From c577f2c6a3b4ddb6ba87a882723c53a248afbeba Mon Sep 17 00:00:00 2001
From: telsoa01 <telmo.soares@arm.com>
Date: Fri, 31 Aug 2018 09:22:23 +0100
Subject: Release 18.08

---
 src/armnnUtils/GraphTopologicalSort.hpp | 86 ++++++++++++++++++++++++---------
 1 file changed, 63 insertions(+), 23 deletions(-)

(limited to 'src/armnnUtils/GraphTopologicalSort.hpp')

diff --git a/src/armnnUtils/GraphTopologicalSort.hpp b/src/armnnUtils/GraphTopologicalSort.hpp
index f455289567..86eb4cc030 100644
--- a/src/armnnUtils/GraphTopologicalSort.hpp
+++ b/src/armnnUtils/GraphTopologicalSort.hpp
@@ -5,11 +5,14 @@
 #pragma once
 
 #include <boost/assert.hpp>
+#include <boost/optional.hpp>
 
 #include <functional>
 #include <map>
+#include <stack>
 #include <vector>
 
+
 namespace armnnUtils
 {
 
@@ -22,51 +25,88 @@ enum class NodeState
     Visited,
 };
 
-template<typename TNodeId>
-bool Visit(
-    TNodeId current,
-    std::function<std::vector<TNodeId>(TNodeId)> getIncomingEdges,
-    std::vector<TNodeId>& outSorted,
-    std::map<TNodeId, NodeState>& nodeStates)
+
+template <typename TNodeId>
+boost::optional<TNodeId> GetNextChild(TNodeId node,
+                                      std::function<std::vector<TNodeId>(TNodeId)> getIncomingEdges,
+                                      std::map<TNodeId, NodeState>& nodeStates)
 {
-    auto currentStateIt = nodeStates.find(current);
-    if (currentStateIt != nodeStates.end())
+    for (TNodeId childNode : getIncomingEdges(node))
     {
-        if (currentStateIt->second == NodeState::Visited)
-        {
-            return true;
-        }
-        if (currentStateIt->second == NodeState::Visiting)
+        if (nodeStates.find(childNode) == nodeStates.end())
         {
-            return false;
+            return childNode;
         }
         else
         {
-            BOOST_ASSERT(false);
+            if (nodeStates.find(childNode)->second == NodeState::Visiting)
+            {
+                return childNode;
+            }
         }
     }
 
-    nodeStates[current] = NodeState::Visiting;
+    return {};
+}
 
-    for (TNodeId inputNode : getIncomingEdges(current))
+template<typename TNodeId>
+bool TopologicallySort(
+    TNodeId initialNode,
+    std::function<std::vector<TNodeId>(TNodeId)> getIncomingEdges,
+    std::vector<TNodeId>& outSorted,
+    std::map<TNodeId, NodeState>& nodeStates)
+{
+    std::stack<TNodeId> nodeStack;
+
+    // If the node is never visited we should search it
+    if (nodeStates.find(initialNode) == nodeStates.end())
     {
-        Visit(inputNode, getIncomingEdges, outSorted, nodeStates);
+        nodeStack.push(initialNode);
     }
 
-    nodeStates[current] = NodeState::Visited;
+    while (!nodeStack.empty())
+    {
+        TNodeId current = nodeStack.top();
+
+        nodeStates[current] = NodeState::Visiting;
+
+        boost::optional<TNodeId> nextChildOfCurrent = GetNextChild(current, getIncomingEdges, nodeStates);
+
+        if (nextChildOfCurrent)
+        {
+            TNodeId nextChild = nextChildOfCurrent.get();
+
+            // If the child has not been searched, add to the stack and iterate over this node
+            if (nodeStates.find(nextChild) == nodeStates.end())
+            {
+                nodeStack.push(nextChild);
+                continue;
+            }
+
+            // If we re-encounter a node being visited there is a cycle
+            if (nodeStates[nextChild] == NodeState::Visiting)
+            {
+                return false;
+            }
+        }
+
+        nodeStack.pop();
+
+        nodeStates[current] = NodeState::Visited;
+        outSorted.push_back(current);
+    }
 
-    outSorted.push_back(current);
     return true;
 }
 
 }
 
-// Sorts an directed acyclic graph (DAG) into a flat list such that all inputs to a node are before the node itself.
+// Sorts a directed acyclic graph (DAG) into a flat list such that all inputs to a node are before the node itself.
 // Returns true if successful or false if there is an error in the graph structure (e.g. it contains a cycle).
 // The graph is defined entirely by the "getIncomingEdges" function which the user provides. For a given node,
 // it must return the list of nodes which are required to come before it.
 // "targetNodes" is the list of nodes where the search begins - i.e. the nodes that you want to evaluate.
-// The implementation is based on https://en.wikipedia.org/wiki/Topological_sorting#Depth-first_search
+// This is an iterative implementation based on https://en.wikipedia.org/wiki/Topological_sorting#Depth-first_search
 template<typename TNodeId, typename TTargetNodes>
 bool GraphTopologicalSort(
     const TTargetNodes& targetNodes,
@@ -78,7 +118,7 @@ bool GraphTopologicalSort(
 
     for (TNodeId targetNode : targetNodes)
     {
-        if (!Visit(targetNode, getIncomingEdges, outSorted, nodeStates))
+        if (!TopologicallySort(targetNode, getIncomingEdges, outSorted, nodeStates))
         {
             return false;
         }
-- 
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