.
    pub actions: Style,
    /// A cell whose value is itself a link.
    pub link: Style,
    /// The row under the cursor, for a caller rendering with a
    /// [`TableState`](ratatui::widgets::TableState).
    pub selected: Style,
    /// Cells between columns. Counted when deciding what fits, so a table that
    /// narrows and a table that draws agree about the room available.
    pub column_spacing: u16,
    /// The caret drawn after the heading of an ascending column.
    ///
    /// Defaults to [`Sort::glyph`], which is where the spelling lives now: three
    /// renderers holding the same literal agreed by coincidence. Still a knob,
    /// because a terminal is the one host that may not be able to draw it — a
    /// font without the geometric-shapes block leaves a box, and `"^"` is a
    /// better caret than a tofu.
    ///
    /// Bare, with no leading space: the gap is [`heading`]'s, written once for
    /// all three states rather than baked into two strings and forgotten in the
    /// third.
    pub ascending: &'static str,
    /// The caret drawn after the heading of a descending column.
    pub descending: &'static str,
}

impl Default for TableStyle {
    fn default() -> Self {
        Self {
            header: Style::new().add_modifier(Modifier::BOLD),
            sorted: Style::new().add_modifier(Modifier::BOLD),
            // Nothing of its own. A cell style patches the row's, so a colour
            // is the only thing that could separate this from the header row it
            // sits in, and the colourless default has none to spend: the idle
            // caret is what says the heading answers a press. `from_theme` is
            // where the three tones are real.
            sortable: Style::new(),
            value: Style::new(),
            tokens: Style::new(),
            actions: Style::new(),
            link: Style::new().add_modifier(Modifier::UNDERLINED),
            selected: Style::new().add_modifier(Modifier::REVERSED),
            column_spacing: 1,
            ascending: Sort::Ascending.glyph(),
            descending: Sort::Descending.glyph(),
        }
    }
}

impl TableStyle {
    /// The house table, from a loaded theme.
    ///
    /// This is the lift `mnw-cli` and `viewer` were each doing by hand: a muted
    /// bold heading, the ordered column brought back up to primary, actions and
    /// links on the action colour rather than on the cell's text colour, and
    /// selection carried by the background alone.
    ///
    /// Selection carries no foreground on purpose. A row can be red for a failed
    /// upload or green for a published item, and repainting its text on
    /// selection loses that distinction on exactly the row the user is looking
    /// at. `mnw-cli`'s `selected_style` found this and its comment says so;
    /// this is that comment's code, in the library, once.
    #[cfg(feature = "theme")]
    #[must_use]
    pub fn from_theme(theme: &crate::Theme) -> Self {
        Self {
            header: Style::new()
                .fg(theme.content_muted)
                .add_modifier(Modifier::BOLD),
            sorted: Style::new()
                .fg(theme.content_primary)
                .add_modifier(Modifier::BOLD),
            sortable: Style::new().fg(theme.content_secondary),
            value: Style::new().fg(theme.content_primary),
            // A token paints its own background, and a tone underneath it would
            // fight the one sitting on it. Secondary is what shows through the
            // gaps.
            tokens: Style::new().fg(theme.content_secondary),
            actions: Style::new().fg(theme.action_primary),
            link: Style::new()
                .fg(theme.action_primary)
                .add_modifier(Modifier::UNDERLINED),
            selected: Style::new()
                .bg(theme.surface_raised)
                .add_modifier(Modifier::BOLD),
            column_spacing: 1,
            ascending: Sort::Ascending.glyph(),
            descending: Sort::Descending.glyph(),
        }
    }

    /// The style a cell of this part takes.
    ///
    /// [`CellPart`] is `#[non_exhaustive]`, and a member added upstream lands on
    /// [`value`](Self::value): a part this renderer has not learned draws as
    /// text, which is a cell rendering plainly rather than a build that stops.
    /// Grep this when adopting a new `makeover-layout`.
    #[must_use]
    pub fn for_part(&self, part: Option<CellPart>) -> Style {
        match part {
            Some(CellPart::Tokens) => self.tokens,
            Some(CellPart::Actions) => self.actions,
            Some(CellPart::Link) => self.link,
            _ => self.value,
        }
    }
}

