Implement iterative graph compression

src/tam/network.js

@@ -340,7 +340,7 @@ different sequence of nodes in two or more lines.`);
  * Create a graph for navigating between stops.
  * @param {Array.<Object>} elementsList List of nodes retrieved from OSM.
  * @param {Object.<string,Object>} elementsById OSM nodes indexed by their ID.
- * @return {Object.<string,Object.<string,NavigationEdge>} Resulting graph.
+ * @return {Object} Resulting graph and reverse arcs.
  */
 const createNavigationGraph = (elementsList, elementsById) => {
     const navigation = {};
@@ -350,7 +350,7 @@ const createNavigationGraph = (elementsList, elementsById) => {
     for (const obj of elementsList) {
         if (obj.type === "node") {
             navigation[obj.id] = {};
-            navigationReverse[obj.id] = {};
+            navigationReverse[obj.id] = new Set();
         }
     }
 
@@ -360,27 +360,27 @@ const createNavigationGraph = (elementsList, elementsById) => {
             const oneWay = osm.isOneWay(obj);
 
             for (let i = 0; i + 1 < obj.nodes.length; ++i) {
-                const from = obj.nodes[i];
+                const from = obj.nodes[i].toString();
-                let to = obj.nodes[i + 1];
+                let to = obj.nodes[i + 1].toString();
-                let path = [from.toString(), to.toString()];
+                let path = [from, to];
 
                 // Make sure we can’t jump between rails at railway crossings
                 if (i + 2 < obj.nodes.length
                         && osm.isRailwayCrossing(elementsById[to])) {
-                    const next = obj.nodes[i + 2];
+                    const next = obj.nodes[i + 2].toString();
-                    path = [from.toString(), to.toString(), next.toString()];
+                    path = [from, to, next];
                     to = next;
                     i += 1;
                 }
 
                 navigation[from][to] = path;
-                navigationReverse[to][from] = true;
+                navigationReverse[to].add(from);
 
                 if (!oneWay) {
                     const reversePath = [...path];
                     reversePath.reverse();
                     navigation[to][from] = reversePath;
-                    navigationReverse[from][to] = true;
+                    navigationReverse[from].add(to);
                 }
             }
         }
@@ -390,29 +390,22 @@ const createNavigationGraph = (elementsList, elementsById) => {
 };
 
 /**
- * Remove and relink nodes that connect only two nodes or less.
+ * Identify intermediate nodes of the navigation graph that can be simplified.
- * @param {Object.<string,Stop>} stops List of stops.
+ * @param {Set.<string>} stopsSet OSM IDs of stop nodes.
  * @param {Navigation} navigation Input navigation graph.
- * @param {Object.<string,Object.<string,boolean>>} navigationReverse
+ * @param {Object.<string,Set.<string>>} navigationReverse Reverse arcs.
- * Backward edges of the navigation graph.
+ * @return {Set.<string>} Set of compressible nodes.
  */
-const compressNavigationGraph = (stops, navigation, navigationReverse) => {
+const findCompressibleNodes = (stopsSet, navigation, navigationReverse) => {
-    const stopsReverse = Object.fromEntries(
+    const compressible = new Set();
-        Object.entries(stops).map(([id, stop]) => [stop.properties.node, id])
-    );
 
-    let removedDeadEnds = true;
-    const nodesToCompress = {};
-
-    while (removedDeadEnds) {
-        // Identify nodes to be compressed
     for (const nodeId in navigation) {
-            if (nodeId in stopsReverse) {
+        if (stopsSet.has(nodeId)) {
             // Keep stop nodes
             continue;
         }
 
-            const entries = new Set(Object.keys(navigationReverse[nodeId]));
+        const entries = navigationReverse[nodeId];
         const exits = new Set(Object.keys(navigation[nodeId]));
 
         // Keep split nodes, i.e. nodes with at least two exit nodes
@@ -462,26 +455,34 @@ const compressNavigationGraph = (stops, navigation, navigationReverse) => {
         }
 
         // Compress all other nodes
-            nodesToCompress[nodeId] = true;
+        compressible.add(nodeId);
     }
 
-        // Find nodes that cannot be used to directly link up two kept nodes
+    return compressible;
-        const usedNodes = {};
+};
 
