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src/graph/executor/algo/SubgraphExecutor.h

@@ -9,15 +9,32 @@
 #include "graph/executor/Executor.h"
 
 // Subgraph receive result from GetNeightbors
-//
-//
-//
+// There are two Main functions
+// First : Extract the deduplicated destination VID from GetNeighbors
+// Second: Delete previously visited edges and save the result(iter) to the variable `resultVar`
 //
 // Member:
-// historyVids_ : is hash table
-// KEY   : the ID of the visited destination Vertex
-// VALUE : the number of steps to visit the KEY (starting vertex is 0)
+// `historyVids_` : is hash table
+//    KEY   : the VID of the visited destination Vertex
+//    VALUE : the number of steps to visit the KEY (starting vertex is 0)
+// since each vertex will only be visited once, if it is a one-way edge expansion, there will be no
+// duplicate edges. we only need to focus on the case of two-way expansion
+//
+// How to delete edges:
+//  determine whether a loop is formed by the number of steps. If the destination vid has been
+//  visited, and the number of steps of the destination vid differs by 2 from the current steps, it
+//  is judged that a loop is formed, the edge needs to be deleted
 //
+// For example: Topology is below
+// a->c, a->b, b->a, b->c
+// statamate: get subgraph from 'a' both edge yield vertices as nodes, edges as relationships
+// first steps :  a->b, a->c, a<-b, all edges need to save
+// second steps:  b->a, b<-a, b->c, c<-a
+// since it is a two-way expansion, the negative edge has already been visited,
+// so b<-a & c<-a are deleted
+// b->a : the number of steps of the destination vid `a` is 0, and the current steps is 2. it can be
+// determined that a loop is formed, so this edge also needs to be deleted.
+// b->c : determined by the number of steps that no loop is formed, so keep it
 
 namespace nebula {
 namespace graph {