diff --git a/java/src/main/java/org/softwareheritage/graph/algo/Traversal.java b/java/src/main/java/org/softwareheritage/graph/algo/Traversal.java
index df422f7..4c2a433 100644
--- a/java/src/main/java/org/softwareheritage/graph/algo/Traversal.java
+++ b/java/src/main/java/org/softwareheritage/graph/algo/Traversal.java
@@ -1,457 +1,449 @@
 package org.softwareheritage.graph.algo;
 
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Iterator;
 import java.util.LinkedList;
 import java.util.Map;
 import java.util.Queue;
 import java.util.Random;
 import java.util.Stack;
 
 import it.unimi.dsi.bits.LongArrayBitVector;
 
 import org.softwareheritage.graph.AllowedEdges;
 import org.softwareheritage.graph.Endpoint;
 import org.softwareheritage.graph.Graph;
 import org.softwareheritage.graph.Neighbors;
 import org.softwareheritage.graph.Node;
 
 /**
  * Traversal algorithms on the compressed graph.
  * <p>
  * Internal implementation of the traversal API endpoints. These methods only input/output internal
  * long ids, which are converted in the {@link Endpoint} higher-level class to Software Heritage
  * PID.
  *
  * @author The Software Heritage developers
  * @see org.softwareheritage.graph.Endpoint
  */
 
 public class Traversal {
     /** Graph used in the traversal */
     Graph graph;
     /** Boolean to specify the use of the transposed graph */
     boolean useTransposed;
     /** Graph edge restriction */
     AllowedEdges edges;
 
     /** Hash set storing if we have visited a node */
     HashSet<Long> visited;
     /** Hash map storing parent node id for each nodes during a traversal */
     Map<Long, Long> parentNode;
     /** Number of edges accessed during traversal */
     long nbEdgesAccessed;
 
     /** random number generator, for random walks */
     Random rng;
 
     /**
      * Constructor.
      *
      * @param graph graph used in the traversal
      * @param direction a string (either "forward" or "backward") specifying edge orientation
      * @param edgesFmt a formatted string describing <a
      * href="https://docs.softwareheritage.org/devel/swh-graph/api.html#terminology">allowed edges</a>
      */
     public Traversal(Graph graph, String direction, String edgesFmt) {
         if (!direction.matches("forward|backward")) {
             throw new IllegalArgumentException("Unknown traversal direction: " + direction);
         }
 
         this.graph = graph;
         this.useTransposed = (direction.equals("backward"));
         this.edges = new AllowedEdges(graph, edgesFmt);
 
         long nbNodes = graph.getNbNodes();
         this.visited = new HashSet<>();
         this.parentNode = new HashMap<>();
         this.nbEdgesAccessed = 0;
         this.rng = new Random();
     }
 
     /**
      * Returns number of accessed edges during traversal.
      *
      * @return number of edges accessed in last traversal
      */
     public long getNbEdgesAccessed() {
         return nbEdgesAccessed;
     }
 
     /**
      * Returns number of accessed nodes during traversal.
      *
      * @return number of nodes accessed in last traversal
      */
     public long getNbNodesAccessed() {
         return this.visited.size();
     }
 
     /**
      * Push version of {@link leaves}: will fire passed callback for each leaf.
      */
     public void leavesVisitor(long srcNodeId, NodeIdConsumer cb) {
         Stack<Long> stack = new Stack<Long>();
         this.nbEdgesAccessed = 0;
 
         stack.push(srcNodeId);
         visited.add(srcNodeId);
 
         while (!stack.isEmpty()) {
             long currentNodeId = stack.pop();
 
             long neighborsCnt = 0;
             nbEdgesAccessed += graph.degree(currentNodeId, useTransposed);
             for (long neighborNodeId : new Neighbors(graph, useTransposed, edges, currentNodeId)) {
                 neighborsCnt++;
                 if (!visited.contains(neighborNodeId)) {
                     stack.push(neighborNodeId);
                     visited.add(neighborNodeId);
                 }
             }
 
             if (neighborsCnt == 0) {
                 cb.accept(currentNodeId);
             }
         }
     }
 
