geojsonvt 0.1.0

A Rust library for slicing GeoJSON data into vector tile on the fly.
Documentation 
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use crate::types::VtPoint;

/// Simplifies a polyline using the [Douglas-Peucker algorithm](https://en.wikipedia.org/wiki/Ramer–Douglas–Peucker_algorithm)
///
/// # Arguments
/// * `points` - A mutable vector of [`VtPoint`] representing the original polyline.
///              The z-coordinate will be modified to store simplification metrics.
/// * `tolerance` - Simplification tolerance (squared value). Points with square distance
///                less than this value may be removed.
///
/// # Examples
/// ```ignore
/// use geojsonvt::simplify::simplify;
/// use geojsonvt::types::VtPoint;
///
/// let mut points = vec![
///     VtPoint::new(0.0, 0.0, 0.0),
///     VtPoint::new(1.0, 1.0, 0.0),
///     VtPoint::new(2.0, 2.0, 0.0),
///     VtPoint::new(3.0, 1.0, 0.0),
///     VtPoint::new(4.0, 0.0, 0.0),
/// ];
///
/// simplify(&mut points, 0.5);
/// ```
pub fn simplify(points: &mut [VtPoint], tolerance: f64) {
    let len = points.len() - 1;
    if points.is_empty() {
        return;
    }
    points.first_mut().unwrap().z = 1.;
    points.last_mut().unwrap().z = 1.;
    douglas_peucker(points, 0, len, tolerance * tolerance);
}

fn douglas_peucker(points: &mut [VtPoint], first: usize, last: usize, sq_tolerance: f64) {
    let mut max_sq_dist = sq_tolerance;
    let mut index = 0;
    let mid = ((first + last) as i64) / 2;
    let mut min_pos_to_mid = (last - first) as i64;

    for i in first + 1..last {
        let sq_dist = point_segment_dist(&points[i], &points[first], &points[last]);
        if sq_dist > max_sq_dist {
            index = i;
            max_sq_dist = sq_dist;
        } else if sq_dist == max_sq_dist {
            let pos_to_mid = (i as i64 - mid).abs();
            if pos_to_mid < min_pos_to_mid {
                index = i;
                min_pos_to_mid = pos_to_mid;
            }
        }
    }
    if max_sq_dist > sq_tolerance {
        points[index].z = max_sq_dist;
        if index - first > 1 {
            douglas_peucker(points, first, index, sq_tolerance);
        }
        if last - index > 1 {
            douglas_peucker(points, index, last, sq_tolerance);
        }
    }
}

fn point_segment_dist(p: &VtPoint, a: &VtPoint, b: &VtPoint) -> f64 {
    let mut x = a.x;
    let mut y = a.y;
    let mut dx = b.x - x;
    let mut dy = b.y - y;
    if dx != 0. || dy != 0. {
        let t = ((p.x - x) * dx + (p.y - y) * dy) / (dx * dx + dy * dy);
        if t > 1. {
            x = b.x;
            y = b.y;
        } else if t > 0. {
            x += dx * t;
            y += dy * t;
        }
    }
    dx = p.x - x;
    dy = p.y - y;
    dx * dx + dy * dy
}

