ggplot_rs/render/

svg_backend.rs

//! A self-contained SVG `DrawBackend` — a *second* backend (no plotters),
//! proving the `DrawBackend` abstraction: the same `PlotRenderer` drives it.
//!
//! It emits SVG elements directly, so SVG output needs no glyph rasterization
//! (text is `<text>` with font attributes the viewer renders).

use super::backend::{
    DrawBackend, FillPattern, FontFace, LineEnd, LineStyle, PointShape, PointStyle, RectStyle,
    TextAnchor, TextStyle,
};
use super::{Rect, RenderError};

/// Accumulates SVG markup for a plot rendered via [`DrawBackend`].
///
/// Every number written to an attribute is finite: marks whose geometry is
/// `NaN`/`±inf` are dropped (a polyline keeps its finite vertices), and
/// non-finite style values fall back to safe defaults. All data-derived
/// attribute values and text go through [`escape`].
pub struct SvgBackend {
    plot_area: Rect,
    total_area: Rect,
    body: String,
    tooltip: Option<String>,
    axis_key: Option<String>,
    series_key: Option<String>,
    value_key: Option<String>,
    mark_part: Option<String>,
    mark_attrs: Vec<(String, String)>,
    /// Geom of the layer being drawn (see [`DrawBackend::set_mark_geom`]).
    mark_geom: Option<String>,
    root_attrs: Vec<(String, String)>,
    warnings: Vec<String>,
    /// Emit the root `data-plot` panel rect (off for composite pages, whose
    /// root is not a single panel).
    plot_attr: bool,
    /// B2-4: whether `GGPlot::svg_formatted_values` is on — see
    /// [`DrawBackend::wants_formatted_values`].
    formatted_values: bool,
    /// `data-fvalue` for the next drawn mark(s) — see
    /// [`DrawBackend::set_mark_fvalue`].
    mark_fvalue: Option<String>,
    /// `data-fx` for the next drawn mark(s) — see
    /// [`DrawBackend::set_mark_fx`].
    mark_fx: Option<String>,
    /// Patterns registered so far via [`Self::pattern_url`] (#206),
    /// deduped by equality: `(pattern, "<id_prefix>-pat-<n>")`. Drained
    /// into a `<defs>` block by [`Self::pattern_defs`] on `finish`/
    /// `finish_fragment`.
    patterns: Vec<(FillPattern, String)>,
    /// This backend instance's id prefix for `<defs>` — process-unique
    /// (a monotonic counter), so inlining several rendered SVGs into one
    /// HTML page never lets one's `url(#…)` resolve into another's
    /// `<pattern>` (SVG ids share one DOM-wide namespace once inlined).
    id_prefix: String,
}

/// Source for [`SvgBackend::id_prefix`] — process-wide, so every backend
/// instance gets a distinct prefix regardless of thread.
static NEXT_ID_PREFIX: std::sync::atomic::AtomicU64 = std::sync::atomic::AtomicU64::new(0);

impl SvgBackend {
    pub fn new(width: u32, height: u32, plot_area: Rect) -> Self {
        SvgBackend {
            plot_area,
            total_area: Rect {
                x: 0.0,
                y: 0.0,
                width: width as f64,
                height: height as f64,
            },
            body: String::new(),
            tooltip: None,
            axis_key: None,
            series_key: None,
            value_key: None,
            mark_part: None,
            mark_attrs: Vec::new(),
            mark_geom: None,
            root_attrs: Vec::new(),
            warnings: Vec::new(),
            plot_attr: true,
            formatted_values: false,
            mark_fvalue: None,
            mark_fx: None,
            patterns: Vec::new(),
            id_prefix: format!(
                "gg{:x}",
                NEXT_ID_PREFIX.fetch_add(1, std::sync::atomic::Ordering::Relaxed)
            ),
        }
    }

    /// The `url(#…)` fill value for `pattern`, registering its `<defs>
    /// <pattern>` the first time this exact pattern is seen on this
    /// backend (later calls with an equal pattern reuse the same id).
    fn pattern_url(&mut self, pattern: &FillPattern) -> String {
        if let Some((_, id)) = self.patterns.iter().find(|(p, _)| p == pattern) {
            return format!("url(#{id})");
        }
        let id = format!("{}-pat-{}", self.id_prefix, self.patterns.len());
        self.patterns.push((*pattern, id.clone()));
        format!("url(#{id})")
    }

    /// Shared by [`DrawBackend::draw_rect`]/[`DrawBackend::draw_rect_pattern`]
    /// — `fill` is already resolved (a hex colour or a pattern's `url(#…)`).
    fn rect_markup(
        &mut self,
        (x0, y0): (f64, f64),
        (x1, y1): (f64, f64),
        fill: &str,
        style: &RectStyle,
    ) -> Result<(), RenderError> {
        if !(x0.is_finite() && y0.is_finite() && x1.is_finite() && y1.is_finite()) {
            return Ok(());
        }
        let (x, y) = (x0.min(x1), y0.min(y1));
        let (w, h) = ((x1 - x0).abs(), (y1 - y0).abs());
        let stroke = style.stroke.map(rgb).unwrap_or_else(|| "none".into());
        // SVG `<rect>` takes rx/ry natively — clamp to half the shorter side
        // so an oversized radius draws a "stadium" instead of a malformed box.
        let r = style.corner_radius.max(0.0).min(w / 2.0).min(h / 2.0);
        let radius = if r > 0.0 {
            format!(" rx=\"{r:.2}\" ry=\"{r:.2}\"")
        } else {
            String::new()
        };
        let attrs = format!(
            "x=\"{x:.2}\" y=\"{y:.2}\" width=\"{w:.2}\" height=\"{h:.2}\"{radius} fill=\"{fill}\"{} \
             stroke=\"{stroke}\" stroke-width=\"{:.2}\"",
            opacity_attr("fill", style.alpha),
            finite_or(style.stroke_width, 0.0)
        );
        self.push_mark("rect", "bar", &attrs);
        Ok(())
    }

    /// Shared by [`DrawBackend::draw_polygon`]/[`DrawBackend::draw_polygon_pattern`].
    fn polygon_markup(
        &mut self,
        pts: &[(f64, f64)],
        fill: &str,
        style: &RectStyle,
    ) -> Result<(), RenderError> {
        let pts = finite_pts(pts);
        if pts.len() < 3 {
            return Ok(());
        }
        let stroke = style.stroke.map(rgb).unwrap_or_else(|| "none".into());
        let attrs = format!(
            "points=\"{}\" fill=\"{fill}\"{} stroke=\"{stroke}\" stroke-width=\"{:.2}\"",
            points(&pts),
            opacity_attr("fill", style.alpha),
            finite_or(style.stroke_width, 0.0)
        );
        self.push_mark("polygon", "band", &attrs);
        Ok(())
    }