/// The heading, with the caret if this column is ordered by or offers to be.
///
/// A column [`sorted`](Column::sorted) but not
/// [`sortable`](Column::sortable) still gets its caret. Both combinations mean
/// something, which is why the description holds the two fields apart: a list
/// ordered by a key the user cannot change is a real thing, and the caret is how
/// it says so.
///
/// A column sortable and *not* sorted draws the idle mark, in the ascending
/// spelling because that is the direction a first press takes. The tone is what
/// separates it from the column in force, and [`header`] picks that; here the
/// point is the width. This is what closes the reflow: pressing a heading used
/// to widen its column by two cells and shift every column after it, because
/// [`measure`] sizes from this function and the caret appeared with the press.
fn heading<'a>(column: &Column<'a>, style: &TableStyle) -> Line<'a> {
    let caret = match column.sorted {
        Some(Sort::Ascending) => style.ascending,
        Some(Sort::Descending) => style.descending,
        None if column.sortable => style.ascending,
        None => return Line::from(column.name),
    };
    // The gap, once, rather than inside each of the two style strings. A
    // consumer swapping the glyph for an ASCII one does not have to remember to
    // bring a space with it.
    Line::from(format!("{} {caret}", column.name))
}

/// How wide a column wants to be, in cells, at its narrowest.
///
/// The floor for a fill column rather than its appetite, because narrowing asks
/// what a layout costs at minimum and a fill column costs its floor.
fn min_width<'a, R>(column: &Column<'a>, rows: &[R], sizing: &Sizing<'_>, style: &TableStyle) -> u16
where
    R: AsRef<[Cell<'a>]>,
{
    match column.width {
        Width::Content => measure(column, rows, style),
        Width::Fixed => sizing.length_for(column.name),
        // Includes a width added to the description since this renderer was
        // built. Taking the slack above a floor is the behaviour that makes no
        // claim, which is the same fallback the webview renderer's `auto` track
        // is chosen to be.
        _ => sizing.length_for(column.name),
    }
}

/// The widest thing in a column, heading included.
///
/// The heading counts because it is drawn: a column sized to its cells alone
/// truncates its own name, and a two-character column called `duration` reads as
/// `du`. The caret counts for the same reason, which is why this measures
/// [`heading`] rather than [`Column::name`].
fn measure<'a, R>(column: &Column<'a>, rows: &[R], style: &TableStyle) -> u16
where
    R: AsRef<[Cell<'a>]>,
{
    let widest = rows
        .iter()
        .filter_map(|row| {
            row.as_ref()
                .iter()
                .find(|cell| cell.column == column.name)
                .map(|cell| cell.content.width())
        })
        .max()
        .unwrap_or(0);
    u16::try_from(widest.max(heading(column, style).width())).unwrap_or(u16::MAX)
}

/// Whether the columns kept at `cutoff` fit in `width`.
fn fits<'a, R>(
    columns: &[Column<'a>],
    rows: &[R],
    sizing: &Sizing<'_>,
    style: &TableStyle,
    cutoff: Priority,
    width: u16,
) -> bool
where
    R: AsRef<[Cell<'a>]>,
{
    let kept: Vec<&Column<'a>> = columns.iter().filter(|c| c.kept_at(cutoff)).collect();
    let gaps = u32::from(style.column_spacing) * (kept.len().saturating_sub(1)) as u32;
    let tracks: u32 = kept
        .iter()
        .map(|c| u32::from(min_width(c, rows, sizing, style)))
        .sum();
    tracks + gaps <= u32::from(width)
}

/// The weakest cutoff whose columns fit in `width`.
///
/// Raised until the layout fits, and never past [`Priority::Essential`]: the
/// essential columns are what makes a row identify itself, so a window too
/// narrow for them gets them truncated rather than dropped. Nothing here counts
/// positions, so which column drops is a property of the column.
#[must_use]
pub fn cutoff_for<'a, R>(
    columns: &[Column<'a>],
    rows: &[R],
    sizing: &Sizing<'_>,
    style: &TableStyle,
    width: u16,
) -> Priority
where
    R: AsRef<[Cell<'a>]>,
{
    for cutoff in CUTOFFS {
        if fits(columns, rows, sizing, style, cutoff, width) {
            return cutoff;
        }
    }
    Priority::Essential
}