     /**
      * Returns the leaves of a subgraph rooted at the specified source node.
      *
      * @param srcNodeId source node
      * @return list of node ids corresponding to the leaves
      */
     public ArrayList<Long> leaves(long srcNodeId) {
         ArrayList<Long> nodeIds = new ArrayList<Long>();
         leavesVisitor(srcNodeId, (nodeId) -> nodeIds.add(nodeId));
         return nodeIds;
     }
 
     /**
      * Push version of {@link neighbors}: will fire passed callback on each
      * neighbor.
      */
     public void neighborsVisitor(long srcNodeId, NodeIdConsumer cb) {
         this.nbEdgesAccessed = graph.degree(srcNodeId, useTransposed);
         for (long neighborNodeId : new Neighbors(graph, useTransposed, edges, srcNodeId)) {
             cb.accept(neighborNodeId);
         }
     }
 
     /**
      * Returns node direct neighbors (linked with exactly one edge).
      *
      * @param srcNodeId source node
      * @return list of node ids corresponding to the neighbors
      */
     public ArrayList<Long> neighbors(long srcNodeId) {
         ArrayList<Long> nodeIds = new ArrayList<Long>();
         neighborsVisitor(srcNodeId, (nodeId) -> nodeIds.add(nodeId));
         return nodeIds;
     }
 
     /**
      * Push version of {@link visitNodes}: will fire passed callback on each
      * visited node.
      */
     public void visitNodesVisitor(long srcNodeId, NodeIdConsumer cb) {
         Stack<Long> stack = new Stack<Long>();
         this.nbEdgesAccessed = 0;
 
         stack.push(srcNodeId);
         visited.add(srcNodeId);
 
         while (!stack.isEmpty()) {
             long currentNodeId = stack.pop();
             cb.accept(currentNodeId);
 
             nbEdgesAccessed += graph.degree(currentNodeId, useTransposed);
             for (long neighborNodeId : new Neighbors(graph, useTransposed, edges, currentNodeId)) {
                 if (!visited.contains(neighborNodeId)) {
                     stack.push(neighborNodeId);
                     visited.add(neighborNodeId);
                 }
             }
         }
     }
 
     /**
      * Performs a graph traversal and returns explored nodes.
      *
      * @param srcNodeId source node
      * @return list of explored node ids
      */
     public ArrayList<Long> visitNodes(long srcNodeId) {
         ArrayList<Long> nodeIds = new ArrayList<Long>();
         visitNodesVisitor(srcNodeId, (nodeId) -> nodeIds.add(nodeId));
         return nodeIds;
     }
 
     /**
      * Push version of {@link visitPaths}: will fire passed callback on each
      * discovered (complete) path.
      */
     public void visitPathsVisitor(long srcNodeId, PathConsumer cb) {
         Stack<Long> currentPath = new Stack<Long>();
         this.nbEdgesAccessed = 0;
         visitPathsInternalVisitor(srcNodeId, currentPath, cb);
     }
 
     /**
      * Performs a graph traversal and returns explored paths.
      *
      * @param srcNodeId source node
      * @return list of explored paths (represented as a list of node ids)
      */
     public ArrayList<ArrayList<Long>> visitPaths(long srcNodeId) {
         ArrayList<ArrayList<Long>> paths = new ArrayList<>();
         visitPathsVisitor(srcNodeId, (path) -> paths.add(path));
         return paths;
     }
 
     private void visitPathsInternalVisitor(long currentNodeId,
                                            Stack<Long> currentPath,
                                            PathConsumer cb) {
         currentPath.push(currentNodeId);
 
         long visitedNeighbors = 0;
         nbEdgesAccessed += graph.degree(currentNodeId, useTransposed);
         for (long neighborNodeId : new Neighbors(graph, useTransposed, edges, currentNodeId)) {
             visitPathsInternalVisitor(neighborNodeId, currentPath, cb);
             visitedNeighbors++;
         }
 
