tinhorn 0.1.1

//! All ratatui rendering lives here.

use ratatui::layout::{Alignment, Constraint, Flex, Layout, Rect};
use ratatui::style::{Color, Modifier, Style, Stylize};
use ratatui::text::{Line, Span};
use ratatui::widgets::{Block, Borders, Clear, Padding, Paragraph};
use ratatui::Frame;

use crate::app::{App, Die, Pane, Particle, Stats, DIE_H, DIE_W};

/// Per-die colour palette; dice cycle through it by index.
const PALETTE: [Color; 8] = [
    Color::Cyan,
    Color::Yellow,
    Color::Green,
    Color::Magenta,
    Color::Red,
    Color::Blue,
    Color::LightGreen,
    Color::LightMagenta,
];

fn die_color(idx: usize) -> Color {
    PALETTE[idx % PALETTE.len()]
}

pub fn render(frame: &mut Frame, app: &mut App) {
    let area = frame.area();
    let chunks = Layout::vertical([
        Constraint::Min(5),    // bouncing arena
        Constraint::Length(4), // results
        Constraint::Length(1), // input field
        Constraint::Length(1), // help
    ])
    .split(area);

    render_arena(frame, app, chunks[0]);
    render_results(frame, app, chunks[1]);
    render_input(frame, app, chunks[2]);
    render_help(frame, app, chunks[3]);

    // A pop-out pane floats on top of everything when one is toggled open.
    match app.pane {
        Pane::None => {}
        Pane::Help => render_help_overlay(frame, area),
        Pane::History => render_history_overlay(frame, app, area),
        Pane::Stats => render_stats_overlay(frame, app, area),
    }
}

/// The one tier ladder for a throw's power. The arena title and the release
/// echo both read it, so the two labels on screen can never disagree about
/// the same release.
#[derive(Clone, Copy, PartialEq)]
enum ThrowTier {
    Lob,
    Toss,
    Rocket,
    Peak,
}

fn throw_tier(power: f32) -> ThrowTier {
    if power >= 0.92 {
        ThrowTier::Peak
    } else if power >= 0.70 {
        ThrowTier::Rocket
    } else if power >= 0.33 {
        ThrowTier::Toss
    } else {
        ThrowTier::Lob
    }
}

fn render_arena(frame: &mut Frame, app: &mut App, area: Rect) {
    let title = if app.shaking() {
        " 🎲  tinhorn — shaking… ".to_string()
    } else if app.all_settled() {
        " 🎲  tinhorn — settled ".to_string()
    } else if app.spawned {
        // Name the throw for what it was; a plain Tab roll just "rolls".
        match app.last_throw.map(|t| throw_tier(t.power)) {
            Some(ThrowTier::Lob) => " 🎲  tinhorn — a timid lob… ".to_string(),
            Some(ThrowTier::Toss) => " 🎲  tinhorn — a clean toss… ".to_string(),
            Some(ThrowTier::Rocket | ThrowTier::Peak) => {
                " 🎲  tinhorn — a rocket throw… ".to_string()
            }
            None => " 🎲  tinhorn — rolling… ".to_string(),
        }
    } else {
        " 🎲  tinhorn ".to_string()
    };
    let block = Block::default()
        .borders(Borders::ALL)
        .border_style(Style::default().fg(Color::DarkGray))
        .title(title.bold());

    let inner = block.inner(area);
    frame.render_widget(block, area);

    // Hand the arena size back to the simulation.
    app.arena_w = inner.width as f32;
    app.arena_h = inner.height as f32;

    if inner.width < DIE_W as u16 || inner.height < DIE_H as u16 {
        return;
    }

    // A hard throw rattles the whole box for its first instants: every die is
    // drawn shifted by the same jitter, so the arena itself seems to shudder.
    let jitter = match app.last_throw {
        Some(throw) if app.tremor() > 0.0 => {
            let phase = (throw.age * 30.0) as i32;
            match phase % 4 {
                0 => (1i32, 0i32),
                1 => (0, 1),
                2 => (-1, 0),
                _ => (0, -1),
            }
        }
        _ => (0, 0),
    };

    if app.spawned {
        let buf = frame.buffer_mut();
        for die in &app.dice {
            draw_die(buf, inner, die, jitter);
        }
        for p in &app.particles {
            draw_particle(buf, inner, p);
        }
    }