/// The tracks for the columns kept at `cutoff`.
///
/// Only the surviving tracks, which is what keeps the track list and the hiding
/// in agreement. A caller that dropped a cell but left its track would get a
/// column of empty space, which is the other half of the goingson bug the
/// webview renderer's `grid_template_columns` names.
#[must_use]
pub fn constraints<'a, R>(
    columns: &[Column<'a>],
    rows: &[R],
    sizing: &Sizing<'_>,
    style: &TableStyle,
    cutoff: Priority,
) -> Vec<Constraint>
where
    R: AsRef<[Cell<'a>]>,
{
    columns
        .iter()
        .filter(|column| column.kept_at(cutoff))
        .map(|column| match column.width {
            // Takes what it needs and no more, which is a fixed track once the
            // needing has been measured.
            Width::Content => Constraint::Length(measure(column, rows, style)),
            Width::Fixed => Constraint::Length(sizing.length_for(column.name)),
            // `Min` and not `Fill`: a fill column absorbs the slack *and* keeps
            // its floor, which is what `minmax(len, 1fr)` says at the webview
            // renderer. `Fill` would let it collapse below the floor when a
            // fixed column takes the room.
            _ => Constraint::Min(sizing.length_for(column.name)),
        })
        .collect()
}

/// One row's cells, in column order.
///
/// Ordered by the columns and not by the cells, so a row cannot silently
/// disagree with its table about what comes where. A column with no cell gets an
/// empty cell, which keeps the tracks aligned; a cell naming no column is
/// dropped, because there is nowhere to put it. That is
/// `makeover-webview`'s `cells_html` rule, and it has to be the same rule or the
/// two renderers disagree about a row they were handed identically.
#[must_use]
pub fn row<'a>(
    columns: &[Column<'a>],
    cells: &[Cell<'a>],
    style: &TableStyle,
    cutoff: Priority,
) -> Row<'a> {
    Row::new(
        columns
            .iter()
            .filter(|column| column.kept_at(cutoff))
            .map(|column| {
                let found = cells.iter().find(|cell| cell.column == column.name);
                let part = found.and_then(|cell| cell.part);
                let content = found.map_or_else(Line::default, |cell| cell.content.clone());
                TrackCell::from(content).style(style.for_part(part))
            })
            .collect::<Vec<_>>(),
    )
}

/// The heading row for the columns kept at `cutoff`.
///
/// Exposed beside [`table`] because a caller assembling its own
/// [`Table`] still has to draw a header that agrees with the body about what
/// just disappeared. Assembling it a second time by hand is how they stop
/// agreeing.
#[must_use]
pub fn header<'a>(columns: &[Column<'a>], style: &TableStyle, cutoff: Priority) -> Row<'a> {
    Row::new(
        columns
            .iter()
            .filter(|column| column.kept_at(cutoff))
            .map(|column| {
                // Three states, three tones (wiki `three-tone-convention`). In
                // force, offering, and not a control at all -- and the middle
                // one is the state that had nowhere to be said, so a heading
                // you could press looked exactly like one you could not.
                let tone = match (column.sorted, column.sortable) {
                    (Some(_), _) => style.sorted,
                    (None, true) => style.sortable,
                    (None, false) => style.header,
                };
                TrackCell::from(heading(column, style)).style(tone)
            })
            .collect::<Vec<_>>(),
    )
    .style(style.header)
}

/// A described table, sized and narrowed for `width`.
///
/// Hands back a [`Table`] rather than drawing one. Selection and scroll live in
/// the app's [`TableState`](ratatui::widgets::TableState), and the row highlight
/// is already set from [`TableStyle::selected`], so a caller renders this with
/// `render_stateful_widget` and gets the house selection without saying anything
/// further.
///
/// `width` is the area the table will be drawn in, which is what narrowing is
/// decided against. Pass the [`Rect`](ratatui::layout::Rect) width that
/// [`frame`](crate::frame) handed back rather than the region's own, or the
/// table budgets for the two cells the edge took.
#[must_use]
pub fn table<'a, R>(
    columns: &[Column<'a>],
    rows: &[R],
    sizing: &Sizing<'_>,
    style: &TableStyle,
    width: u16,
) -> Table<'a>
where
    R: AsRef<[Cell<'a>]>,
{
    let cutoff = cutoff_for(columns, rows, sizing, style, width);
    let widths = constraints(columns, rows, sizing, style, cutoff);
    let body: Vec<Row<'a>> = rows
        .iter()
        .map(|cells| row(columns, cells.as_ref(), style, cutoff))
        .collect();