         if (visitedNeighbors == 0) {
             ArrayList<Long> path = new ArrayList<Long>();
             for (long nodeId : currentPath) {
                 path.add(nodeId);
             }
             cb.accept(path);
         }
 
         currentPath.pop();
     }
 
     /**
      * Performs a graph traversal with backtracking, and returns the first
      * found path from source to destination.
      *
      * @param srcNodeId source node
      * @param dst destination (either a node or a node type)
      * @return found path as a list of node ids
      */
     public <T> ArrayList<Long> walk(long srcNodeId, T dst, String visitOrder) {
         long dstNodeId = -1;
         if (visitOrder.equals("dfs")) {
             dstNodeId = walkInternalDFS(srcNodeId, dst);
         } else if (visitOrder.equals("bfs")) {
             dstNodeId = walkInternalBFS(srcNodeId, dst);
         } else {
             throw new IllegalArgumentException("Unknown visit order: " + visitOrder);
         }
 
         if (dstNodeId == -1) {
             throw new IllegalArgumentException("Cannot find destination: " + dst);
         }
 
         ArrayList<Long> nodeIds = backtracking(srcNodeId, dstNodeId);
         return nodeIds;
     }
 
     /**
      * Performs a random walk (picking a random successor at each step) from
      * source to destination.
      *
      * @param srcNodeId source node
      * @param dst destination (either a node or a node type)
      * @return found path as a list of node ids or an empty path to indicate
      * that no suitable path have been found
      */
     public <T> ArrayList<Long> randomWalk(long srcNodeId, T dst) {
         return randomWalk(srcNodeId, dst, 0);
     }
 
     /**
      * Performs a stubborn random walk (picking a random successor at each
      * step) from source to destination. The walk is "stubborn" in the sense
      * that it will not give up the first time if a satisfying target node is
      * found, but it will retry up to a limited amount of times.
      *
      * @param srcNodeId source node
      * @param dst destination (either a node or a node type)
      * @param retries number of times to retry; 0 means no retries (single walk)
      * @return found path as a list of node ids or an empty path to indicate
      * that no suitable path have been found
      */
     public <T> ArrayList<Long> randomWalk(long srcNodeId, T dst, int retries) {
         long curNodeId = srcNodeId;
         ArrayList<Long> path = new ArrayList<Long>();
         this.nbEdgesAccessed = 0;
         boolean found;
 
         if (retries < 0) {
             throw new IllegalArgumentException("Negative number of retries given: " + retries);
         }
 
         while (true) {
             path.add(curNodeId);
-            long nbNeighbors = graph.degree(curNodeId, useTransposed);
-            if (nbNeighbors == 0) {
+            Neighbors neighbors = new Neighbors(graph, useTransposed, edges, curNodeId);
+            curNodeId = randomPick(neighbors.iterator());
+            if (curNodeId < 0) {
                 found = false;
                 break;
             }
-            Neighbors neighbors = new Neighbors(graph, useTransposed, edges, curNodeId);
-            Iterator<Long> successors = neighbors.iterator();
-
-            curNodeId = randomPick(successors, nbNeighbors);
             if (isDstNode(curNodeId, dst)) {
                 path.add(curNodeId);
                 found = true;
                 break;
             }
         }
 
         if (found) {
             return path;
         } else if (retries > 0) {  // try again
             return randomWalk(srcNodeId, dst, retries - 1);
         } else {  // not found and no retries left
             path.clear();
             return path;
         }
     }
 