    // The cup rattles along the arena floor while a shake is in progress,
    // drawn over whatever the last roll left lying around.
    if app.shaking() {
        draw_cup(frame.buffer_mut(), inner, app);
    }

    // The release echo: the caught power, frozen at the top of the arena so
    // you learn what your timing bought you while the dice are still flying.
    if let Some(throw) = app.release_echo() {
        draw_release_echo(frame.buffer_mut(), inner, throw);
    }
}

/// Width of the power meter in cells — the live one in the cup and the frozen
/// release echo, which must stay pixel-identical to the meter it echoes.
const METER_WIDTH: usize = 14;

/// The meter itself: `power` as filled-vs-empty cells.
fn power_bar(power: f32) -> String {
    let filled = ((power * METER_WIDTH as f32).round() as usize).min(METER_WIDTH);
    "▓".repeat(filled) + &"░".repeat(METER_WIDTH - filled)
}

/// The frozen caught-power readout shown right after a release: the meter as
/// it was the instant you let go, graded and named. Fades before it vanishes.
fn draw_release_echo(buf: &mut ratatui::buffer::Buffer, inner: Rect, throw: crate::app::Throw) {
    let (word, color) = release_grade(throw.power);
    let label = format!("caught {} {word}", power_bar(throw.power));
    let w = label.chars().count() as u16;
    if inner.width < w || inner.height < 2 {
        return;
    }
    let x = inner.x + (inner.width - w) / 2;
    // Old (dying) echo dims; a fresh one is bold.
    let mut style = Style::default().fg(color);
    if throw.age > 0.9 {
        style = style.add_modifier(Modifier::DIM);
    } else {
        style = style.add_modifier(Modifier::BOLD);
    }
    buf.set_string(x, inner.y, label, style);
}

/// How the release reads on the meter: the wording of the catch. The peak is
/// the prize; a lob is its own reward.
fn release_grade(power: f32) -> (&'static str, Color) {
    match throw_tier(power) {
        ThrowTier::Peak => ("— the peak!", Color::Yellow),
        ThrowTier::Rocket => ("— a rocket", Color::Red),
        ThrowTier::Toss => ("— a clean toss", Color::Cyan),
        ThrowTier::Lob => ("— a timid lob", Color::DarkGray),
    }
}

/// One celebration glyph: gold sparks for a crit, grey dust for a fumble,
/// dimming as it dies.
fn draw_particle(buf: &mut ratatui::buffer::Buffer, inner: Rect, p: &Particle) {
    let x = inner.x as i32 + p.x.round() as i32;
    let y = inner.y as i32 + p.y.round() as i32;
    if x < inner.x as i32
        || x >= inner.right() as i32
        || y < inner.y as i32
        || y >= inner.bottom() as i32
    {
        return;
    }
    let mut style = Style::default().fg(if p.bright {
        Color::Yellow
    } else {
        Color::DarkGray
    });
    if p.fade() > 0.55 {
        style = style.add_modifier(Modifier::DIM);
    } else if p.bright {
        style = style.add_modifier(Modifier::BOLD);
    }
    buf.set_string(x as u16, y as u16, p.glyph.to_string(), style);
}

/// The dice cup and its power meter, drawn while shaking (the Throw). The cup
/// sways with [`App::cup_x`]; the glyphs inside rattle frame to frame; the
/// meter above is fixed and colour-codes the power you'd release right now.
fn draw_cup(buf: &mut ratatui::buffer::Buffer, inner: Rect, app: &App) {
    let power = app.power();

    // Rattling dice inside the cup: two quarter-block glyphs that jump around
    // as the shake clock advances. Purely cosmetic, so no RNG is consumed.
    const RATTLE: [char; 6] = ['▖', '▘', '▝', '▗', '▚', '▞'];
    let tick = (app.shake_t() * 14.0) as usize;
    let (g1, g2) = (RATTLE[tick % 6], RATTLE[(tick + 3) % 6]);