    /// Shared by [`DrawBackend::draw_polygon_rings`]/
    /// [`DrawBackend::draw_polygon_rings_pattern`].
    fn polygon_rings_markup(
        &mut self,
        rings: &[Vec<(f64, f64)>],
        fill: &str,
        style: &RectStyle,
    ) -> Result<(), RenderError> {
        let mut d = String::new();
        for ring in rings {
            let pts = finite_pts(ring);
            if pts.len() < 3 {
                continue;
            }
            d.push('M');
            d.push_str(&format!("{},{}", num(pts[0].0), num(pts[0].1)));
            for p in &pts[1..] {
                d.push('L');
                d.push_str(&format!("{},{}", num(p.0), num(p.1)));
            }
            d.push('Z');
        }
        if d.is_empty() {
            return Ok(());
        }
        let stroke = style.stroke.map(rgb).unwrap_or_else(|| "none".into());
        let attrs = format!(
            "d=\"{d}\" fill-rule=\"evenodd\" fill=\"{fill}\"{} stroke=\"{stroke}\" stroke-width=\"{:.2}\"",
            opacity_attr("fill", style.alpha),
            finite_or(style.stroke_width, 0.0)
        );
        self.push_mark("path", "band", &attrs);
        Ok(())
    }

    /// The `<defs>` block for every pattern [`Self::pattern_url`]
    /// registered (empty when none were) — a `<pattern>` tile per the
    /// stripe's own cycle, rotated by `patternTransform`, a `bg`-filled
    /// tile with a narrower `fg` stripe at its edge (#206).
    fn pattern_defs(&self) -> String {
        if self.patterns.is_empty() {
            return String::new();
        }
        let mut defs = String::from("<defs>");
        for (p, id) in &self.patterns {
            let FillPattern::Stripe {
                fg,
                bg,
                size_px,
                angle_deg,
            } = p;
            let size = if *size_px > 0.0 { *size_px } else { 1.0 };
            defs.push_str(&format!(
                "<pattern id=\"{id}\" width=\"{size}\" height=\"{size}\" patternUnits=\"userSpaceOnUse\" patternTransform=\"rotate({angle_deg})\"><rect width=\"{size}\" height=\"{size}\" fill=\"{}\"/><rect width=\"{:.3}\" height=\"{size}\" fill=\"{}\"/></pattern>",
                rgb(*bg),
                size * 0.3,
                rgb(*fg),
            ));
        }
        defs.push_str("</defs>");
        defs
    }

    /// Turn `data-fvalue`/`data-fx` on for the whole render
    /// ([`GGPlot::svg_formatted_values`](crate::plot::GGPlot::svg_formatted_values)).
    /// Default off (set once, before drawing).
    pub(crate) fn set_formatted_values(&mut self, on: bool) {
        self.formatted_values = on;
    }

    /// Add extra attributes to the root `<svg>` element (e.g. the
    /// `data-domain` family from [`root_data_attrs`]). Names must be plain
    /// attribute names; values are escaped on output.
    pub fn set_root_attrs(&mut self, attrs: Vec<(String, String)>) {
        self.root_attrs = attrs;
    }

    /// Append extra attributes to the root `<svg>` element, after any
    /// already set by [`set_root_attrs`](Self::set_root_attrs) (e.g. the
    /// `data-domain` family from [`root_data_attrs`]). Unlike
    /// `set_root_attrs`, this does not replace what is already there.
    /// Callers (`GGPlot::svg_attr`/`PlotGrid::svg_attr`, via
    /// [`validate_root_attrs`]) are responsible for name validation and
    /// collision checks; this just appends. Values are escaped on output.
    pub fn extend_root_attrs(&mut self, attrs: Vec<(String, String)>) {
        self.root_attrs.extend(attrs);
    }

    /// Omit the root `data-plot` attribute (composite pages).
    pub(crate) fn without_plot_attr(&mut self) {
        self.plot_attr = false;
    }

    /// Append pre-rendered, already-escaped SVG markup (e.g. a nested plot
    /// fragment) to the body. Crate-internal: callers guarantee well-formedness.
    pub(crate) fn push_raw(&mut self, markup: &str) {
        self.body.push_str(markup);
    }

    /// The accumulated body markup (without the root element).
    pub(crate) fn body(&self) -> &str {
        &self.body
    }

    /// Take the warnings reported while drawing (see [`DrawBackend::warn`]).
    pub fn take_warnings(&mut self) -> Vec<String> {
        std::mem::take(&mut self.warnings)
    }

    /// ` class="gg-mark gg-<kind>" data-geom="<geom>"` while a layer's geom is
    /// drawing; empty for axes, grid lines, legends and backgrounds.
    fn mark_class(&self, kind: &str) -> String {
        match &self.mark_geom {
            Some(g) => format!(" class=\"gg-mark gg-{kind}\" data-geom=\"{}\"", escape(g)),
            None => String::new(),
        }
    }

