GLE-10323 fix(algo): fix tg_maxflow algorithm (#179)

Author: DavidFan2000

algorithms/Path/maxflow/tg_maxflow.gsql

@@ -3,8 +3,8 @@ CREATE QUERY tg_maxflow(VERTEX source, VERTEX sink, Set<STRING> v_type, SET<STRI
    BOOL print_results = TRUE, BOOL display_edges = TRUE, STRING file_path = "")  SYNTAX V1 { 
 
     /*
-    First Author: <First Author Name>
-    First Commit Date:  <First Commit Date>
+    First Author: David Zelong Fan
+    First Commit Date:  Jun 6, 2025
 
     Recent Author: <Recent Commit Author Name>
     Recent Commit Date: <Recent Commit Date>
@@ -50,178 +50,125 @@ CREATE QUERY tg_maxflow(VERTEX source, VERTEX sink, Set<STRING> v_type, SET<STRI
             file to write CSV output to
     */
 
-  TYPEDEF TUPLE<INT prev_flow, BOOL is_forward, VERTEX prev> tb_node;
-  GroupByAccum<VERTEX source, VERTEX targ, SumAccum<FLOAT> flow> @@group_by_flow_accum;
-  SetAccum<VERTEX> @@curr_set;
+  // variables used in max flow computation
+  GroupByAccum<VERTEX v_from, VERTEX v_to, SumAccum<INT> flow> @@flow_gb;
+  GroupByAccum<VERTEX v_from, VERTEX v_to, SumAccum<INT> flow> @@residual_gb;
+  MinAccum<INT> @@path_flow;
+
+  // variables used in bfs
+  ListAccum<VERTEX> @path_list;
+  OrAccum @end_point;
+  ListAccum<ListAccum<VERTEX>> @@total_path_list;
+  ListAccum<VERTEX> @@shortest_path;
+
+  // variables used for printing
+  FILE f(file_path);
   SetAccum<EDGE> @@edges_set;
-  HeapAccum<tb_node>(1, prev_flow DESC) @trace_back_heap;
-                              
-  MaxAccum<FLOAT> @@max_cap_threshold;
-  SumAccum<FLOAT> @@sum_max_flow = 0;
-  MinAccum<FLOAT> @@min_flow;
-  OrAccum @or_is_visited, @@or_is_found;
-  BOOL minimum_reached = FALSE;
-	FILE f(file_path);
-  @@max_cap_threshold = min_flow_threshhold;
-         
-  IF cap_type NOT IN ("UINT", "INT", "FLOAT", "DOUBLE") THEN
-      PRINT "weight_type must be UINT, INT, FLOAT, or DOUBLE" AS errMsg;
-      RETURN;
-  END;    
-  
-  ##### Initialize #####
-  init = {v_type};       
-  init = SELECT s
-         FROM init:s - (e_type_set:e) - v_type:t
-         ACCUM 
-             FLOAT fl = 0,
-             CASE cap_type 
-	         WHEN "UINT" THEN
-	             fl = e.getAttr(cap_attr, "UINT")
-                 WHEN "INT" THEN
-		     fl = e.getAttr(cap_attr, "INT")
-                 WHEN "FLOAT" THEN
-                     fl = e.getAttr(cap_attr, "FLOAT")
-                 WHEN "DOUBLE" THEN
-                     fl = e.getAttr(cap_attr, "DOUBLE")
-                 END,
-             @@group_by_flow_accum += (s, t -> 0),
-             IF s == source THEN 
-	         @@max_cap_threshold += fl 
-	     END;
-
-  //used for determining minimum flow of path, s.t. minimum flow > cap_threshold
-  @@max_cap_threshold = pow(3, float_to_int(log(@@max_cap_threshold)/log(3)));
-  
-  ##### Push one flow at a time until there is residudal graph is disconnected #####
+  SumAccum<INT> @@sum_max_flow = 0;
+
+  // initialize flow and residual graph
+  all_vertices = {v_type};
+  all_vertices = SELECT s
+         FROM all_vertices:s - (e_type_set:e) - v_type:t
+         ACCUM
+            FLOAT cap = 0,
+            CASE cap_type 
+              WHEN "UINT" THEN
+                cap = e.getAttr(cap_attr, "UINT")
+              WHEN "INT" THEN
+                cap = e.getAttr(cap_attr, "INT")
+              WHEN "FLOAT" THEN
+                cap = e.getAttr(cap_attr, "FLOAT")