    // On an arena too narrow for the cup, drawing anyway would paint over the
    // border chrome (max_x saturates below inner.x); skip the theater instead.
    let cup_w: u16 = 8;
    if inner.width < cup_w {
        return;
    }
    let max_x = inner.right() - cup_w;
    let cx = (inner.x as f32 + app.cup_x() - cup_w as f32 / 2.0).round() as i32;
    let cx = (cx.max(inner.x as i32) as u16).min(max_x);
    let cup_style = Style::default()
        .fg(Color::Yellow)
        .add_modifier(Modifier::BOLD);

    if inner.height >= 2 {
        let top = inner.bottom() - 2;
        buf.set_string(cx, top, format!("(  {g1}{g2}  )"), cup_style);
        buf.set_string(cx, top + 1, " ╲____╱ ", cup_style);
    }

    // The power meter sits centred above the cup's travel, steady so your eye
    // can time the release against it. Skipped when it wouldn't fit — like
    // the cup, it must never spill over the arena border.
    if inner.height >= 5 {
        let label = format!("power {} throw ↵", power_bar(power));
        let w = label.chars().count() as u16;
        if inner.width < w {
            return;
        }
        let x = inner.x + (inner.width - w) / 2;
        let y = inner.bottom() - 4;
        let bar_color = if power < 0.5 {
            Color::Green
        } else if power < 0.85 {
            Color::Yellow
        } else {
            Color::Red
        };
        buf.set_string(x, y, label, Style::default().fg(bar_color));
    }
}

/// Paint a single die into the buffer at its current position, drawn as the
/// 2D silhouette of the matching polyhedron (triangle for d4, square for d6,
/// diamond for d8, kite for d10, pentagon for d12, hexagon for d20).
/// `jitter` shifts the whole drawing — the throw's brief screen-shake.
fn draw_die(buf: &mut ratatui::buffer::Buffer, inner: Rect, die: &Die, jitter: (i32, i32)) {
    let max_x = inner.right().saturating_sub(DIE_W as u16);
    let max_y = inner.bottom().saturating_sub(DIE_H as u16);

    let jx = (inner.x as i32 + die.x.round() as i32 + jitter.0).max(inner.x as i32) as u16;
    let jy = (inner.y as i32 + die.y.round() as i32 + jitter.1).max(inner.y as i32) as u16;
    let bx = jx.clamp(inner.x, max_x);
    let by = jy.clamp(inner.y, max_y);

    // A dropped die (kept == false) is still thrown and animated, but rendered
    // greyed-and-dimmed so you can watch e.g. advantage discard the lower d20.
    let style = if !die.kept {
        Style::default()
            .fg(Color::DarkGray)
            .add_modifier(Modifier::DIM)
    } else if die.settled {
        Style::default()
            .fg(die_color(die.color_idx))
            .add_modifier(Modifier::BOLD)
    } else {
        Style::default()
            .fg(die_color(die.color_idx))
            .add_modifier(Modifier::DIM)
    };

    let rows = die_shape(die.sides, die.shown);
    for (i, row) in rows.iter().enumerate() {
        buf.set_string(bx, by + i as u16, row, style);
    }
}

/// The four-row, six-cell-wide ASCII silhouette for a die of `sides`, with its
/// face value laid into the body. Non-standard dice fall back to a plain box.
fn die_shape(sides: u32, value: u32) -> [String; 4] {
    // Standard polyhedral faces top out at 20, so a 2-wide slot is enough; the
    // fallback box is given a 4-wide slot for the occasional d100 etc.
    let v = format!("{value:^2}");
    match sides {
        4 => [
            "  ╱╲  ".into(),
            " ╱  ╲ ".into(),
            format!("╱ {v} ╲"),
            "‾‾‾‾‾‾".into(),
        ],
        6 => [
            "┌────┐".into(),
            "│    │".into(),
            format!("│ {v} │"),
            "└────┘".into(),
        ],
        8 => [
            "  ╱╲  ".into(),
            " ╱  ╲ ".into(),
            format!(" ╲{v}╱ "),
            "  ╲╱  ".into(),
        ],
        10 => [
            " ╱‾‾╲ ".into(),
            "╱    ╲".into(),
            format!("╲ {v} ╱"),
            "  ╲╱  ".into(),
        ],
        12 => [
            "  ╱╲  ".into(),
            " ╱  ╲ ".into(),
            format!("│ {v} │"),
            "└────┘".into(),
        ],
        20 => [
            " ╱‾‾╲ ".into(),
            format!("│ {v} │"),
            "│    │".into(),
            " ╲__╱ ".into(),
        ],
        _ => {
            let v = format!("{value:^4}");
            [
                "┌────┐".into(),
                "│    │".into(),
                format!("│{v}│"),
                "└────┘".into(),
            ]
        }
    }
}