    Table::new(body, widths)
        .header(header(columns, style, cutoff))
        .column_spacing(style.column_spacing)
        .row_highlight_style(style.selected)
}

/// Whether a table drawn at `width` would leave anything overflowing.
///
/// True only when the essential columns alone do not fit, since that is the one
/// case narrowing cannot answer. A caller that would rather show fewer rows than
/// truncate a cell can ask this and draw something else.
#[must_use]
pub fn overflows<'a, R>(
    columns: &[Column<'a>],
    rows: &[R],
    sizing: &Sizing<'_>,
    style: &TableStyle,
    width: u16,
) -> bool
where
    R: AsRef<[Cell<'a>]>,
{
    !fits(columns, rows, sizing, style, Priority::Essential, width)
}

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

    fn columns() -> Vec<Column<'static>> {
        vec![
            Column {
                name: "name",
                width: Width::Fill,
                priority: Priority::Essential,
                sortable: true,
                sorted: Some(Sort::Ascending),
            },
            Column {
                name: "size",
                width: Width::Fixed,
                priority: Priority::Secondary,
                sortable: true,
                sorted: None,
            },
            Column {
                name: "note",
                width: Width::Content,
                priority: Priority::Optional,
                sortable: false,
                sorted: None,
            },
        ]
    }

    fn sizing() -> Sizing<'static> {
        Sizing {
            lengths: &[("name", 10), ("size", 6)],
            fallback: 4,
        }
    }

    fn rows() -> Vec<Vec<Cell<'static>>> {
        vec![
            vec![
                Cell::new("name", "alpha"),
                Cell::new("size", "1kb"),
                Cell::new("note", "a longer note"),
            ],
            vec![Cell::new("name", "beta"), Cell::new("size", "2kb")],
        ]
    }

    fn cell_text(row: &Row<'_>) -> Vec<String> {
        // Rendering is the only way to read a ratatui Row back, and reading it
        // back is the point: these tests assert what a user sees.
        use ratatui::layout::Rect;
        use ratatui::widgets::Widget;
        let mut buf = ratatui::buffer::Buffer::empty(Rect::new(0, 0, 60, 1));
        Table::new(vec![row.clone()], [Constraint::Length(18); 3])
            .column_spacing(1)
            .render(Rect::new(0, 0, 60, 1), &mut buf);
        (0..3)
            .map(|i| {
                let start = i * 19;
                (start..start + 18)
                    .map(|x| buf[(x as u16, 0)].symbol())
                    .collect::<String>()
                    .trim_end()
                    .to_owned()
            })
            .collect()
    }

    /// The foreground each of the three heading cells was drawn in.
    ///
    /// Read off a rendered buffer for [`cell_text`]'s reason: a ratatui `Row`
    /// hands nothing back, and what is asserted is what a user sees.
    fn cell_colors(row: &Row<'_>) -> Vec<Option<ratatui::style::Color>> {
        use ratatui::layout::Rect;
        use ratatui::widgets::Widget;
        let mut buf = ratatui::buffer::Buffer::empty(Rect::new(0, 0, 60, 1));
        Table::new(vec![row.clone()], [Constraint::Length(18); 3])
            .column_spacing(1)
            .render(Rect::new(0, 0, 60, 1), &mut buf);
        (0..3).map(|i| buf[(i * 19, 0)].fg).map(Some).collect()
    }

    #[test]
    fn cells_are_ordered_by_the_columns_and_not_by_the_row() {
        // The row hands them over backwards. The table decides the order, which
        // is what stops a row silently disagreeing with its own header.
        let cols = columns();
        let out_of_order = vec![
            Cell::new("note", "third"),
            Cell::new("name", "first"),
            Cell::new("size", "second"),
        ];
        let drawn = row(
            &cols,
            &out_of_order,
            &TableStyle::default(),
            Priority::Optional,
        );
        assert_eq!(cell_text(&drawn), vec!["first", "second", "third"]);
    }

