If you ever want to encode a poly line against Google Earth / OSM Valhalla standards

Polyline.java

import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Gatherers;

public final class Polyline {

	private final List<Point> points = new ArrayList<>();

	private final int scale;

	// Valhalla uses a scale of 6, Google is stupid and uses 5.
	Polyline() {
		this(6);
	}

	Polyline(int scale) {
		this.scale = (int) Math.pow(10, scale);
	}

	void add(double x, double y) {
		this.points.add(new Point((int) Math.floor(x * this.scale), (int) Math.floor(y * this.scale)));
	}

	@Override
	public String toString() {
		if (this.points.isEmpty()) {
			return "";
		}
		var initial = this.points.getFirst().encode();
		if (this.points.size() == 1) {
			return initial;
		}
		var remainder = this.points.stream()
			.gather(Gatherers.windowSliding(2))
			.map(pair -> pair.getLast().offset(pair.getFirst()).encode())
			.collect(Collectors.joining());
		return initial + remainder;
	}

	public static void main(String... a) {

		var polyline = new Polyline(5);
		polyline.add(38.5, -120.2);
		polyline.add(40.7, -120.95);
		polyline.add(43.252, -126.453);
		System.err.println(polyline);
	}

	record Point(int x, int y) {

		private Point offset(Point other) {
			return new Point(this.x - other.x(), this.y - other.y());
		}

		private String encode() {
			return encode0(this.x) + encode0(this.y);
		}

		private String encode0(int v) {
			var num = v << 1;
			if (v < 0) {
				num = ~num;
			}

			var result = new StringBuilder();
			while (num >= 0x20) {
				int nextValue = (0x20 | (num & 0x1f)) + 63;
				result.append((char) (nextValue));
				num >>= 5;
			}

			num += 63;
			result.append((char) (num));
			return result.toString();
		}
	}
}