fn render_results(frame: &mut Frame, app: &App, area: Rect) {
    let block = Block::default()
        .borders(Borders::ALL)
        .border_style(Style::default().fg(Color::DarkGray))
        .title(" result ".bold());
    let inner = block.inner(area);
    frame.render_widget(block, area);

    // Error state takes over the panel.
    if let Some(err) = &app.error {
        let p = Paragraph::new(Line::from(vec![
            Span::styled("⚠ ", Style::default().fg(Color::Red)),
            Span::styled(err.clone(), Style::default().fg(Color::Red)),
        ]));
        frame.render_widget(p, inner);
        return;
    }

    if app.dice.is_empty() {
        let p = Paragraph::new(Span::styled(
            "type a dice expression below — Enter does the rest",
            Style::default().fg(Color::DarkGray),
        ));
        frame.render_widget(p, inner);
        return;
    }

    let settled = app.all_settled();

    // Line 1: one coloured chip per die.
    let mut chips: Vec<Span> = Vec::new();
    for (i, die) in app.dice.iter().enumerate() {
        if i > 0 {
            chips.push(Span::raw(" "));
        }
        let val = if settled { die.final_value } else { die.shown };
        if die.kept {
            let mut style = Style::default().fg(die_color(die.color_idx));
            if settled {
                style = style.add_modifier(Modifier::BOLD);
            }
            chips.push(Span::styled(format!("[{val}]"), style));
        } else {
            // Dropped die: greyed and dimmed so it reads as discarded.
            chips.push(Span::styled(
                format!("[{val}]"),
                Style::default()
                    .fg(Color::DarkGray)
                    .add_modifier(Modifier::DIM),
            ));
        }
    }
    if app.modifier != 0 {
        let sign = if app.modifier > 0 { "+" } else { "−" };
        chips.push(Span::styled(
            format!("  {sign}{}", app.modifier.abs()),
            Style::default().fg(Color::Gray),
        ));
    }

    // Line 2: the total — and, when the roll is staked, the verdict.
    let total = if settled {
        app.total()
    } else {
        app.live_total()
    };
    let total_style = if settled {
        Style::default()
            .fg(Color::Black)
            .bg(Color::Green)
            .add_modifier(Modifier::BOLD)
    } else {
        Style::default().fg(Color::DarkGray)
    };
    let mut total_spans = vec![
        Span::styled("  Σ total ", Style::default().fg(Color::Gray)),
        Span::styled(format!(" {total} "), total_style),
        Span::raw("   "),
    ];

    match (app.target, app.verdict()) {
        // Settled with stakes: slam the verdict (the wording is shared with
        // the CLI's verbose breakdown so the two can't drift).
        (Some(target), Some((success, margin))) => {
            let bg = if success { Color::Green } else { Color::Red };
            total_spans.push(Span::styled(
                format!("vs {target}  "),
                Style::default().fg(Color::Gray),
            ));
            total_spans.push(Span::styled(
                format!(" {} ", crate::app::verdict_text(success, margin)),
                Style::default()
                    .fg(Color::Black)
                    .bg(bg)
                    .add_modifier(Modifier::BOLD),
            ));
        }
        // Stakes declared, dice still falling: show what's at stake.
        (Some(target), None) => {
            total_spans.push(Span::styled(
                format!("vs {target}  (rolling…)"),
                Style::default().fg(Color::DarkGray),
            ));
        }
        _ => {
            total_spans.push(Span::styled(
                if settled { "" } else { "(rolling…)" },
                Style::default().fg(Color::DarkGray),
            ));
        }
    }