    /// Emit `<tag attrs/>`, or `<tag attrs data-…><title>tip</title></tag>`
    /// when a tooltip / mark metadata is set (native SVG hover + host grouping).
    ///
    /// `kind` is the data-mark class suffix (see [`Self::mark_class`]) used
    /// when no mark part is set. While a mark part *is* set (e.g. an
    /// arrowhead), the part name is the effective kind instead — an
    /// arrowhead gets `gg-mark gg-arrow`, not its parent's `gg-band`/`gg-line`.
    fn push_mark(&mut self, tag: &str, kind: &str, attrs: &str) {
        // Built-ins first, then user `data-*` attrs — re-validated here as
        // defence in depth even though `geom::set_row_extras` already
        // derives them only from validated `data_attr` mappings, because
        // `set_mark_attrs` is a public `DrawBackend` method any caller could
        // invoke directly. A later entry with the same name replaces an
        // earlier one rather than duplicating it — including `mark_part`'s
        // own `data-part`, added last, so it always wins over anything a
        // caller supplied under that name (an element never carries the
        // same attribute twice, which is an XML parse error).
        let mut pairs: Vec<(String, String)> = Vec::new();
        for (name, val) in [
            ("data-x", &self.axis_key),
            ("data-series", &self.series_key),
            ("data-value", &self.value_key),
        ] {
            if let Some(v) = val {
                pairs.push((name.to_string(), v.clone()));
            }
        }
        // B2-4: `data-fx` only when it says something `data-x` doesn't —
        // most marks have a plain numeric x, where the "formatted" value is
        // the same string, and a redundant duplicate attribute is noise.
        if let Some(fx) = &self.mark_fx {
            if self.axis_key.as_deref() != Some(fx.as_str()) {
                pairs.push(("data-fx".to_string(), fx.clone()));
            }
        }
        if let Some(fvalue) = &self.mark_fvalue {
            pairs.push(("data-fvalue".to_string(), fvalue.clone()));
        }
        for (name, val) in &self.mark_attrs {
            if !is_valid_mark_attr_name(name) {
                continue;
            }
            match pairs.iter_mut().find(|(n, _)| n == name) {
                Some(existing) => existing.1 = val.clone(),
                None => pairs.push((name.clone(), val.clone())),
            }
        }
        // A mark part (e.g. an arrowhead) is decorative: no `<title>`, but it
        // still carries `data-part` plus `aria-hidden` so assistive tech
        // skips it (the parent mark already carries the semantic tooltip).
        // Routed through the same dedup as the other attrs.
        if let Some(part) = &self.mark_part {
            match pairs.iter_mut().find(|(n, _)| n == "data-part") {
                Some(existing) => existing.1 = part.clone(),
                None => pairs.push(("data-part".to_string(), part.clone())),
            }
        }
        let mut data = String::new();
        for (name, val) in &pairs {
            data.push_str(&format!(" {name}=\"{}\"", escape(val)));
        }
        if self.mark_part.is_some() {
            data.push_str(" aria-hidden=\"true\"");
        }
        // The class goes last so existing attribute sequences stay
        // byte-identical. Exactly one `class` attribute is ever written here
        // (no `data_attr` mapping can be named bare `class` — see
        // `is_valid_mark_attr_name` — so nothing in `pairs` can collide with it).
        let effective_kind: &str = self.mark_part.as_deref().unwrap_or(kind);
        data.push_str(&self.mark_class(effective_kind));
        let mark = if self.mark_part.is_some() {
            format!("<{tag} {attrs}{data}/>")
        } else {
            match &self.tooltip {
                Some(t) => format!("<{tag} {attrs}{data}><title>{}</title></{tag}>", escape(t)),
                None => format!("<{tag} {attrs}{data}/>"),
            }
        };
        self.body.push_str(&mark);
    }

    fn root_open(&self, prefix: &str) -> String {
        let p = &self.plot_area;
        let (w, h) = (self.total_area.width as i64, self.total_area.height as i64);
        let mut extra = String::new();
        for (k, v) in &self.root_attrs {
            extra.push_str(&format!(" {k}=\"{}\"", escape(v)));
        }
        let plot = if self.plot_attr {
            format!(
                " data-plot=\"{} {} {} {}\"",
                num(p.x),
                num(p.y),
                num(p.width),
                num(p.height),
            )
        } else {
            String::new()
        };
        format!("<svg {prefix}width=\"{w}\" height=\"{h}\" viewBox=\"0 0 {w} {h}\"{plot}{extra}>")
    }

    /// Wrap the accumulated elements in a complete `<svg>` document. The
    /// `data-plot` attribute records the panel (data) area in viewBox units so a
    /// host can map screen coordinates back to data (e.g. for zoom/crosshair).
    pub fn finish(self) -> String {
        let open = self.root_open("xmlns=\"http://www.w3.org/2000/svg\" ");
        let defs = self.pattern_defs();
        format!("{open}{defs}{}</svg>", self.body)
    }

    /// Wrap the accumulated elements in a *nested* `<svg>` positioned at
    /// `(x, y)` in a parent SVG's user space — no `xmlns` (it inherits the
    /// parent's namespace), so the result can be pasted into a larger SVG as-is.
    /// `data-plot` stays in this fragment's own viewBox units.
    pub fn finish_fragment(self, x: f64, y: f64) -> String {
        let open = self.root_open(&format!("x=\"{}\" y=\"{}\" ", num(x), num(y)));
        let defs = self.pattern_defs();
        format!("{open}{defs}{}</svg>", self.body)
    }

    /// Emit one `<polyline>` for an already-finite run of points — see
    /// [`DrawBackend::draw_line`]'s splitting at non-finite vertices.
    fn draw_line_run(&mut self, pts: &[(f64, f64)], style: &LineStyle) {
        let dash = match style
            .linetype
            .pattern()
            .iter()
            .flat_map(|(d, g)| [*d, *g])
            .map(|v| format!("{v}"))
            .collect::<Vec<_>>()
            .join(",")
        {
            s if s.is_empty() => String::new(),
            s => format!(" stroke-dasharray=\"{s}\""),
        };
        // `Butt` is SVG's own default: omitting the attribute keeps default
        // output byte-identical.
        let linecap = match style.lineend {
            LineEnd::Butt => "",
            LineEnd::Round => " stroke-linecap=\"round\"",
            LineEnd::Square => " stroke-linecap=\"square\"",
        };
        let attrs = format!(
            "points=\"{}\" fill=\"none\" stroke=\"{}\" stroke-width=\"{:.2}\"{}{}{}",
            points(pts),
            rgb(style.color),
            finite_or(style.width, 1.0),
            opacity_attr("stroke", style.alpha),
            dash,
            linecap
        );
        self.push_mark("polyline", "line", &attrs);
    }
}

/// Format a coordinate with two decimals; non-finite becomes `0` (callers
/// drop marks with non-finite geometry before this, so this is a last resort).
fn num(v: f64) -> String {
    if v.is_finite() {
        format!("{v:.2}")
    } else {
        "0".to_string()
    }
}

fn finite_or(v: f64, fallback: f64) -> f64 {
    if v.is_finite() {
        v
    } else {
        fallback
    }
}

/// ` {prefix}-opacity="{alpha:.3}"` (with its own leading space), or the
/// empty string when `alpha` — after the usual non-finite fallback to fully
/// opaque — formats to `"1.000"`. Full opacity is the SVG default, so
/// omitting it keeps default output byte-identical; an explicit `alpha = 1`
/// is indistinguishable from "not set" and also omits the no-op attribute.
fn opacity_attr(prefix: &str, alpha: f64) -> String {
    let a = finite_or(alpha, 1.0);
    if format!("{a:.3}") == "1.000" {
        String::new()
    } else {
        format!(" {prefix}-opacity=\"{a:.3}\"")
    }
}