+              WHEN "DOUBLE" THEN
+                cap = e.getAttr(cap_attr, "DOUBLE")
+            END,
+            @@flow_gb += (s, t -> 0),
+            @@residual_gb += (s, t -> cap)
+        ;
+
+  // mark the target node as true
+  endset = {sink};
+  endset = SELECT s
+          From endset:s
+          ACCUM s.@end_point = true;
+
   WHILE TRUE DO
-      //initilize 
-      
-      init = SELECT s
-             FROM init:s
-             POST-ACCUM s.@or_is_visited = FALSE,
-                        s.@trace_back_heap = tb_node(GSQL_INT_MIN, FALSE, source);
-    
-      start = {source};
-      start = SELECT s 
-              FROM start:s 
-              POST-ACCUM s.@or_is_visited = TRUE;
-  
-      @@or_is_found = False;
-    
-      //BFS to find feasible path from source -> sink
-      WHILE NOT @@or_is_found AND start.size() > 0 DO
-          forwd = SELECT t
-                  FROM start:s - (e_type_set:e) - v_type:t
-                  WHERE NOT t.@or_is_visited
-                  ACCUM 
-                      FLOAT fl = 0,
-                      CASE cap_type
-                          WHEN "UINT" THEN
-			      fl = e.getAttr(cap_attr, "UINT")
-                          WHEN "INT" THEN
-			      fl = e.getAttr(cap_attr, "INT")
-                          WHEN "FLOAT" THEN
-                              fl = e.getAttr(cap_attr, "FLOAT")
-                          WHEN "DOUBLE" THEN
-                              fl = e.getAttr(cap_attr, "DOUBLE")
-                      END, 
-                      IF fl - @@group_by_flow_accum.get(s, t).flow >= @@max_cap_threshold THEN
-                          t.@trace_back_heap += tb_node(fl - @@group_by_flow_accum.get(s, t).flow, TRUE, s),
-                          t.@or_is_visited += TRUE,
-                          @@or_is_found += t == sink
-                      END
-                  HAVING t.@or_is_visited;
-  
-          bacwd = SELECT t
-                  FROM start:s - (reverse_e_type_set) - v_type:t
-                  WHERE NOT t.@or_is_visited
-                  ACCUM 
-                      IF @@group_by_flow_accum.get(t, s).flow >= @@max_cap_threshold THEN
-                          t.@trace_back_heap += tb_node(@@group_by_flow_accum.get(t, s).flow, FALSE, s),
-                          t.@or_is_visited += TRUE,
-                          @@or_is_found += t == sink
-                      END
-                  HAVING t.@or_is_visited;
-  
-          start = forwd UNION bacwd;
-      END;
-    
-      //done when residual graph is disconnected                           
-      IF NOT @@or_is_found AND minimum_reached THEN
-          BREAK;
-      END;  
-    
-      //reduce cap_threshold to look for more path options
-      IF NOT @@or_is_found THEN
-          @@max_cap_threshold = float_to_int(@@max_cap_threshold/3);
-          IF @@max_cap_threshold < min_flow_threshhold THEN 
-              @@max_cap_threshold = min_flow_threshhold; 
-              minimum_reached = TRUE; 
-          END;
-                               
-          CONTINUE;
-      END;
-    
-      //find bottleneck 
-      @@curr_set.clear();
-      @@curr_set += sink;
-      @@min_flow = GSQL_INT_MAX;
+    // Run BFS, starting from the initial node
+    SourceSet = {source};
+    SourceSet = SELECT s 
+            FROM SourceSet:s
+            ACCUM s.@path_list = [s];
 