    // Maxed dice and 1s get named, whatever the stakes. A d20's own crit
    // keeps its beloved name; anything else is a crit on its merits.
    if settled {
        let crits = app.crit_dice().count();
        if crits > 0 {
            let all_d20 = app.crit_dice().all(|d| d.sides == 20);
            let mut label = if all_d20 {
                "  ✦ natural 20".to_string()
            } else {
                "  ✦ crit".to_string()
            };
            if crits > 1 {
                label.push_str(&format!(" ×{crits}"));
            }
            total_spans.push(Span::styled(
                label,
                Style::default()
                    .fg(Color::Yellow)
                    .add_modifier(Modifier::BOLD),
            ));
        }
        let fumbles = app.fumble_dice().count();
        if fumbles > 0 {
            let all_d20 = app.fumble_dice().all(|d| d.sides == 20);
            let mut label = if all_d20 {
                "  natural 1".to_string()
            } else {
                "  fumble".to_string()
            };
            if fumbles > 1 {
                label.push_str(&format!(" ×{fumbles}"));
            }
            total_spans.push(Span::styled(
                label,
                Style::default().fg(Color::Red).add_modifier(Modifier::DIM),
            ));
        }
    }

    let p = Paragraph::new(vec![Line::from(chips), Line::from(total_spans)]);
    frame.render_widget(p, inner);
}

fn render_help(frame: &mut Frame, app: &App, area: Rect) {
    let key = |k| Span::styled(k, Style::default().fg(Color::Cyan).bold());
    let help = if app.shaking() {
        // Mid-shake the bar narrows to the only choices that exist.
        Line::from(vec![
            Span::styled(" › shaking…  ", Style::default().fg(Color::Yellow)),
            key("Enter"),
            Span::raw(" · "),
            key("Tab"),
            Span::raw(" throw  "),
            key("Esc"),
            Span::raw(" put them down"),
        ])
    } else {
        let mut spans = vec![
            Span::styled(" › ", Style::default().fg(Color::Cyan)),
            key("Enter"),
            Span::raw(format!(" {}  ", app.mode.label())),
            key("Tab"),
            Span::raw(" mode  "),
            key("?"),
            Span::raw(" help  "),
            key("^H"),
            Span::raw(" history  "),
            key("^S"),
            Span::raw(" stats  "),
        ];
        spans.push(key("^Q"));
        spans.push(Span::raw(if app.muted {
            " unmute 🔇  "
        } else {
            " mute  "
        }));
        spans.push(key("Esc"));
        spans.push(Span::raw(" quit"));
        Line::from(spans)
    };
    frame.render_widget(Paragraph::new(help), area);
}

/// One row of the syntax table in the help overlay: an example on the left and
/// its meaning on the right.
fn syntax_row<'a>(example: &'a str, meaning: &'a str) -> Line<'a> {
    Line::from(vec![
        Span::raw("  "),
        Span::styled(format!("{example:<11}"), Style::default().fg(Color::Cyan)),
        Span::styled(meaning, Style::default().fg(Color::Gray)),
    ])
}

/// A bold yellow section heading line for the overlays.
fn heading(text: impl Into<String>) -> Line<'static> {
    Line::from(Span::styled(
        text.into(),
        Style::default()
            .fg(Color::Yellow)
            .add_modifier(Modifier::BOLD),
    ))
}

/// The italic dim "press … to close" footer shared by every pane.
fn close_hint() -> Line<'static> {
    Line::from(Span::styled(
        "  Esc · q to close",
        Style::default()
            .fg(Color::DarkGray)
            .add_modifier(Modifier::ITALIC),
    ))
}

/// Draw a centred, bordered panel of `lines` titled `title` over the UI. Sizes
/// itself to its content (capped to the frame), blanks what's behind it, and
/// trims overflow from the bottom so it never spills past its border. Shared by
/// all three pop-out panes.
fn overlay_panel(frame: &mut Frame, area: Rect, title: &str, mut lines: Vec<Line>) {
    let content_h = lines.len() as u16;
    let inner_w = lines.iter().map(Line::width).max().unwrap_or(0) as u16;
    let panel_w = (inner_w + 4).min(area.width); // +4 for borders + side padding
    let panel_h = (content_h + 2).min(area.height); // +2 for top/bottom border

    let rect = centered(panel_w, panel_h, area);

    let block = Block::default()
        .borders(Borders::ALL)
        .border_style(Style::default().fg(Color::Cyan))
        .padding(Padding::horizontal(1))
        .title(title.to_string().bold());

    let max_lines = block.inner(rect).height as usize;
    if lines.len() > max_lines {
        lines.truncate(max_lines);
    }

    let para = Paragraph::new(lines)
        .block(block)
        .alignment(Alignment::Left);
    frame.render_widget(Clear, rect); // blank whatever's behind the panel
    frame.render_widget(para, rect);
}