/// Root `<svg>` data attributes describing the trained position scales, for
/// hosts that map screen coordinates back to data (zoom, crosshair, brushing):
///
/// - `data-xdomain="x0 x1"` / `data-ydomain="y0 y1"` — the data values at the
///   panel's left/right (bottom/top) edges, i.e. the trained limits *after*
///   expansion, for a continuous (numeric or date-time, epoch seconds) axis.
///   Values are in the scale's transformed space (e.g. log10 units for
///   `scale_x_log10`).
/// - `data-domain="x0 x1 y0 y1"` — both of the above, emitted only when both
///   axes are continuous.
/// - `data-xlevels` / `data-ylevels` — a JSON array of the level labels for a
///   discrete axis, in axis order.
/// - `data-flip="true"` under `coord_flip` (x is then drawn vertically).
///
/// The `x`/`y` names refer to the x/y *aesthetics*. Omitted entirely for
/// plots with free facet scales (each panel has its own domain).
pub fn root_data_attrs(built: &crate::build::BuiltPlot) -> Vec<(String, String)> {
    use crate::aes::Aesthetic;
    let mut out = Vec::new();
    if !built.panel_scales.is_empty() {
        return out;
    }
    let axis = |aes: &Aesthetic| -> (Option<(f64, f64)>, Option<String>) {
        match built.scales.get(aes) {
            Some(s) if s.is_discrete() => {
                let labels: Vec<String> = s
                    .breaks()
                    .into_iter()
                    .map(|(_, l)| json_string(&l))
                    .collect();
                (None, Some(format!("[{}]", labels.join(","))))
            }
            Some(s) => (s.expanded_domain(), None),
            None => (None, None),
        }
    };
    let (xd, xl) = axis(&Aesthetic::X);
    let (yd, yl) = axis(&Aesthetic::Y);
    if let (Some((x0, x1)), Some((y0, y1))) = (xd, yd) {
        out.push((
            "data-domain".into(),
            format!("{} {} {} {}", g(x0), g(x1), g(y0), g(y1)),
        ));
    }
    if let Some((a, b)) = xd {
        out.push(("data-xdomain".into(), format!("{} {}", g(a), g(b))));
    }
    if let Some((a, b)) = yd {
        out.push(("data-ydomain".into(), format!("{} {}", g(a), g(b))));
    }
    if let Some(l) = xl {
        out.push(("data-xlevels".into(), l));
    }
    if let Some(l) = yl {
        out.push(("data-ylevels".into(), l));
    }
    if built.coord.is_flipped() {
        out.push(("data-flip".into(), "true".into()));
    }
    out
}

/// Shortest round-trip formatting of a finite number (domains are filtered to
/// finite values before this is called).
fn g(v: f64) -> String {
    format!("{v}")
}

/// Minimal JSON string literal (quotes, backslashes and control characters
/// escaped); the result is attribute-escaped again on output.
fn json_string(s: &str) -> String {
    let mut out = String::with_capacity(s.len() + 2);
    out.push('"');
    for c in s.chars() {
        match c {
            '"' => out.push_str("\\\""),
            '\\' => out.push_str("\\\\"),
            '\n' => out.push_str("\\n"),
            '\r' => out.push_str("\\r"),
            '\t' => out.push_str("\\t"),
            c if (c as u32) < 0x20 => out.push_str(&format!("\\u{:04x}", c as u32)),
            c => out.push(c),
        }
    }
    out.push('"');
    out
}

/// Allow-listed root-level attribute names for `GGPlot::svg_attr`/
/// `PlotGrid::svg_attr`: the fixed names `role`, `id`, `class`, `lang`, or
/// `data-*`/`aria-*` with the same charset-restricted suffix `data_attr`
/// uses (`^[a-z0-9][a-z0-9-]*$`, excluding the reserved `part`/`geom`
/// suffixes — irrelevant at the root, but no reason to special-case them
/// out). Anything else is rejected.
fn is_allowed_root_attr_name(name: &str) -> bool {
    matches!(name, "role" | "id" | "class" | "lang")
        || name
            .strip_prefix("data-")
            .is_some_and(crate::aes::is_valid_data_attr_name)
        || name
            .strip_prefix("aria-")
            .is_some_and(crate::aes::is_valid_data_attr_name)
}

/// Validate and filter a caller's requested root `<svg>` attributes
/// (`GGPlot::svg_attr`/`PlotGrid::svg_attr`), checked centrally here and
/// used by both callers rather than re-implemented per type. A name outside
/// [`is_allowed_root_attr_name`]'s allow-list, or one already present in
/// `existing` (what ggplot-rs itself is about to write to the same root —
/// e.g. `data-plot`/`data-domain`/`data-grid`, or `data-warnings` when
/// `svg_warnings_attr` is also on), is dropped with a warning instead of
/// silently overwriting or duplicating it. `label` names the caller
/// (`"plot"`/`"grid"`) in the warning text.
pub(crate) fn validate_root_attrs(
    requested: &[(String, String)],
    existing: &[(String, String)],
    label: &str,
    warnings: &mut Vec<String>,
) -> Vec<(String, String)> {
    let built_in: Vec<&str> = existing.iter().map(|(k, _)| k.as_str()).collect();
    let mut used: Vec<&str> = built_in.clone();
    let mut out = Vec::new();
    for (name, value) in requested {
        if !is_allowed_root_attr_name(name) {
            warnings.push(format!(
                "{label}: svg_attr(\"{name}\") is not on the allow-list (data-*, aria-*, role, id, class, lang) — ignored"
            ));
            continue;
        }
        if used.contains(&name.as_str()) {
            // Review PR#141 nit #9: distinguish an actual collision with
            // something ggplot-rs itself writes (`data-plot`, `data-grid`,
            // …) from a *second* `svg_attr` call for the same name — the
            // latter is the caller's own earlier value, not ggplot-rs's.
            if built_in.contains(&name.as_str()) {
                warnings.push(format!(
                    "{label}: svg_attr(\"{name}\") collides with an attribute ggplot-rs already writes to the root — ignored"
                ));
            } else {
                warnings.push(format!(
                    "{label}: svg_attr(\"{name}\") was already set by an earlier svg_attr call — ignored"
                ));
            }
            continue;
        }
        used.push(name.as_str());
        out.push((name.clone(), value.clone()));
    }
    out
}

