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---
title: Neo4J contribution
date: 2018-03-06
layout: post
---
Written in 2026, backdated to 2018.
Before v3.4.0, Neo4J shipped with Dijkstra's shortest path search. The
algorithm was too slow for our marine vessel tracking application.
Forked and added A* search. Used the haversine function to steer the search:
```
private double computeHeuristic(
final double lat1, final double lon1,
final double lat2, final double lon2) {
final int earthRadius = 6371;
final double kmToNM = 0.539957;
final double latDistance = Math.toRadians(lat2 - lat1);
final double lonDistance = Math.toRadians(lon2 - lon1);
final double a = Math.sin(latDistance / 2)
* Math.sin(latDistance / 2)
+ Math.cos(Math.toRadians(lat1))
* Math.cos(Math.toRadians(lat2))
* Math.sin(lonDistance / 2)
* Math.sin(lonDistance / 2);
final double c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));
return earthRadius * c * kmToNM;
}
```
Core search loop updates costs when a better path is found:
```
private void updateCosts(
final int source, final int target,
final double newCost, final double heuristic) {
final double oldCost = gCosts.getOrDefault(target, Double.MAX_VALUE);
if (newCost < oldCost) {
gCosts.put(target, newCost);
fCosts.put(target, newCost + heuristic);
path.put(target, source);
}
}
```
Upstreamed the changes.
GitHub release: <a
href="https://github.com/neo4j-contrib/neo4j-graph-algorithms/releases/tag/3.4.0.0"
class="external" target="_blank" rel="noopener noreferrer">Neo4J v3.4.0</a> |
<a
href="https://github.com/neo4j-contrib/neo4j-graph-algorithms/blob/bd9732d9a690319552e134708692acb5a0d6b37c/algo/src/main/java/org/neo4j/graphalgo/impl/ShortestPathAStar.java"
class="external" target="_blank" rel="noopener noreferrer">Full source</a>.
|