+/**
+ * Remove nodes that are not used to link up two kept nodes.
+ * @param {Navigation} navigation Input navigation graph.
+ * @param {Object.<string,Set.<string>>} navigationReverse Reverse arcs.
+ * @param {Set.<string>} compressible Set of nodes that will not be kept.
+ * @return {boolean} True if some dead-ends were removed.
+ */
+const removeDeadEnds = (navigation, navigationReverse, compressible) => {
+    let didRemove = false;
+
+    // Find dead-ends starting from kept nodes
     for (const beginId in navigation) {
-            if (beginId in nodesToCompress) {
+        if (compressible.has(beginId)) {
             continue;
         }
 
-            usedNodes[beginId] = true;
-
-            // Start a DFS from each node to be kept
         const begin = navigation[beginId];
         const stack = [];
         const parent = {[beginId]: beginId};
 
         for (const succId in begin) {
-                if (succId in nodesToCompress) {
+            if (compressible.has(succId)) {
                 stack.push(succId);
                 parent[succId] = beginId;
             }
@@ -491,57 +492,103 @@ const compressNavigationGraph = (stops, navigation, navigationReverse) => {
             const endId = stack.pop();
             const end = navigation[endId];
 
-                if (!(endId in nodesToCompress)) {
+            if (compressible.has(endId)) {
-                    let trackback = parent[endId];
+                let hasSuccessor = false;
+
+                for (const succId in end) {
+                    if (succId !== parent[endId]) {
+                        parent[succId] = endId;
+                        stack.push(succId);
+                        hasSuccessor = true;
+                    }
+                }
+
+                if (!hasSuccessor) {
+                    // Remove the dead-end path
+                    let trackback = endId;
 
                     while (trackback !== beginId) {
-                        usedNodes[trackback] = true;
+                        navigationReverse[trackback].delete(parent[trackback]);
+                        delete navigation[parent[trackback]][trackback];
                         trackback = parent[trackback];
                     }
-                } else {
+
+                    didRemove = true;
+                }
+            }
+        }
+    }
+
+    // Find dead-ends starting from compressible source nodes
+    for (const beginId in navigation) {
+        if (!compressible.has(beginId)) {
+            continue;
+        }
+
+        if (navigationReverse[beginId].size > 0) {
+            continue;
+        }
+
+        const begin = navigation[beginId];
+        const stack = [];
+        const parent = {[beginId]: beginId};
+
+        for (const succId in begin) {
+            stack.push(succId);
+            parent[succId] = beginId;
+        }
+
+        while (stack.length > 0) {
+            const endId = stack.pop();
+            const end = navigation[endId];
+
+            if (compressible.has(endId)) {
                 for (const succId in end) {
                     if (succId !== parent[endId]) {
                         parent[succId] = endId;
                         stack.push(succId);
                     }
                 }
+            } else {
+                // Remove the dead-end path
+                let trackback = endId;
+
+                while (trackback !== beginId) {
+                    navigationReverse[trackback].delete(parent[trackback]);
+                    delete navigation[parent[trackback]][trackback];
+                    trackback = parent[trackback];
+                }
+
+                didRemove = true;
             }
         }
     }
 
-        // Remove dead-end nodes
+    return didRemove;
-        removedDeadEnds = false;
+};
+
+/**
+ * Compress the given set of nodes.
+ * @param {Navigation} navigation Input navigation graph.
+ * @param {Object.<string,Set.<string>>} navigationReverse Reverse arcs.
+ * @param {Set.<string>} compressible Set of nodes to compress.
+ * @return {boolean} True if some nodes were compressed.
+ */
+const removeCompressibleNodes = (navigation, navigationReverse, compressible) => {
+    let didCompress = false;
 
     for (const beginId in navigation) {
-            if (!(beginId in usedNodes)) {
+        if (compressible.has(beginId)) {
-                for (const neighborId in navigation[beginId]) {
-                    delete navigationReverse[neighborId][beginId];
-                }
-
-                for (const neighborId in navigationReverse[beginId]) {
-                    delete navigation[neighborId][beginId];
-                }
-
-                delete navigation[beginId];
-                delete navigationReverse[beginId];
-                removedDeadEnds = true;
-            }
-        }
-    }
-
-    // Perform node compression
-    for (const beginId in navigation) {
-        if (beginId in nodesToCompress) {
             continue;
         }
 