/// `["warning one", "warning two"]` — a minimal hand-rolled JSON array of
/// strings for `svg_warnings_attr` (no serde dependency, kept out of the
/// `sf`-feature dependency tree — see [`json_string`]).
pub(crate) fn warnings_json(warnings: &[String]) -> String {
    let items: Vec<String> = warnings.iter().map(|w| json_string(w)).collect();
    format!("[{}]", items.join(","))
}

/// Defence-in-depth gate for `push_mark`'s `mark_attrs`: `^data-[a-z0-9][a-z0-9-]*$`.
/// `geom::set_row_extras` only ever supplies already-validated names, but
/// `set_mark_attrs` is a public `DrawBackend` method any caller (a custom
/// `Geom` impl, or misuse from outside the crate) could invoke directly with
/// an arbitrary string, so this is checked again right before it reaches
/// the wire — never trust a single validation layer for attribute-name
/// injection.
fn is_valid_mark_attr_name(name: &str) -> bool {
    match name.strip_prefix("data-") {
        Some(suffix) => crate::aes::is_valid_data_attr_name(suffix),
        None => false,
    }
}

fn rgb((r, g, b): (u8, u8, u8)) -> String {
    format!("#{r:02X}{g:02X}{b:02X}")
}

/// Escape text for use in SVG element content *and* quoted attribute values.
/// Quotes must be escaped too: data values land in `data-x="…"`, and the SVG is
/// routinely inlined into HTML, so an unescaped `"` would let data inject
/// attributes (e.g. event handlers). Characters that are not legal in XML 1.0
/// (C0 controls other than tab/LF/CR) are dropped so the document stays
/// well-formed.
fn escape(s: &str) -> String {
    let mut out = String::with_capacity(s.len());
    for c in s.chars() {
        match c {
            '&' => out.push_str("&amp;"),
            '<' => out.push_str("&lt;"),
            '>' => out.push_str("&gt;"),
            '"' => out.push_str("&quot;"),
            '\'' => out.push_str("&#39;"),
            '\t' | '\n' | '\r' => out.push(c),
            c if (c as u32) < 0x20 || c == '\u{FFFE}' || c == '\u{FFFF}' => {}
            c => out.push(c),
        }
    }
    out
}

fn points(pts: &[(f64, f64)]) -> String {
    pts.iter()
        .map(|(x, y)| format!("{x:.2},{y:.2}"))
        .collect::<Vec<_>>()
        .join(" ")
}

fn finite_pts(pts: &[(f64, f64)]) -> Vec<(f64, f64)> {
    pts.iter()
        .copied()
        .filter(|(x, y)| x.is_finite() && y.is_finite())
        .collect()
}

/// Split `pts` into maximal runs of consecutive finite vertices, dropping
/// any run shorter than 2 points. Matches ggplot2's NA-break handling for
/// lines: a non-finite vertex breaks the line instead of being silently
/// skipped, which would otherwise visually bridge across the gap — but a
/// single remaining observation isn't a line segment either (ggplot2 drops
/// a one-point path piece the same way), so it draws nothing rather than
/// an invisible 1-point `<polyline>` that would still carry a `<title>`
/// and hover attributes.
fn finite_runs(pts: &[(f64, f64)]) -> Vec<Vec<(f64, f64)>> {
    let mut runs = Vec::new();
    let mut cur: Vec<(f64, f64)> = Vec::new();
    for &(x, y) in pts {
        if x.is_finite() && y.is_finite() {
            cur.push((x, y));
        } else if !cur.is_empty() {
            if cur.len() >= 2 {
                runs.push(std::mem::take(&mut cur));
            } else {
                cur.clear();
            }
        }
    }
    if cur.len() >= 2 {
        runs.push(cur);
    }
    runs
}

impl DrawBackend for SvgBackend {
    fn plot_area(&self) -> Rect {
        self.plot_area.clone()
    }
    fn total_area(&self) -> Rect {
        self.total_area.clone()
    }

    fn set_tooltip(&mut self, tooltip: Option<String>) {
        self.tooltip = tooltip;
    }

    fn set_mark_axis(&mut self, key: Option<String>) {
        self.axis_key = key;
    }

    fn set_mark_series(&mut self, series: Option<String>) {
        self.series_key = series;
    }

    fn set_mark_value(&mut self, value: Option<String>) {
        self.value_key = value;
    }

    fn set_mark_geom(&mut self, geom: Option<&str>) {
        self.mark_geom = geom.map(str::to_string);
    }

    fn warn(&mut self, message: String) {
        self.warnings.push(message);
    }

    fn set_mark_part(&mut self, part: Option<&str>) {
        self.mark_part = part.map(|s| s.to_string());
    }

    fn set_mark_attrs(&mut self, attrs: Vec<(String, String)>) {
        self.mark_attrs = attrs;
    }

    fn wants_formatted_values(&self) -> bool {
        self.formatted_values
    }

    fn set_mark_fvalue(&mut self, value: Option<String>) {
        self.mark_fvalue = value;
    }

    fn set_mark_fx(&mut self, value: Option<String>) {
        self.mark_fx = value;
    }

    fn draw_circle(
        &mut self,
        (cx, cy): (f64, f64),
        radius: f64,
        style: &PointStyle,
    ) -> Result<(), RenderError> {
        if !(cx.is_finite() && cy.is_finite() && radius.is_finite()) {
            return Ok(());
        }
        let attrs = format!(
            "cx=\"{cx:.2}\" cy=\"{cy:.2}\" r=\"{:.2}\" fill=\"{}\"{}",
            radius.max(0.0),
            rgb(style.color),
            opacity_attr("fill", style.alpha)
        );
        self.push_mark("circle", "point", &attrs);
        Ok(())
    }

