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Fix predecessor DAG construction in k-shortest paths algorithm #2

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Merged
networmix merged 1 commit into from
Feb 8, 2026
Merged

Fix predecessor DAG construction in k-shortest paths algorithm #2

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12 changes: 7 additions & 5 deletions src/k_shortest_paths.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -203,9 +203,10 @@ std::vector<std::pair<std::vector<Cost>, PredDAG>> k_shortest_paths(
for (std::size_t v = 1; v < dag.parent_offsets.size(); ++v) dag.parent_offsets[v] += dag.parent_offsets[v-1];
dag.parents.resize(static_cast<std::size_t>(dag.parent_offsets.back()));
dag.via_edges.resize(static_cast<std::size_t>(dag.parent_offsets.back()));
std::size_t idx = 0;
for (std::size_t i = 1; i < P.nodes.size(); ++i) {
dag.parents[idx] = P.nodes[i-1]; dag.via_edges[idx] = P.edges[i-1]; ++idx;
auto v = P.nodes[i];
auto base = static_cast<std::size_t>(dag.parent_offsets[static_cast<std::size_t>(v)]);
dag.parents[base] = P.nodes[i-1]; dag.via_edges[base] = P.edges[i-1];
}
Comment on lines 206 to 210
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Copilot AI Feb 8, 2026

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Consider adding a regression test that validates the returned PredDAG encodes the concrete path in true CSR form (parents/via_edges stored in the range parent_offsets[v]:parent_offsets[v+1] for each node v). The previous sequential fill could pass basic size/monotonicity checks but still produce incorrect parent lookups; a test that reconstructs the path from the DAG (e.g., via resolve_to_paths or by directly checking parents[parent_offsets[v]]) on a path whose node IDs are not in increasing order would catch this.

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}
items.emplace_back(std::move(dist), std::move(dag));
Expand Down Expand Up @@ -249,7 +250,7 @@ std::vector<std::pair<std::vector<Cost>, PredDAG>> k_shortest_paths(
std::vector<std::uint8_t> edge_mask_local;
edge_mask_local.assign(static_cast<std::size_t>(g.num_edges()), static_cast<std::uint8_t>(1));
for (auto const& P : paths) {
if (P.nodes.size() > j && std::equal(P.nodes.begin(), P.nodes.begin() + j, last.nodes.begin())) {
if (P.nodes.size() > j && std::equal(P.nodes.begin(), P.nodes.begin() + static_cast<std::ptrdiff_t>(j + 1), last.nodes.begin())) {
// Exclude edge at position j for this path
if (P.edges.size() > j) {
edge_mask_local[static_cast<std::size_t>(P.edges[j])] = 0;
Expand Down Expand Up @@ -337,9 +338,10 @@ std::vector<std::pair<std::vector<Cost>, PredDAG>> k_shortest_paths(
for (std::size_t v = 1; v < dag.parent_offsets.size(); ++v) dag.parent_offsets[v] += dag.parent_offsets[v-1];
dag.parents.resize(static_cast<std::size_t>(dag.parent_offsets.back()));
dag.via_edges.resize(static_cast<std::size_t>(dag.parent_offsets.back()));
std::size_t idx = 0;
for (std::size_t i = 1; i < P.nodes.size(); ++i) {
dag.parents[idx] = P.nodes[i-1]; dag.via_edges[idx] = P.edges[i-1]; ++idx;
auto v = P.nodes[i];
auto base = static_cast<std::size_t>(dag.parent_offsets[static_cast<std::size_t>(v)]);
dag.parents[base] = P.nodes[i-1]; dag.via_edges[base] = P.edges[i-1];
}
}
items.emplace_back(std::move(dist), std::move(dag));
Expand Down