Pgr contraction hierarchies

[AuBousquet]

Changes proposed in this pull request:

• A new algorithm is added to implement contraction hierarchies (first pre-processing step for the contraction hierarchies shortest path algorithm);
• Shortest path algorithm is then a bidirectional Dijkstra-like shortest path, which will be implemented in a second time.

@pgRouting/admins

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[AuBousquet]

https://github.com/coderabbitai summary
https://github.com/coderabbitai review

[cvvergara]

@coderabbitai review
@coderabbitai summary

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🧩 Analysis chain

🏁 Script executed:

#!/bin/bash
# Get basic info about the PR
echo "=== PR INFO ==="
gh pr view 2867 --json title,body,author,baseRefName,headRefName,state,isDraft

# Get files changed in the PR
echo -e "\n=== FILES CHANGED ==="
gh pr view 2867 --json files | jq -r '.files[] | .path'

# Get detailed diff stats
echo -e "\n=== DIFF STATS ==="
gh pr view 2867 --json files | jq -r '.files[] | "File: \(.path)\nAdditions: \(.additions)\nDeletions: \(.deletions)\nStatus: \(.status)\n"'

# Get the PR diff
echo -e "\n=== PR DIFF ==="
gh pr diff 2867

# Get repository information
echo -e "\n=== REPOSITORY INFO ==="
gh repo view pgRouting/pgrouting --json description,defaultBranchRef

Length of output: 74825

---

🏁 Script executed:

#!/bin/bash

# Fetch a better PR description
echo "=== PR DESCRIPTION ==="
gh pr view 2867 --json body

# Get the PR summary
echo -e "\n=== PR SUMMARY ==="
gh pr view 2867 --json title,additions,deletions,changedFiles

# Get file structure of the contraction directory and related files to understand the context
echo -e "\n=== CONTRACTION DIRECTORY STRUCTURE ==="
gh pr view 2867 --json files | jq -r '.files[] | select(.path | startswith("include/contraction") or startswith("src/contraction")) | .path'

# Get more information about the repository
echo -e "\n=== REPOSITORY INFO ==="
gh repo view pgRouting/pgrouting --json description,defaultBranchRef

# Look at the main implementation file to understand the algorithm
echo -e "\n=== MAIN IMPLEMENTATION FILE ==="
if gh pr view 2867 --json files | jq -r '.files[] | select(.path == "include/contraction/contractionHierarchies.hpp") | .path' | grep -q "contractionHierarchies.hpp"; then
  gh pr view 2867 --json files | jq -r '.files[] | select(.path == "include/contraction/contractionHierarchies.hpp") | .additions'
fi

# Get information about existing contraction methods for context
echo -e "\n=== EXISTING CONTRACTION METHODS ==="
gh pr view 2867 --json files | jq -r '.files[] | select(.path == "doc/contraction/contraction-family.rst") | .path'
if [ $? -eq 0 ]; then
  cat doc/contraction/contraction-family.rst 2>/dev/null || echo "File not found in current checkout"
fi

Length of output: 4521

---

@cvvergara I've reviewed PR #2867 and prepared a summary as requested:

Summary of PR #2867 - Contraction Hierarchies

This pull request implements the Contraction Hierarchies algorithm, which is a preprocessing technique used to significantly speed up shortest path queries in large graphs like road networks. The implementation adds a new experimental function pgr_contractionHierarchies.

Key additions:

• A preprocessing algorithm that creates a hierarchical structure of the graph
• Shortcut edge generation to preserve optimal paths while contracting vertices
• Support for both directed and undirected graphs
• Ability to specify forbidden vertices that should not be contracted
• Comprehensive documentation with examples
• Test coverage including edge cases

This is the first step toward a complete contraction hierarchies implementation, with a specialized bidirectional Dijkstra algorithm planned for a future PR.

Review:

The implementation is well-structured, following pgRouting's architecture patterns. It includes:

1. Core algorithm in C++ (include/contraction/contractionHierarchies.hpp)
2. PostgreSQL integration through C wrappers
3. Thorough documentation with clear examples
4. Test cases including edge cases and performance tests

The code builds on existing contraction functionality but stands as its own implementation rather than being integrated with pgr_contraction.

The functionality is correctly marked as experimental, which is appropriate for this new addition.

Would you like me to dive deeper into any specific aspect of this PR?

[cvvergara]

Closing this PR in favor of #2868