    /// Native shapes: filled polygons for square/triangle/diamond, a single
    /// stroked `<path>` for the `+` / `×` glyphs (so each point stays one
    /// hoverable mark carrying its `data-*` attributes).
    fn draw_shape(
        &mut self,
        (cx, cy): (f64, f64),
        radius: f64,
        style: &PointStyle,
    ) -> Result<(), RenderError> {
        if !(cx.is_finite() && cy.is_finite() && radius.is_finite()) {
            return Ok(());
        }
        let r = radius.max(0.0);
        let color = rgb(style.color);
        let fill_opacity = opacity_attr("fill", style.alpha);
        let stroke_opacity = opacity_attr("stroke", style.alpha);
        let poly = |pts: &[(f64, f64)]| {
            if style.filled {
                format!("points=\"{}\" fill=\"{color}\"{fill_opacity}", points(pts))
            } else {
                format!(
                    "points=\"{}\" fill=\"none\" stroke=\"{color}\"{stroke_opacity}",
                    points(pts)
                )
            }
        };
        match style.shape {
            PointShape::Circle => return self.draw_circle((cx, cy), radius, style),
            PointShape::Square => {
                let a = poly(&[
                    (cx - r, cy - r),
                    (cx + r, cy - r),
                    (cx + r, cy + r),
                    (cx - r, cy + r),
                ]);
                self.push_mark("polygon", "point", &a);
            }
            PointShape::Triangle => {
                let a = poly(&[(cx, cy - r), (cx + r, cy + r), (cx - r, cy + r)]);
                self.push_mark("polygon", "point", &a);
            }
            PointShape::Diamond => {
                let a = poly(&[(cx, cy - r), (cx + r, cy), (cx, cy + r), (cx - r, cy)]);
                self.push_mark("polygon", "point", &a);
            }
            PointShape::Plus | PointShape::Cross => {
                let d = if style.shape == PointShape::Plus {
                    format!(
                        "M{:.2} {cy:.2}H{:.2}M{cx:.2} {:.2}V{:.2}",
                        cx - r,
                        cx + r,
                        cy - r,
                        cy + r
                    )
                } else {
                    format!(
                        "M{:.2} {:.2}L{:.2} {:.2}M{:.2} {:.2}L{:.2} {:.2}",
                        cx - r,
                        cy - r,
                        cx + r,
                        cy + r,
                        cx - r,
                        cy + r,
                        cx + r,
                        cy - r
                    )
                };
                let a = format!(
                    "d=\"{d}\" fill=\"none\" stroke=\"{color}\" stroke-width=\"{:.2}\"{stroke_opacity}",
                    (r / 2.5).clamp(1.0, 3.0)
                );
                self.push_mark("path", "point", &a);
            }
        }
        Ok(())
    }

    fn draw_line(&mut self, pts: &[(f64, f64)], style: &LineStyle) -> Result<(), RenderError> {
        // Split at non-finite vertices instead of silently dropping them:
        // dropping-and-joining would visually bridge the gap, but ggplot2
        // treats a non-finite (NA) vertex as a break in the line (see
        // `finite_runs`). Each run becomes its own `<polyline>` mark, with
        // the same style and the same data-x/series/value/part attrs.
        for run in finite_runs(pts) {
            self.draw_line_run(&run, style);
        }
        Ok(())
    }

    /// Emit a native `<path d="M.. Q..">` quadratic Bézier (one real curve
    /// instead of a flattened polyline). Non-finite input draws nothing.
    fn draw_quadratic(
        &mut self,
        p0: (f64, f64),
        ctrl: (f64, f64),
        p1: (f64, f64),
        style: &LineStyle,
    ) -> Result<(), RenderError> {
        if !(p0.0.is_finite()
            && p0.1.is_finite()
            && ctrl.0.is_finite()
            && ctrl.1.is_finite()
            && p1.0.is_finite()
            && p1.1.is_finite())
        {
            return Ok(());
        }
        let dash = match style
            .linetype
            .pattern()
            .iter()
            .flat_map(|(d, g)| [*d, *g])
            .map(|v| format!("{v}"))
            .collect::<Vec<_>>()
            .join(",")
        {
            s if s.is_empty() => String::new(),
            s => format!(" stroke-dasharray=\"{s}\""),
        };
        let linecap = match style.lineend {
            LineEnd::Butt => "",
            LineEnd::Round => " stroke-linecap=\"round\"",
            LineEnd::Square => " stroke-linecap=\"square\"",
        };
        let d = format!(
            "M{:.2} {:.2} Q{:.2} {:.2} {:.2} {:.2}",
            p0.0, p0.1, ctrl.0, ctrl.1, p1.0, p1.1
        );
        let attrs = format!(
            "d=\"{d}\" fill=\"none\" stroke=\"{}\" stroke-width=\"{:.2}\"{}{}{}",
            rgb(style.color),
            finite_or(style.width, 1.0),
            opacity_attr("stroke", style.alpha),
            dash,
            linecap
        );
        self.push_mark("path", "line", &attrs);
        Ok(())
    }

    fn draw_rect(
        &mut self,
        top_left: (f64, f64),
        bottom_right: (f64, f64),
        style: &RectStyle,
    ) -> Result<(), RenderError> {
        let fill = style.fill.map(rgb).unwrap_or_else(|| "none".into());
        self.rect_markup(top_left, bottom_right, &fill, style)
    }

    /// #206: same markup [`draw_rect`](Self::draw_rect) builds, filled
    /// with `pattern`'s registered `url(#…)` instead of `style.fill`.
    fn draw_rect_pattern(
        &mut self,
        top_left: (f64, f64),
        bottom_right: (f64, f64),
        pattern: &FillPattern,
        style: &RectStyle,
    ) -> Result<(), RenderError> {
        let fill = self.pattern_url(pattern);
        self.rect_markup(top_left, bottom_right, &fill, style)
    }

    fn draw_polygon(&mut self, pts: &[(f64, f64)], style: &RectStyle) -> Result<(), RenderError> {
        let fill = style.fill.map(rgb).unwrap_or_else(|| "none".into());
        self.polygon_markup(pts, &fill, style)
    }

    /// #206: same markup [`draw_polygon`](Self::draw_polygon) builds,
    /// filled with `pattern`'s registered `url(#…)` instead of `style.fill`.
    fn draw_polygon_pattern(
        &mut self,
        pts: &[(f64, f64)],
        pattern: &FillPattern,
        style: &RectStyle,
    ) -> Result<(), RenderError> {
        let fill = self.pattern_url(pattern);
        self.polygon_markup(pts, &fill, style)
    }

    /// One `<path>`, every ring a subpath (`M…Z`), `fill-rule="evenodd"` so
    /// holes (rings after the first) are cut out of the fill — SVG's native
    /// equivalent of the even-odd rasteriser the pixel/plotters backends run
    /// (#178). A ring with fewer than 3 finite points is dropped; the whole
    /// shape draws nothing when every ring is.
    fn draw_polygon_rings(
        &mut self,
        rings: &[Vec<(f64, f64)>],
        style: &RectStyle,
    ) -> Result<(), RenderError> {
        let fill = style.fill.map(rgb).unwrap_or_else(|| "none".into());
        self.polygon_rings_markup(rings, &fill, style)
    }