     /**
-     * Randomly choose an element from an iterator
+     * Randomly choose an element from an iterator over Longs using reservoir
+     * sampling
      *
      * @param elements iterator over selection domain
-     * @param lenght total length of elements iterated upon
-     * @return randomly chosen element
+     * @return randomly chosen element or -1 if no suitable element was found
      */
-    private <T> T randomPick(Iterator<T> elements, long length) {
-        long elementsToSkip = Math.round(rng.nextFloat() * (length - 1));
-        long skippedElements = -1;
-        T e;
+    private long randomPick(Iterator<Long> elements) {
+        long curPick = -1;
+        long seenCandidates = 0;
 
         while (elements.hasNext()) {
-            e = elements.next();
-            skippedElements++;
-            this.nbEdgesAccessed++;
-            if (skippedElements < elementsToSkip) {
-                continue;
-            } else {
-                return e;
+            seenCandidates++;
+            if (Math.round(rng.nextFloat() * (seenCandidates - 1)) == 0) {
+                curPick = elements.next();
             }
         }
 
-        throw new IllegalStateException("Skipped past all available elements");
+        return curPick;
     }
 
     /**
      * Internal DFS function of {@link #walk}.
      *
      * @param srcNodeId source node
      * @param dst destination (either a node or a node type)
      * @return final destination node or -1 if no path found
      */
     private <T> long walkInternalDFS(long srcNodeId, T dst) {
         Stack<Long> stack = new Stack<Long>();
         this.nbEdgesAccessed = 0;
 
         stack.push(srcNodeId);
         visited.add(srcNodeId);
 
         while (!stack.isEmpty()) {
             long currentNodeId = stack.pop();
             if (isDstNode(currentNodeId, dst)) {
                 return currentNodeId;
             }
 
             nbEdgesAccessed += graph.degree(currentNodeId, useTransposed);
             for (long neighborNodeId : new Neighbors(graph, useTransposed, edges, currentNodeId)) {
                 if (!visited.contains(neighborNodeId)) {
                     stack.push(neighborNodeId);
                     visited.add(neighborNodeId);
                     parentNode.put(neighborNodeId, currentNodeId);
                 }
             }
         }
 
         return -1;
     }
 
     /**
      * Internal BFS function of {@link #walk}.
      *
      * @param srcNodeId source node
      * @param dst destination (either a node or a node type)
      * @return final destination node or -1 if no path found
      */
     private <T> long walkInternalBFS(long srcNodeId, T dst) {
         Queue<Long> queue = new LinkedList<Long>();
         this.nbEdgesAccessed = 0;
 
         queue.add(srcNodeId);
         visited.add(srcNodeId);
 
         while (!queue.isEmpty()) {
             long currentNodeId = queue.poll();
             if (isDstNode(currentNodeId, dst)) {
                 return currentNodeId;
             }
 
             nbEdgesAccessed += graph.degree(currentNodeId, useTransposed);
             for (long neighborNodeId : new Neighbors(graph, useTransposed, edges, currentNodeId)) {
                 if (!visited.contains(neighborNodeId)) {
                     queue.add(neighborNodeId);
                     visited.add(neighborNodeId);
                     parentNode.put(neighborNodeId, currentNodeId);
                 }
             }
         }
 
         return -1;
     }
 
     /**
      * Internal function of {@link #walk} to check if a node corresponds to the destination.
      *
      * @param nodeId current node
      * @param dst destination (either a node or a node type)
      * @return true if the node is a destination, or false otherwise
      */
     private <T> boolean isDstNode(long nodeId, T dst) {
         if (dst instanceof Long) {
             long dstNodeId = (Long) dst;
             return nodeId == dstNodeId;
         } else if (dst instanceof Node.Type) {
             Node.Type dstType = (Node.Type) dst;
             return graph.getNodeType(nodeId) == dstType;
         } else {
             return false;
         }
     }
 
     /**
      * Internal backtracking function of {@link #walk}.
      *
      * @param srcNodeId source node
      * @param dstNodeId destination node
      * @return the found path, as a list of node ids
      */
     private ArrayList<Long> backtracking(long srcNodeId, long dstNodeId) {
         ArrayList<Long> path = new ArrayList<Long>();
         long currentNodeId = dstNodeId;
         while (currentNodeId != srcNodeId) {
             path.add(currentNodeId);
             currentNodeId = parentNode.get(currentNodeId);
         }
         path.add(srcNodeId);
         Collections.reverse(path);
         return path;
     }
 }