-      WHILE NOT @@curr_set.contains(source) DO
-          start = @@curr_set;
-          @@curr_set.clear();
-          start = SELECT s 
-                  FROM start:s
-                  POST-ACCUM @@min_flow += s.@trace_back_heap.top().prev_flow,
-                             @@curr_set += s.@trace_back_heap.top().prev;
-    
-      END;
-  
-      @@sum_max_flow += @@min_flow;
-    
-      //traceback to source and update flow vertices
-      @@curr_set.clear();
-      @@curr_set += sink;
-      WHILE NOT @@curr_set.contains(source) DO
-          start = @@curr_set;
-          @@curr_set.clear();
-          start = SELECT s 
-                  FROM start:s
-                  POST-ACCUM 
-		      @@curr_set += s.@trace_back_heap.top().prev, 
-                      CASE 
-                          WHEN s.@trace_back_heap.top().is_forward THEN
-                              @@group_by_flow_accum += (s.@trace_back_heap.top().prev, s -> @@min_flow)
-                          ELSE 
-			      @@group_by_flow_accum += (s, s.@trace_back_heap.top().prev -> -@@min_flow)
-                          END;
-      END;
-  END;
+    WHILE SourceSet.size() > 0 and @@total_path_list.size() == 0 DO 
+        SourceSet = SELECT t 
+                    FROM SourceSet:s -((reverse_e_type_set|e_type_set):e)- :t
+                    WHERE @@residual_gb.get(s, t).flow > min_flow_threshhold AND t.@path_list.size() == 0
+                    ACCUM
+                    // choose any path for tie-breaking
+                    IF s.@path_list.size() > 0 THEN
+                        IF t.@end_point == true THEN
+                            @@total_path_list += s.@path_list + [t]
+                        ELSE
+                            t.@path_list = s.@path_list + [t]
+                        END
+                    END
+                ;
+    END; // end of BFS
+
+    // if no augmenting path is found, break because we've reached max flow
+    if @@total_path_list.size() == 0 THEN
+        break;
+    END;
+
+    @@shortest_path = @@total_path_list.get(0);
+
+    @@path_flow = GSQL_INT_MAX;
 
+    // see how much flow we can send
+    FOREACH i IN RANGE[0, @@shortest_path.size() - 2] DO
+      @@path_flow += @@residual_gb.get(@@shortest_path.get(i), @@shortest_path.get(i + 1)).flow;
+    END;
+
+    // send the flow and update the residual graph
+    FOREACH i IN RANGE[0, @@shortest_path.size() - 2] DO
+      @@flow_gb += (@@shortest_path.get(i), @@shortest_path.get(i + 1) -> @@path_flow);
+      @@residual_gb += (@@shortest_path.get(i + 1), @@shortest_path.get(i) -> @@path_flow);
+      @@residual_gb += (@@shortest_path.get(i), @@shortest_path.get(i + 1) -> -@@path_flow);
+    END;
+    @@sum_max_flow += @@path_flow;
+
+    // reset and clear
+    @@total_path_list.clear();
+    all_vertices = SELECT s
+            FROM all_vertices:s
+            POST-ACCUM s.@path_list.clear();
+
+  END;
+  
   ##### Output #####
   IF file_path != "" THEN
       f.println("Maxflow: " + to_string(@@sum_max_flow));
       f.println("From","To","Flow");
-  END;                            
-  start = {source};
-  WHILE start.size() != 0 DO
-      start = SELECT t
-              FROM start:s - (e_type_set:e) - v_type:t
-              WHERE @@group_by_flow_accum.get(s,t).flow >= min_flow_threshhold 
+  END;
+  SourceSet = {source};
+  WHILE SourceSet.size() != 0 DO
+      SourceSet = SELECT t
+              FROM SourceSet:s - (e_type_set:e) - v_type:t
+              WHERE @@flow_gb.get(s,t).flow > min_flow_threshhold 
               ACCUM
                   IF print_results THEN 
-		      @@edges_set += e 
-		  END,
+                    @@edges_set += e 
+                  END,
                   IF file_path != "" THEN 
-		      f.println(s, t, @@group_by_flow_accum.get(s,t).flow) 
-		  END;
+                      f.println(s, t, @@flow_gb.get(s,t).flow) 
+                  END
+              ;
   END;
-  
+
   IF print_results THEN
       PRINT @@sum_max_flow;
       IF display_edges THEN
           PRINT @@edges_set; 
       END;
   END;
-}
+}