Fix Strongly Connected Components Search algorithm

vadyushkins · Dec 25, 2023 · 0115996 · 0115996
1 parent 2d7db89
commit 0115996
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Showing 2 changed files with 35 additions and 9 deletions.
diff --git a/algorithms/sccs/sccs.cpp b/algorithms/sccs/sccs.cpp
@@ -25,6 +25,13 @@ std::vector<std::vector<int64_t>> strongly_connected_components_search(
         }
     }
 
+    std::vector<std::vector<int64_t>> transposed_graph(n);
+    for (int64_t i = 0; i < n; ++i) {
+        for (int64_t j : graph[i]) {
+            transposed_graph[j].push_back(i);
+        }
+    }
+
     visited.assign(n, false);
 
     std::vector<int64_t> strongly_connected_component;
@@ -34,7 +41,7 @@ std::vector<std::vector<int64_t>> strongly_connected_components_search(
         visited[node] = true;
         strongly_connected_component.push_back(node);
 
-        for (int64_t neighbour : graph[node]) {
+        for (int64_t neighbour : transposed_graph[node]) {
             if (false == visited[neighbour]) {
                 compute_strongly_connected_component(neighbour);
             }
@@ -43,8 +50,9 @@ std::vector<std::vector<int64_t>> strongly_connected_components_search(
 
     std::vector<std::vector<int64_t>> result;
     for (int64_t i = n - 1; i >= 0; --i) {
-        if (false == visited[i]) {
-            compute_strongly_connected_component(i);
+        int64_t node = condensation_order[i];
+        if (false == visited[node]) {
+            compute_strongly_connected_component(node);
             result.emplace_back(strongly_connected_component);
             strongly_connected_component.clear();
         }

diff --git a/algorithms/sccs/test_sccs.cpp b/algorithms/sccs/test_sccs.cpp
@@ -13,8 +13,8 @@ TEST(StrongleConnectedComponents, Test0) {
     std::vector<std::vector<int64_t>> strongly_connected_components = strongly_connected_components_search(graph);
 
     std::vector<std::vector<int64_t>> expected_strongly_connected_components = {
-        {3, 2},
-        {1, 0},
+        {0, 1},
+        {2, 3},
     };
 
     ASSERT_EQ(strongly_connected_components, expected_strongly_connected_components);
@@ -33,8 +33,8 @@ TEST(StrongleConnectedComponents, Test1) {
     std::vector<std::vector<int64_t>> strongly_connected_components = strongly_connected_components_search(graph);
 
     std::vector<std::vector<int64_t>> expected_strongly_connected_components = {
-        {5, 3, 4},
-        {2, 1, 0},
+        {0, 1, 2},
+        {3, 5, 4},
     };
 
     ASSERT_EQ(strongly_connected_components, expected_strongly_connected_components);
@@ -49,8 +49,8 @@ TEST(StrongleConnectedComponents, Test2) {
     std::vector<std::vector<int64_t>> strongly_connected_components = strongly_connected_components_search(graph);
 
     std::vector<std::vector<int64_t>> expected_strongly_connected_components = {
-        {1},
         {0},
+        {1},
     };
 
     ASSERT_EQ(strongly_connected_components, expected_strongly_connected_components);
@@ -68,7 +68,25 @@ TEST(StrongleConnectedComponents, Test3) {
     std::vector<std::vector<int64_t>> strongly_connected_components = strongly_connected_components_search(graph);
 
     std::vector<std::vector<int64_t>> expected_strongly_connected_components = {
-        {4, 0, 1, 2, 3},
+        {0, 4, 3, 2, 1},
+    };
+
+    ASSERT_EQ(strongly_connected_components, expected_strongly_connected_components);
+}
+
+TEST(StrongleConnectedComponents, Test4) {
+    std::vector<std::vector<int64_t>> graph = {
+        {},
+        {},
+        {},
+    };
+
+    std::vector<std::vector<int64_t>> strongly_connected_components = strongly_connected_components_search(graph);
+
+    std::vector<std::vector<int64_t>> expected_strongly_connected_components = {
+        {2},
+        {1},
+        {0},
     };
 
     ASSERT_EQ(strongly_connected_components, expected_strongly_connected_components);