graphhopper-0002: AlternativeRouteEdgeCH — IntArrayList.contains() O(P) inside edge loop

File

core/src/main/java/com/graphhopper/routing/AlternativeRouteEdgeCH.java

Severity

HIGH

Lines Affected

Line 190: alternatives.get(0).nodes.contains() × 2 — inside per-edge loop
Lines 197–204: nodesInCurrentAlternativeSetContains() — O(A×P) per edge call

Defective Code

// sharedDistanceWithShortest() — O(E × P) per call
private double sharedDistanceWithShortest(Path path) {
    double sharedDistance = 0.0;
    List<EdgeIteratorState> edges = path.calcEdges();
    for (EdgeIteratorState edge : edges) {
        if (alternatives.get(0).nodes.contains(edge.getBaseNode()) && alternatives.get(0).nodes.contains(edge.getAdjNode())) {
            sharedDistance += edge.getDistance();
        }
    }
    return sharedDistance;
}

// nodesInCurrentAlternativeSetContains() — O(A × P) per call
private boolean nodesInCurrentAlternativeSetContains(int v) {
    for (AlternativeInfo alternative : alternatives) {
        if (alternative.nodes.contains(v)) {  // IntArrayList.contains() = O(P)
            return true;
        }
    }
    return false;
}

Root Cause

Identical to graphhopper-0001. This is the edge-based CH variant of the same algorithm. AlternativeInfo.nodes is an IntArrayList backed by Path.calcNodes(). Every call to contains() performs a linear scan over path nodes.

Complexity Analysis

Path	Per-call	Total calls	Overall
Slow (IntArrayList.contains)	O(P)	O(E × A)	O(E × A × P)
Fast (IntScatterSet.contains)	O(1)	O(E × A)	O(E × A)

Fixed Code

Same fix as graphhopper-0001: add an IntScatterSet nodeSet field to AlternativeInfo, populate it at construction time from nodes, and use nodeSet.contains() in all membership test calls.

public static class AlternativeInfo {
    final double shareWeight;
    final Path path;
    final IntIndexedContainer nodes;
    final IntSet nodeSet;  // ADD

    AlternativeInfo(Path path, double shareWeight) {
        this.path = path;
        this.shareWeight = shareWeight;
        this.nodes = path.calcNodes();
        IntScatterSet set = new IntScatterSet(nodes.size());
        for (int i = 0; i < nodes.size(); i++) set.add(nodes.get(i));
        this.nodeSet = set;
    }
}

Replace all alternative.nodes.contains(v) and alternatives.get(0).nodes.contains(x) with the corresponding nodeSet.contains() calls.

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