Function simplify_svg_polylines

crates/plotlars-plotters/src/render/mod.rs:214–256

Find every `points="..."` attribute in the SVG and, when the polyline has enough vertices, apply Ramer-Douglas-Peucker to remove redundant ones.

(svg: &mut String)

/// Find every `points="..."` attribute in the SVG and, when the polyline has
/// enough vertices, apply Ramer-Douglas-Peucker to remove redundant ones.
fn simplify_svg_polylines(svg: &mut String) {
    // MIN_POINTS: skip simplification for short polylines (axis ticks, error-bar
    // caps, scatter markers); RDP overhead outweighs savings below this size.
    // EPSILON: 0.5 px matches plotters' integer-pixel rounding granularity, so
    // collinear-after-rounding vertices collapse without altering visible shape.
    const MIN_POINTS: usize = 20;
    const EPSILON: f64 = 0.5;

    let mut result = String::with_capacity(svg.len());
    let mut remaining = svg.as_str();
    let mut modified = false;

    while let Some(idx) = remaining.find("points=\"") {
        let prefix_end = idx + 8; // length of `points="`
        result.push_str(&remaining[..prefix_end]);
        remaining = &remaining[prefix_end..];

        if let Some(end) = remaining.find('"') {
            let raw = &remaining[..end];
            // Cheap pre-check: count separators before parsing. A polyline with
            // <MIN_POINTS vertices has <MIN_POINTS spaces between coord pairs.
            let space_count = raw.bytes().filter(|&b| b == b' ').count();
            if space_count >= MIN_POINTS {
                let points = parse_svg_points(raw);
                if points.len() > MIN_POINTS {
                    let simplified = rdp_simplify(&points, EPSILON);
                    result.push_str(&format_svg_points(&simplified));
                    modified = true;
                } else {
                    result.push_str(raw);
                }
            } else {
                result.push_str(raw);
            }
            result.push('"');
            remaining = &remaining[end + 1..];
        }
    }
    if modified {
        result.push_str(remaining);
        *svg = result;
    }
}

fn parse_svg_points(s: &str) -> Vec<(f64, f64)> {
    s.split_whitespace()