    #[test]
    fn a_cell_naming_no_column_is_dropped_and_a_column_with_no_cell_keeps_its_place() {
        let cols = columns();
        let cells = vec![Cell::new("note", "kept"), Cell::new("nonesuch", "lost")];
        let drawn = row(&cols, &cells, &TableStyle::default(), Priority::Optional);
        // Two empty tracks, then the note. The empties are what keeps the third
        // column under the third heading.
        assert_eq!(cell_text(&drawn), vec!["", "", "kept"]);
    }

    #[test]
    fn a_content_column_is_measured_from_its_widest_cell() {
        let style = TableStyle::default();
        let widths = constraints(&columns(), &rows(), &sizing(), &style, Priority::Optional);
        assert_eq!(widths[2], Constraint::Length("a longer note".len() as u16));
    }

    #[test]
    fn a_content_column_never_truncates_its_own_heading() {
        // The cells are two characters wide and the heading is eight. Sizing to
        // the cells alone would draw the column as `du`.
        let cols = vec![Column {
            name: "duration",
            width: Width::Content,
            priority: Priority::Essential,
            sortable: false,
            sorted: None,
        }];
        let rows = vec![vec![Cell::new("duration", "3s")]];
        let widths = constraints(
            &cols,
            &rows,
            &sizing(),
            &TableStyle::default(),
            Priority::Optional,
        );
        assert_eq!(widths[0], Constraint::Length(8));
    }

    #[test]
    fn a_caret_is_part_of_what_a_heading_costs() {
        // Measured off `heading` and not off `name`, or the sorted column is
        // exactly two cells too narrow and drops its own arrow.
        let cols = vec![Column {
            name: "size",
            width: Width::Content,
            priority: Priority::Essential,
            sortable: true,
            sorted: Some(Sort::Descending),
        }];
        let rows: Vec<Vec<Cell<'_>>> = vec![];
        let style = TableStyle::default();
        let widths = constraints(&cols, &rows, &sizing(), &style, Priority::Optional);
        assert_eq!(
            widths[0],
            Constraint::Length(6),
            "size plus a space and a caret"
        );
    }

    #[test]
    fn narrowing_drops_the_optional_column_first_and_the_essential_one_never() {
        let style = TableStyle::default();
        let (cols, rows, sz) = (columns(), rows(), sizing());
        // Everything: 10 + 6 + 13 tracks and two gaps.
        assert_eq!(
            cutoff_for(&cols, &rows, &sz, &style, 40),
            Priority::Optional
        );
        // No room for the note.
        assert_eq!(
            cutoff_for(&cols, &rows, &sz, &style, 20),
            Priority::Secondary
        );
        // No room for the size either.
        assert_eq!(
            cutoff_for(&cols, &rows, &sz, &style, 12),
            Priority::Essential
        );
        // No room for anything, and the essential column stays anyway.
        assert_eq!(
            cutoff_for(&cols, &rows, &sz, &style, 2),
            Priority::Essential
        );
        assert!(overflows(&cols, &rows, &sz, &style, 2));
        assert!(!overflows(&cols, &rows, &sz, &style, 12));
    }

    #[test]
    fn a_column_inserted_left_of_the_cut_does_not_change_what_drops() {
        // The goingson bug, as a test. `nth-child(n+5)` against a seven-column
        // table hides whatever lands at position five, so inserting a column
        // anywhere left of the cut moves it onto a different column with nothing
        // edited and nothing reported.
        //
        // Asserted at a fixed cutoff, because that is where the two ways of
        // addressing a column disagree. A narrower budget SHOULD drop more
        // columns, and does below; what must not change is which ones, in what
        // order, for a given cutoff.
        let dropped = |cols: &[Column<'_>], cutoff| -> Vec<String> {
            cols.iter()
                .filter(|c| !c.kept_at(cutoff))
                .map(|c| c.name.to_owned())
                .collect()
        };
        let before = columns();
        let mut after = vec![Column {
            name: "mark",
            width: Width::Fixed,
            priority: Priority::Essential,
            sortable: false,
            sorted: None,
        }];
        after.extend(columns());

        for cutoff in CUTOFFS {
            assert_eq!(
                dropped(&before, cutoff),
                dropped(&after, cutoff),
                "inserting a column changed what {cutoff:?} drops"
            );
        }
        assert_eq!(dropped(&before, Priority::Secondary), vec!["note"]);
    }