    /// #206: same markup [`draw_polygon_rings`](Self::draw_polygon_rings)
    /// builds (holes cut out under the same even-odd rule), filled with
    /// `pattern`'s registered `url(#…)` instead of `style.fill`.
    fn draw_polygon_rings_pattern(
        &mut self,
        rings: &[Vec<(f64, f64)>],
        pattern: &FillPattern,
        style: &RectStyle,
    ) -> Result<(), RenderError> {
        let fill = self.pattern_url(pattern);
        self.polygon_rings_markup(rings, &fill, style)
    }

    fn draw_text(
        &mut self,
        text: &str,
        pos: (f64, f64),
        style: &TextStyle,
    ) -> Result<(), RenderError> {
        self.draw_text_with_title(text, pos, style, None)
    }

    /// #159: with `title`, wraps the `<text>` in a `<title>` child
    /// carrying the full (escaped) label — e.g. a legend key truncated
    /// with an ellipsis to fit a vertical multi-column grid.
    fn draw_text_with_title(
        &mut self,
        text: &str,
        (x, y): (f64, f64),
        style: &TextStyle,
        title: Option<&str>,
    ) -> Result<(), RenderError> {
        if !(x.is_finite() && y.is_finite()) {
            return Ok(());
        }
        let anchor = match style.anchor {
            TextAnchor::Start => "start",
            TextAnchor::Middle => "middle",
            TextAnchor::End => "end",
        };
        let family = escape(style.family.as_deref().unwrap_or("sans-serif"));
        let weight = if style.face == FontFace::Bold {
            " font-weight=\"bold\""
        } else {
            ""
        };
        let fstyle = if style.face == FontFace::Italic {
            " font-style=\"italic\""
        } else {
            ""
        };
        let angle = finite_or(style.angle, 0.0);
        let transform = if angle.abs() > 0.01 {
            format!(" transform=\"rotate({angle:.1} {x:.2} {y:.2})\"")
        } else {
            String::new()
        };
        let class = self.mark_class("text");
        let title_tag = match title {
            Some(t) => format!("<title>{}</title>", escape(t)),
            None => String::new(),
        };
        self.body.push_str(&format!(
            "<text x=\"{x:.2}\" y=\"{y:.2}\" font-size=\"{:.2}\" text-anchor=\"{anchor}\" \
             dominant-baseline=\"middle\" font-family=\"{family}\"{weight}{fstyle} fill=\"{}\"{transform}{class}>{title_tag}{}</text>",
            finite_or(style.size, 12.0),
            rgb(style.color),
            escape(text)
        ));
        Ok(())
    }

    /// #196: each linked run becomes `<a href="…" target="_blank"
    /// rel="noopener"><tspan text-decoration="underline">…</tspan></a>`
    /// (`rel="noopener"` so the opened tab can't reach back via
    /// `window.opener`); a plain run is a bare `<tspan>`. All runs sit
    /// inside one `<text>`, so `style.anchor` aligns their combined width
    /// exactly as it would a single run's.
    fn draw_text_runs(
        &mut self,
        runs: &[(&str, Option<&str>)],
        (x, y): (f64, f64),
        style: &TextStyle,
    ) -> Result<(), RenderError> {
        if !(x.is_finite() && y.is_finite()) {
            return Ok(());
        }
        let anchor = match style.anchor {
            TextAnchor::Start => "start",
            TextAnchor::Middle => "middle",
            TextAnchor::End => "end",
        };
        let family = escape(style.family.as_deref().unwrap_or("sans-serif"));
        let weight = if style.face == FontFace::Bold {
            " font-weight=\"bold\""
        } else {
            ""
        };
        let fstyle = if style.face == FontFace::Italic {
            " font-style=\"italic\""
        } else {
            ""
        };
        let angle = finite_or(style.angle, 0.0);
        let transform = if angle.abs() > 0.01 {
            format!(" transform=\"rotate({angle:.1} {x:.2} {y:.2})\"")
        } else {
            String::new()
        };
        let class = self.mark_class("text");
        let mut inner = String::new();
        for (text, href) in runs {
            match href {
                Some(u) => inner.push_str(&format!(
                    "<a href=\"{}\" target=\"_blank\" rel=\"noopener\"><tspan text-decoration=\"underline\">{}</tspan></a>",
                    escape(u),
                    escape(text)
                )),
                None => inner.push_str(&format!("<tspan>{}</tspan>", escape(text))),
            }
        }
        self.body.push_str(&format!(
            "<text x=\"{x:.2}\" y=\"{y:.2}\" font-size=\"{:.2}\" text-anchor=\"{anchor}\" \
             dominant-baseline=\"middle\" font-family=\"{family}\"{weight}{fstyle} fill=\"{}\"{transform}{class}>{inner}</text>",
            finite_or(style.size, 12.0),
            rgb(style.color),
        ));
        Ok(())
    }
}

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

    fn backend() -> SvgBackend {
        SvgBackend::new(
            100,
            100,
            Rect {
                x: 0.0,
                y: 0.0,
                width: 100.0,
                height: 100.0,
            },
        )
    }

    #[test]
    fn draw_quadratic_emits_a_path_with_m_and_q() {
        let mut b = backend();
        b.draw_quadratic((0.0, 0.0), (10.0, 20.0), (20.0, 0.0), &LineStyle::default())
            .unwrap();
        let svg = b.finish();
        assert!(svg.contains("<path"));
        assert!(svg.contains("d=\"M0.00 0.00 Q10.00 20.00 20.00 0.00\""));
        assert!(!svg.contains("NaN"));
        assert!(!svg.contains("inf"));
    }

    #[test]
    fn draw_quadratic_drops_non_finite_input() {
        let mut b = backend();
        b.draw_quadratic(
            (f64::NAN, 0.0),
            (10.0, 20.0),
            (20.0, 0.0),
            &LineStyle::default(),
        )
        .unwrap();
        b.draw_quadratic(
            (0.0, 0.0),
            (f64::INFINITY, 20.0),
            (20.0, 0.0),
            &LineStyle::default(),
        )
        .unwrap();
        let svg = b.finish();
        assert!(!svg.contains("<path"));
    }

    #[test]
    fn draw_line_omits_linecap_by_default_but_writes_it_when_set() {
        let mut b = backend();
        b.draw_line(&[(0.0, 0.0), (10.0, 10.0)], &LineStyle::default())
            .unwrap();
        let svg = b.finish();
        assert!(!svg.contains("stroke-linecap"));

        let mut b2 = backend();
        let style = LineStyle {
            lineend: LineEnd::Round,
            ..Default::default()
        };
        b2.draw_line(&[(0.0, 0.0), (10.0, 10.0)], &style).unwrap();
        let svg2 = b2.finish();
        assert!(svg2.contains("stroke-linecap=\"round\""));
    }