/// The dice-notation reference, drawn over the UI when `?` is pressed.
fn render_help_overlay(frame: &mut Frame, area: Rect) {
    let lines = vec![
        heading("Dice"),
        syntax_row("3d6", "three six-sided dice"),
        syntax_row("d20", "one die (count defaults to 1)"),
        syntax_row("d6+d8", "combine different dice"),
        syntax_row("2d20-1", "add or subtract a flat modifier"),
        Line::raw(""),
        heading("Keep / drop"),
        syntax_row("2d20kh1", "advantage — keep the highest 1"),
        syntax_row("2d20kl1", "disadvantage — keep the lowest 1"),
        syntax_row("4d6dl1", "drop the lowest 1 (ability scores)"),
        syntax_row("4d6dh1", "drop the highest 1"),
        Line::raw(""),
        heading("Exploding"),
        syntax_row("3d6!", "a max face rolls another die"),
        syntax_row("d10!>8", "explode on any face over 8 (also !=N, !<N)"),
        Line::raw(""),
        heading("Multiply & stakes"),
        syntax_row("4d6*2", "double this term's sum (modifiers stack)"),
        syntax_row("d20+5 vs 15", "roll against a target — meet it, beat it"),
        Line::raw(""),
        heading("The Throw"),
        syntax_row(
            "Enter",
            "shake the cup; Enter again throws — harder at the peak",
        ),
        syntax_row("Tab", "cycle Enter's mode: shake → roll → insta"),
        syntax_row("Esc", "put them down. Power never touches the values."),
        Line::from(Span::styled(
            "  Separators: +  -  ,  space  or just write dice next to each other.",
            Style::default().fg(Color::DarkGray),
        )),
        close_hint(),
    ];
    overlay_panel(frame, area, " 🎲  dice notation ", lines);
}

/// The roll-history pane: recent rolls, newest first.
fn render_history_overlay(frame: &mut Frame, app: &App, area: Rect) {
    let mut lines: Vec<Line> = Vec::new();

    if app.history.is_empty() {
        lines.push(Line::from(Span::styled(
            "  no rolls yet — shake some dice loose with Enter",
            Style::default().fg(Color::DarkGray),
        )));
    } else {
        // Newest first; show at most what the frame can hold.
        let shown = (area.height as usize).saturating_sub(4).max(1);
        let skipped = app.history.len().saturating_sub(shown);
        for (n, e) in app.history.iter().rev().take(shown).enumerate() {
            let idx = app.history.len() - n; // 1-based, counting from the newest
            let faces = e
                .values
                .iter()
                .map(|v| v.to_string())
                .collect::<Vec<_>>()
                .join(" ");
            lines.push(Line::from(vec![
                Span::styled(
                    format!("  {idx:>3}. "),
                    Style::default().fg(Color::DarkGray),
                ),
                Span::styled(format!("{:<12}", e.expr), Style::default().fg(Color::Cyan)),
                Span::styled(format!("[{faces}]  "), Style::default().fg(Color::Gray)),
                Span::styled(
                    format!("= {}", e.total),
                    Style::default()
                        .fg(Color::Green)
                        .add_modifier(Modifier::BOLD),
                ),
            ]));
        }
        if skipped > 0 {
            lines.push(Line::from(Span::styled(
                format!("  … and {skipped} older"),
                Style::default().fg(Color::DarkGray),
            )));
        }
    }

    lines.push(Line::raw(""));
    lines.push(close_hint());
    overlay_panel(frame, area, " 🎲  history ", lines);
}