    #[test]
    fn a_column_never_outlives_a_more_essential_one() {
        // The ordering claim narrowing rests on: whatever the budget, the set
        // kept is closed upward. A layout that dropped `size` while keeping
        // `note` would be counting something other than priority.
        let style = TableStyle::default();
        let (cols, rows, sz) = (columns(), rows(), sizing());
        for width in 0..48u16 {
            let cutoff = cutoff_for(&cols, &rows, &sz, &style, width);
            let kept: Vec<&str> = cols
                .iter()
                .filter(|c| c.kept_at(cutoff))
                .map(|c| c.name)
                .collect();
            assert!(
                kept.contains(&"name"),
                "the essential column left at {width}"
            );
            if kept.contains(&"note") {
                assert!(
                    kept.contains(&"size"),
                    "optional outlived secondary at {width}"
                );
            }
        }
    }

    #[test]
    fn a_dropped_column_takes_its_track_with_it() {
        // A cell hidden with its track left behind is a column of empty space,
        // which is the half of the goingson bug that survives fixing the other.
        let style = TableStyle::default();
        let widths = constraints(&columns(), &rows(), &sizing(), &style, Priority::Secondary);
        assert_eq!(widths.len(), 2);
        let drawn = row(&columns(), &rows()[0], &style, Priority::Secondary);
        assert_eq!(cell_text(&drawn), vec!["alpha", "1kb", ""]);
    }

    #[test]
    fn a_fill_column_keeps_its_floor_while_taking_the_slack() {
        // `Min` and not `Fill`, which is `minmax(10, 1fr)` at the webview
        // renderer. A `Fill` track collapses under a fixed neighbour.
        let style = TableStyle::default();
        let widths = constraints(&columns(), &rows(), &sizing(), &style, Priority::Optional);
        assert_eq!(widths[0], Constraint::Min(10));
        assert_eq!(widths[1], Constraint::Length(6));
    }

    #[test]
    fn a_column_with_no_length_of_its_own_takes_the_fallback() {
        let cols = vec![Column {
            name: "unlisted",
            width: Width::Fixed,
            priority: Priority::Essential,
            sortable: false,
            sorted: None,
        }];
        let rows: Vec<Vec<Cell<'_>>> = vec![];
        let widths = constraints(
            &cols,
            &rows,
            &sizing(),
            &TableStyle::default(),
            Priority::Optional,
        );
        assert_eq!(widths[0], Constraint::Length(4));
    }

    #[test]
    fn a_heading_carries_a_caret_when_it_is_ordered_by_or_offers_to_be() {
        let style = TableStyle::default();
        let head = header(&columns(), &style, Priority::Optional);
        assert_eq!(
            cell_text(&head),
            vec!["name \u{25B2}", "size \u{25B2}", "note"],
            "in force and offering both carry one; not a control carries none"
        );
    }

    #[test]
    fn the_three_states_of_a_heading_are_three_tones() {
        // wiki `three-tone-convention`. The middle state is the one that had
        // nowhere to be said: a heading you can press looked exactly like one
        // you cannot, and the idle caret alone does not separate them, because
        // a sorted-but-unsortable column draws a caret too.
        use ratatui::style::Color;
        let style = TableStyle {
            sorted: Style::new().fg(Color::Red),
            sortable: Style::new().fg(Color::Green),
            header: Style::new().fg(Color::Blue),
            ..TableStyle::default()
        };
        let drawn = cell_colors(&header(&columns(), &style, Priority::Optional));
        assert_eq!(
            drawn,
            vec![Some(Color::Red), Some(Color::Green), Some(Color::Blue)]
        );