    #[test]
    fn square_linecap_is_written_on_both_draw_line_and_draw_quadratic() {
        let style = LineStyle {
            lineend: LineEnd::Square,
            ..Default::default()
        };
        let mut b = backend();
        b.draw_line(&[(0.0, 0.0), (10.0, 10.0)], &style).unwrap();
        assert!(b.finish().contains("stroke-linecap=\"square\""));

        let mut b2 = backend();
        b2.draw_quadratic((0.0, 0.0), (5.0, 10.0), (10.0, 0.0), &style)
            .unwrap();
        assert!(b2.finish().contains("stroke-linecap=\"square\""));
    }

    /// `push_mark`'s defence-in-depth gate: even if an invalid name reaches
    /// `set_mark_attrs` directly (bypassing `Aes::data_attr`'s own
    /// validation — `set_mark_attrs` is a public `DrawBackend` method any
    /// caller could invoke), it's dropped before it reaches the wire.
    #[test]
    fn set_mark_attrs_defence_in_depth_drops_invalid_names() {
        let mut b = backend();
        b.set_mark_attrs(vec![
            ("not-data-prefixed".to_string(), "x".to_string()),
            ("data-".to_string(), "x".to_string()), // empty suffix
            ("data-Upper".to_string(), "x".to_string()), // uppercase
            ("data-ok".to_string(), "valid".to_string()), // the one legitimate entry
        ]);
        b.draw_circle((1.0, 1.0), 1.0, &PointStyle::default())
            .unwrap();
        let svg = b.finish();
        assert!(!svg.contains("not-data-prefixed"));
        assert!(!svg.contains("data-Upper"));
        assert!(!svg.contains("data-=\""));
        assert!(svg.contains("data-ok=\"valid\""));
    }

    // #206: pattern fills.

    fn stripe() -> FillPattern {
        FillPattern::Stripe {
            fg: (10, 20, 30),
            bg: (200, 210, 220),
            size_px: 4.0,
            angle_deg: 45.0,
        }
    }

    fn pattern_id(svg: &str) -> String {
        let i = svg.find("<pattern id=\"").unwrap() + "<pattern id=\"".len();
        svg[i..i + svg[i..].find('"').unwrap()].to_string()
    }

    #[test]
    fn a_pattern_fill_registers_one_defs_pattern_and_references_it() {
        let mut b = backend();
        b.draw_rect_pattern((0.0, 0.0), (10.0, 10.0), &stripe(), &RectStyle::default())
            .unwrap();
        let svg = b.finish();
        assert_eq!(svg.matches("<pattern ").count(), 1, "{svg}");
        let id = pattern_id(&svg);
        assert!(
            svg.contains(&format!("fill=\"url(#{id})\"")),
            "rect references the registered pattern id {id}: {svg}"
        );
    }

    #[test]
    fn a_rounded_rect_pattern_still_rounds_its_corners() {
        let mut b = backend();
        b.draw_rect_pattern(
            (0.0, 0.0),
            (10.0, 10.0),
            &stripe(),
            &RectStyle {
                corner_radius: 3.0,
                ..Default::default()
            },
        )
        .unwrap();
        let svg = b.finish();
        assert!(svg.contains("rx=\"3.00\""), "{svg}");
        assert!(svg.contains("fill=\"url(#"), "{svg}");
    }

    #[test]
    fn an_equal_pattern_reuses_the_same_defs_entry() {
        let mut b = backend();
        b.draw_rect_pattern((0.0, 0.0), (5.0, 5.0), &stripe(), &RectStyle::default())
            .unwrap();
        b.draw_polygon_pattern(
            &[(0.0, 0.0), (1.0, 0.0), (1.0, 1.0)],
            &stripe(),
            &RectStyle::default(),
        )
        .unwrap();
        let svg = b.finish();
        assert_eq!(
            svg.matches("<pattern ").count(),
            1,
            "two equal patterns share one <defs> entry: {svg}"
        );
    }

    #[test]
    fn different_patterns_get_their_own_defs_entries() {
        let mut b = backend();
        b.draw_rect_pattern((0.0, 0.0), (5.0, 5.0), &stripe(), &RectStyle::default())
            .unwrap();
        let other = FillPattern::Stripe {
            fg: (1, 1, 1),
            bg: (2, 2, 2),
            size_px: 8.0,
            angle_deg: 0.0,
        };
        b.draw_rect_pattern((0.0, 0.0), (5.0, 5.0), &other, &RectStyle::default())
            .unwrap();
        let svg = b.finish();
        assert_eq!(svg.matches("<pattern ").count(), 2, "{svg}");
    }

    #[test]
    fn no_pattern_used_means_no_defs_block_at_all() {
        let mut b = backend();
        b.draw_rect((0.0, 0.0), (5.0, 5.0), &RectStyle::default())
            .unwrap();
        let svg = b.finish();
        assert!(!svg.contains("<defs>"), "{svg}");
    }

    #[test]
    fn pattern_fill_cuts_out_holes_like_a_flat_fill_does() {
        let mut b = backend();
        let outer = vec![(0.0, 0.0), (10.0, 0.0), (10.0, 10.0), (0.0, 10.0)];
        let hole = vec![(2.0, 2.0), (8.0, 2.0), (8.0, 8.0), (2.0, 8.0)];
        b.draw_polygon_rings_pattern(&[outer, hole], &stripe(), &RectStyle::default())
            .unwrap();
        let svg = b.finish();
        assert!(svg.contains("fill-rule=\"evenodd\""), "{svg}");
        assert_eq!(
            svg.matches("<path").count(),
            1,
            "one path, both rings: {svg}"
        );
    }

    #[test]
    fn two_backends_on_one_page_never_collide_on_a_pattern_id() {
        let mut a = backend();
        a.draw_rect_pattern((0.0, 0.0), (5.0, 5.0), &stripe(), &RectStyle::default())
            .unwrap();
        let mut b = backend();
        b.draw_rect_pattern((0.0, 0.0), (5.0, 5.0), &stripe(), &RectStyle::default())
            .unwrap();
        let (sa, sb) = (a.finish(), b.finish());
        assert_ne!(pattern_id(&sa), pattern_id(&sb), "{sa}\n{sb}");
    }
}