-        // Start a DFS from each node to be kept
+        // Start a DFS from each kept node
         const begin = navigation[beginId];
         const stack = [];
         const parent = {[beginId]: beginId};
 
         for (const succId in begin) {
-            if (succId in nodesToCompress) {
+            if (compressible.has(succId)) {
                 stack.push(succId);
                 parent[succId] = beginId;
             }
@@ -551,44 +598,35 @@ const compressNavigationGraph = (stops, navigation, navigationReverse) => {
             const endId = stack.pop();
             const end = navigation[endId];
 
-            if (!(endId in nodesToCompress)) {
+            if (!compressible.has(endId)) {
                 // Found another kept node
-                // Collect and remove intermediate nodes
+                // Collect and remove intermediate path
-                const reversePath = [endId];
+                let path = [];
-                let trackback = parent[endId];
+                let trackback = endId;
-                let oneWay = !(trackback in end);
 
-                while (trackback !== beginId) {
+                do {
-                    reversePath.push(trackback);
+                    const segment = [...navigation[parent[trackback]][trackback]];
-                    oneWay = oneWay || !(parent[trackback] in navigation[trackback]);
+                    segment.reverse();
+                    path = path.concat(segment.slice(0, -1));
+
+                    navigationReverse[trackback].delete(parent[trackback]);
+                    delete navigation[parent[trackback]][trackback];
 
-                    delete navigation[trackback];
-                    delete navigationReverse[trackback];
                     trackback = parent[trackback];
+                } while (trackback !== beginId);
+
+                // Make sure not to add loops if we’re compressing a cycle
+                if (endId !== beginId) {
+                    path.push(beginId);
+                    path.reverse();
+
+                    begin[endId] = path;
+                    navigationReverse[endId].add(beginId);
                 }
 
-                reversePath.push(beginId);
+                didCompress = true;
-                const forwardPath = [...reversePath];
-                forwardPath.reverse();
-
-                // Create edges to link both nodes directly
-                delete begin[forwardPath[1]];
-                delete navigationReverse[endId][reversePath[1]];
-
-                delete end[reversePath[1]];
-                delete navigationReverse[beginId][forwardPath[1]];
-
-                if (!(endId in begin)) {
-                    begin[endId] = forwardPath;
-                    navigationReverse[endId][beginId] = true;
-                }
-
-                if (!oneWay && !(beginId in end)) {
-                    end[beginId] = reversePath;
-                    navigationReverse[beginId][endId] = true;
-                }
             } else {
-                // Continue the traversal down unused nodes
+                // Continue the traversal down compressible nodes
                 let isFirst = true;
 
                 for (const succId in end) {
@@ -606,6 +644,57 @@ non-junction node ${endId}`);
             }
         }
     }
+
+    return didCompress;
+};
+
+/**
+ * Find nodes in the graph that have no exits nor entries and remove them.
+ * @param {Navigation} navigation Input navigation graph.
+ * @param {Object.<string,Set.<string>>} navigationReverse Reverse arcs.
+ */
+const cleanUpIsolatedNodes = (navigation, navigationReverse) => {
+    for (const nodeId in navigation) {
+        if (
+            Object.keys(navigation[nodeId]).length === 0
+            && navigationReverse[nodeId].size === 0
+        ) {
+            delete navigation[nodeId];
+            delete navigationReverse[nodeId];
+        }
+    }
+};
+
+/**
+ * Remove and relink nodes that connect only two nodes or less.
+ * @param {Object.<string,Stop>} stops List of stops.
+ * @param {Navigation} navigation Input navigation graph.
+ * @param {Object.<string,Set.<string>>} navigationReverse Reverse arcs.
+ */
+const compressNavigationGraph = (stops, navigation, navigationReverse) => {
+    const stopsSet = new Set(
+        Object.values(stops).map(stop => stop.properties.node)
+    );
+    let compressible = null;
+    let didCompress = true;
+
+    while (didCompress) {
+        let didRemove = true;
+
+        while (didRemove) {
+            compressible = findCompressibleNodes(
+                stopsSet, navigation, navigationReverse
+            );
+            didRemove = removeDeadEnds(
+                navigation, navigationReverse, compressible
+            );
+        }
+
+        didCompress = removeCompressibleNodes(
+            navigation, navigationReverse, compressible
+        );
+        cleanUpIsolatedNodes(navigation, navigationReverse);
+    }
 };
 
 /**