        // The colourless default separates them by the caret and nothing else,
        // and that is the honest limit rather than an oversight: a cell style
        // patches the row's, so a plain cell under a bold header row is drawn
        // bold whatever it holds. Three tones need three colours, which is what
        // `from_theme` is for.
        let house = TableStyle::default();
        let plain = cell_colors(&header(&columns(), &house, Priority::Optional));
        assert_eq!(plain[0], plain[1], "no colour to spend, so none is claimed");
    }

    #[test]
    fn pressing_a_heading_does_not_move_the_columns_after_it() {
        // The reflow the idle caret closes. `measure` sizes from `heading`, so
        // a caret that appeared with the press widened its own column by two
        // cells and shifted the rest of the row sideways.
        let style = TableStyle::default();
        let offering = Column {
            name: "size",
            width: Width::Content,
            priority: Priority::Secondary,
            sortable: true,
            sorted: None,
        };
        let in_force = Column {
            sorted: Some(Sort::Descending),
            ..offering
        };
        let rows: Vec<Vec<Cell<'_>>> = vec![];
        assert_eq!(
            measure(&offering, &rows, &style),
            measure(&in_force, &rows, &style)
        );

        // And the column that is not a control at all is narrower, which is the
        // width that would be wrong to reserve: it has no caret to draw.
        let inert = Column {
            sortable: false,
            ..offering
        };
        assert!(measure(&inert, &rows, &style) < measure(&offering, &rows, &style));
    }

    #[test]
    fn a_column_sorted_without_being_sortable_still_draws_its_caret() {
        // A list ordered by a key the user cannot change is a real thing to
        // describe, which is why the description holds the two fields apart.
        // Drawing the caret only for a sortable column would collapse them.
        let cols = vec![Column {
            name: "rank",
            width: Width::Content,
            priority: Priority::Essential,
            sortable: false,
            sorted: Some(Sort::Descending),
        }];
        let head = header(&cols, &TableStyle::default(), Priority::Optional);
        assert_eq!(cell_text(&head), vec!["rank \u{25BC}", "", ""]);
    }

    #[test]
    fn the_parts_a_cell_can_be_are_styled_apart() {
        // The drift `CellPart` exists to end: one style for a whole cell paints
        // a control as though it were text.
        let style = TableStyle::default();
        assert_eq!(style.for_part(Some(CellPart::Value)), style.value);
        assert_eq!(style.for_part(Some(CellPart::Tokens)), style.tokens);
        assert_eq!(style.for_part(Some(CellPart::Actions)), style.actions);
        assert_eq!(style.for_part(Some(CellPart::Link)), style.link);
        assert_ne!(style.for_part(Some(CellPart::Link)), style.value);
        // A cell mixing parts says nothing, and takes the text style.
        assert_eq!(style.for_part(None), style.value);
    }

    #[test]
    fn a_table_narrows_itself_from_the_width_it_is_given() {
        // The whole path in one call, which is what a consumer actually uses.
        let style = TableStyle::default();
        let wide = table(&columns(), &rows(), &sizing(), &style, 40);
        let narrow = table(&columns(), &rows(), &sizing(), &style, 20);
        use ratatui::layout::Rect;
        use ratatui::widgets::Widget;

        let mut buf = ratatui::buffer::Buffer::empty(Rect::new(0, 0, 40, 3));
        wide.render(Rect::new(0, 0, 40, 3), &mut buf);
        let head: String = (0..40).map(|x| buf[(x, 0)].symbol()).collect();
        assert!(head.contains("note"));

        let mut buf = ratatui::buffer::Buffer::empty(Rect::new(0, 0, 20, 3));
        narrow.render(Rect::new(0, 0, 20, 3), &mut buf);
        let head: String = (0..20).map(|x| buf[(x, 0)].symbol()).collect();
        assert!(!head.contains("note"), "the optional column is gone");
        assert!(head.contains("name"), "the essential one is not");
    }

    #[test]
    fn selection_is_carried_by_the_background_alone() {
        // A row can be red for a failure or green for a success, and a
        // foreground on the selection loses that on exactly the row being looked
        // at. Asserted on the default so a caller who supplies no theme still
        // gets the rule.
        let style = TableStyle::default();
        assert!(style.selected.fg.is_none());
    }
}