gpx.studio/website/src/lib/components/toolbar/tools/routing/simplify.ts

import { ramerDouglasPeucker, type GPXFile, type TrackSegment } from 'gpx';

const earthRadius = 6371008.8;

export const MIN_ANCHOR_ZOOM = 0;
export const MAX_ANCHOR_ZOOM = 22;

export function getZoomLevelForDistance(latitude: number, distance?: number): number {
    if (distance === undefined) {
        return MIN_ANCHOR_ZOOM;
    }

    const rad = Math.PI / 180;
    const lat = latitude * rad;

    return Math.min(
        MAX_ANCHOR_ZOOM,
        Math.max(MIN_ANCHOR_ZOOM, Math.round(Math.log2((earthRadius * Math.cos(lat)) / distance)))
    );
}

export function updateAnchorPoints(file: GPXFile) {
    let segments = file.getSegments();

    for (let segment of segments) {
        if (!segment._data.anchors) {
            // New segment, compute anchor points for it
            computeAnchorPoints(segment);
            continue;
        }

        if (segment.trkpt.length > 0) {
            // Ensure first and last points are anchors and always visible
            segment.trkpt[0]._data.anchor = true;
            segment.trkpt[0]._data.zoom = 0;
            segment.trkpt[segment.trkpt.length - 1]._data.anchor = true;
            segment.trkpt[segment.trkpt.length - 1]._data.zoom = 0;
        }
    }
}

function computeAnchorPoints(segment: TrackSegment) {
    let points = segment.trkpt;
    let anchors = ramerDouglasPeucker(points, 1);
    anchors.forEach((anchor) => {
        let point = anchor.point;
        point._data.anchor = true;
        point._data.zoom = getZoomLevelForDistance(point.getLatitude(), anchor.distance);
    });
    segment._data.anchors = true;
}