/// The statistics pane: theoretical odds for the current expression plus a
/// summary of the rolls actually made this session.
fn render_stats_overlay(frame: &mut Frame, app: &mut App, area: Rect) {
    let lines = match app.stats() {
        Ok(s) => stats_lines(&s),
        Err(e) => vec![
            Line::from(Span::styled(
                "  can't compute stats — the expression doesn't parse:",
                Style::default().fg(Color::Red),
            )),
            Line::from(Span::styled(
                format!("  {e}"),
                Style::default().fg(Color::Red),
            )),
            Line::raw(""),
            close_hint(),
        ],
    };
    overlay_panel(frame, area, " 🎲  statistics ", lines);
}

/// Lay out the statistics into display lines: a header, the theoretical
/// min/max/mean, a small distribution curve, and the session summary.
fn stats_lines(s: &Stats) -> Vec<Line<'static>> {
    let mut lines = vec![
        heading(format!("Odds for  {}", s.expr)),
        Line::from(vec![
            Span::raw("  "),
            Span::styled(
                format!("min {}   max {}   avg {:.1}", s.min, s.max, s.mean),
                Style::default().fg(Color::Gray),
            ),
        ]),
        Line::from(Span::styled(
            format!("  (estimated from {} samples)", s.samples),
            Style::default().fg(Color::DarkGray),
        )),
        Line::raw(""),
    ];

    // A staked expression leads with what matters: the odds of making it.
    if let (Some(target), Some(odds)) = (s.target, s.success_odds) {
        lines.insert(
            1,
            Line::from(vec![
                Span::raw("  "),
                Span::styled(format!("vs {target}: "), Style::default().fg(Color::Gray)),
                Span::styled(
                    format!("{:.0}% to succeed", odds * 100.0),
                    Style::default()
                        .fg(Color::Green)
                        .add_modifier(Modifier::BOLD),
                ),
            ]),
        );
    }

    // A little horizontal distribution: one bar per bucket, scaled to the peak.
    if !s.dist.is_empty() {
        let peak = s
            .dist
            .iter()
            .map(|b| b.fraction)
            .fold(0.0_f64, f64::max)
            .max(1e-9);
        for b in &s.dist {
            let filled = (b.fraction / peak * 18.0).round() as usize;
            let bar: String = "█".repeat(filled);
            lines.push(Line::from(vec![
                Span::styled(
                    format!("  {:>4} ", b.total),
                    Style::default().fg(Color::DarkGray),
                ),
                Span::styled(bar, Style::default().fg(Color::Cyan)),
                Span::styled(
                    format!(" {:>4.1}%", b.fraction * 100.0),
                    Style::default().fg(Color::Gray),
                ),
            ]));
        }
        lines.push(Line::raw(""));
    }

    // Session summary for this exact expression.
    lines.push(heading("This session"));
    if s.session.count == 0 {
        lines.push(Line::from(Span::styled(
            format!(
                "  no rolls of {} yet ({} rolls total)",
                s.expr, s.total_rolls
            ),
            Style::default().fg(Color::DarkGray),
        )));
    } else {
        lines.push(Line::from(Span::styled(
            format!(
                "  {} rolls   low {}   high {}   mean {:.1}",
                s.session.count, s.session.min, s.session.max, s.session.mean
            ),
            Style::default().fg(Color::Gray),
        )));
    }

    lines.push(Line::raw(""));
    lines.push(close_hint());
    lines
}

/// A `w`×`h` rectangle centred inside `area` (clamped to fit).
fn centered(w: u16, h: u16, area: Rect) -> Rect {
    let [row] = Layout::vertical([Constraint::Length(h)])
        .flex(Flex::Center)
        .areas(area);
    let [cell] = Layout::horizontal([Constraint::Length(w)])
        .flex(Flex::Center)
        .areas(row);
    cell
}

/// The editable dice expression, with a block cursor.
fn render_input(frame: &mut Frame, app: &App, area: Rect) {
    let p = Paragraph::new(Line::from(vec![
        Span::styled("dice ▸ ", Style::default().fg(Color::Cyan).bold()),
        Span::raw(app.input.clone()),
        Span::styled("█", Style::default().fg(Color::Cyan)),
    ]));
    frame.render_widget(p, area);
}