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/* Distributed under the OSI-approved BSD 3-Clause License. See accompanying
file Copyright.txt or https://cmake.org/licensing for details. */
#include "cmComputeComponentGraph.h"
#include <algorithm>
#include <cassert>
cmComputeComponentGraph::cmComputeComponentGraph(Graph const& input)
: InputGraph(input)
{
// Identify components.
this->Tarjan();
// Compute the component graph.
this->ComponentGraph.resize(0);
this->ComponentGraph.resize(this->Components.size());
this->TransferEdges();
}
cmComputeComponentGraph::~cmComputeComponentGraph() = default;
void cmComputeComponentGraph::Tarjan()
{
int n = static_cast<int>(this->InputGraph.size());
TarjanEntry entry = { 0, 0 };
this->TarjanEntries.resize(0);
this->TarjanEntries.resize(n, entry);
this->TarjanComponents.resize(0);
this->TarjanComponents.resize(n, -1);
this->TarjanWalkId = 0;
this->TarjanVisited.resize(0);
this->TarjanVisited.resize(n, 0);
for (int i = 0; i < n; ++i) {
// Start a new DFS from this node if it has never been visited.
if (!this->TarjanVisited[i]) {
assert(this->TarjanStack.empty());
++this->TarjanWalkId;
this->TarjanIndex = 0;
this->TarjanVisit(i);
}
}
}
void cmComputeComponentGraph::TarjanVisit(int i)
{
// We are now visiting this node.
this->TarjanVisited[i] = this->TarjanWalkId;
// Initialize the entry.
this->TarjanEntries[i].Root = i;
this->TarjanComponents[i] = -1;
this->TarjanEntries[i].VisitIndex = ++this->TarjanIndex;
this->TarjanStack.push(i);
// Follow outgoing edges.
EdgeList const& nl = this->InputGraph[i];
for (cmGraphEdge const& ni : nl) {
int j = ni;
// Ignore edges to nodes that have been reached by a previous DFS
// walk. Since we did not reach the current node from that walk
// it must not belong to the same component and it has already
// been assigned to a component.
if (this->TarjanVisited[j] > 0 &&
this->TarjanVisited[j] < this->TarjanWalkId) {
continue;
}
// Visit the destination if it has not yet been visited.
if (!this->TarjanVisited[j]) {
this->TarjanVisit(j);
}
// If the destination has not yet been assigned to a component,
// check if it has a better root for the current object.
if (this->TarjanComponents[j] < 0) {
if (this->TarjanEntries[this->TarjanEntries[j].Root].VisitIndex <
this->TarjanEntries[this->TarjanEntries[i].Root].VisitIndex) {
this->TarjanEntries[i].Root = this->TarjanEntries[j].Root;
}
}
}
// Check if we have found a component.
if (this->TarjanEntries[i].Root == i) {
// Yes. Create it.
int c = static_cast<int>(this->Components.size());
this->Components.emplace_back();
NodeList& component = this->Components[c];
// Populate the component list.
int j;
do {
// Get the next member of the component.
j = this->TarjanStack.top();
this->TarjanStack.pop();
// Assign the member to the component.
this->TarjanComponents[j] = c;
this->TarjanEntries[j].Root = i;
// Store the node in its component.
component.push_back(j);
} while (j != i);
// Sort the component members for clarity.
std::sort(component.begin(), component.end());
}
}
void cmComputeComponentGraph::TransferEdges()
{
// Map inter-component edges in the original graph to edges in the
// component graph.
int n = static_cast<int>(this->InputGraph.size());
for (int i = 0; i < n; ++i) {
int i_component = this->TarjanComponents[i];
EdgeList const& nl = this->InputGraph[i];
for (cmGraphEdge const& ni : nl) {
int j = ni;
int j_component = this->TarjanComponents[j];
if (i_component != j_component) {
// We do not attempt to combine duplicate edges, but instead
// store the inter-component edges with suitable multiplicity.
this->ComponentGraph[i_component].emplace_back(
j_component, ni.IsStrong(), ni.GetBacktrace());
}
}
}
}
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