#[cfg(test)]
mod tests {
    use super::simplify;
    use crate::types::VtPoint;
    fn create_points_from_array(points: &[[f64; 2]]) -> Vec<VtPoint> {
        points
            .iter()
            .map(|p| VtPoint::new(p[0], p[1], 0.))
            .collect::<Vec<_>>()
    }
    #[test]
    fn test_simplify() {
        let raw_points = [
            [0.22455, 0.25015],
            [0.22691, 0.24419],
            [0.23331, 0.24145],
            [0.23498, 0.23606],
            [0.24421, 0.23276],
            [0.26259, 0.21531],
            [0.26776, 0.21381],
            [0.27357, 0.20184],
            [0.27312, 0.19216],
            [0.27762, 0.18903],
            [0.28036, 0.18141],
            [0.28651, 0.17774],
            [0.29241, 0.15937],
            [0.29691, 0.15564],
            [0.31495, 0.15137],
            [0.31975, 0.14516],
            [0.33033, 0.13757],
            [0.34148, 0.13996],
            [0.36998, 0.13789],
            [0.38739, 0.14251],
            [0.39128, 0.13939],
            [0.40952, 0.14114],
            [0.41482, 0.13975],
            [0.42772, 0.12730],
            [0.43960, 0.11974],
            [0.47493, 0.10787],
            [0.48651, 0.10675],
            [0.48920, 0.10945],
            [0.49379, 0.10863],
            [0.50474, 0.11966],
            [0.51296, 0.12235],
            [0.51863, 0.12089],
            [0.52409, 0.12688],
            [0.52957, 0.12786],
            [0.53421, 0.14093],
            [0.53927, 0.14724],
            [0.56769, 0.14891],
            [0.57525, 0.15726],
            [0.58062, 0.15815],
            [0.60153, 0.15685],
            [0.61774, 0.15986],
            [0.62200, 0.16704],
            [0.62955, 0.19460],
            [0.63890, 0.19561],
            [0.64126, 0.20081],
            [0.65177, 0.20456],
            [0.67155, 0.22255],
            [0.68368, 0.21745],
            [0.69525, 0.21915],
            [0.70064, 0.21798],
            [0.70312, 0.21436],
            [0.71226, 0.21587],
            [0.72149, 0.21281],
            [0.72781, 0.21336],
            [0.72998, 0.20873],
            [0.73532, 0.20820],
            [0.73994, 0.20477],
            [0.76998, 0.20842],
            [0.77960, 0.21687],
            [0.78420, 0.21816],
            [0.80024, 0.21462],
            [0.81053, 0.21973],
            [0.81719, 0.22682],
            [0.82077, 0.23617],
            [0.82723, 0.23616],
            [0.82989, 0.23989],
            [0.85100, 0.24894],
            [0.85988, 0.25549],
            [0.86521, 0.26853],
            [0.85795, 0.28030],
            [0.86548, 0.29145],
            [0.86681, 0.29866],
            [0.86468, 0.30271],
            [0.86779, 0.30617],
            [0.85987, 0.31137],
            [0.86008, 0.31435],
            [0.85829, 0.31494],
            [0.85810, 0.32760],
            [0.85454, 0.33540],
            [0.86092, 0.34300],
            [0.85643, 0.35015],
            [0.85142, 0.35296],
            [0.84984, 0.35959],
            [0.85456, 0.36553],
            [0.84974, 0.37038],
            [0.84409, 0.37189],
            [0.84475, 0.38044],
            [0.84152, 0.38367],
            [0.83957, 0.39040],
            [0.84559, 0.39905],
            [0.84840, 0.40755],
            [0.84371, 0.41130],
            [0.84409, 0.41988],
            [0.83951, 0.43276],
            [0.84133, 0.44104],
            [0.84762, 0.44922],
            [0.84716, 0.45844],
            [0.85138, 0.46279],
            [0.85397, 0.47115],
            [0.86636, 0.48077],
        ];
        let raw_result = [
            [0.22455, 0.25015],
            [0.26776, 0.21381],
            [0.29691, 0.15564],
            [0.33033, 0.13757],
            [0.40952, 0.14114],
            [0.4396, 0.11974],
            [0.48651, 0.10675],
            [0.52957, 0.12786],
            [0.53927, 0.14724],
            [0.56769, 0.14891],
            [0.61774, 0.15986],
            [0.62955, 0.1946],
            [0.67155, 0.22255],
            [0.72781, 0.21336],
            [0.73994, 0.20477],
            [0.76998, 0.20842],
            [0.7842, 0.21816],
            [0.80024, 0.21462],
            [0.82077, 0.23617],
            [0.85988, 0.25549],
            [0.86521, 0.26853],
            [0.85795, 0.2803],
            [0.86779, 0.30617],
            [0.85829, 0.31494],
            [0.85454, 0.3354],
            [0.86092, 0.343],
            [0.84984, 0.35959],
            [0.85456, 0.36553],
            [0.84409, 0.37189],
            [0.83957, 0.3904],
            [0.8484, 0.40755],
            [0.83951, 0.43276],
            [0.85397, 0.47115],
            [0.86636, 0.48077],
        ];
        let points = create_points_from_array(&raw_points);
        let result = create_points_from_array(&raw_result);
        let mut simplified = points.clone();
        simplify(&mut simplified, 0.001 * 0.001);
        let selected = simplified
            .into_iter()
            .filter(|p| p.z > 0.005 * 0.005)
            .collect::<Vec<_>>();

        assert_eq!(selected.len(), result.len());
        for (p1, p2) in selected.iter().zip(result.iter()) {
            assert_eq!([p1.x, p2.y], [p2.x, p2.y]);
        }
    }
}