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| 1 | +--- |
| 2 | +title: Expanding the C++ Graph Backend - DFS and Bellman-Ford |
| 3 | +date: 2025-10-22 23:25:31 +0530 |
| 4 | +categories: [Season of Commits] |
| 5 | +tags: [Week 10] |
| 6 | +--- |
| 7 | + |
| 8 | +This week’s progress focused on extending the C++ graph backend in PyDataStructs with two major algorithmic implementations, Depth-First Search (DFS) and the Bellman-Ford shortest path algorithm. The [PR](https://github.com/codezonediitj/pydatastructs/pull/707) builds upon the existing backend architecture established in earlier commits, further accelerating computation on large graphs while maintaining full compatibility with the Python interface. |
| 9 | + |
| 10 | +--- |
| 11 | + |
| 12 | +### 1. Adding DFS and Bellman-Ford to the C++ Backend |
| 13 | + |
| 14 | +The primary goal of this update was to move more core graph algorithms from Python to C++ for improved performance and scalability. With this PR, both **DFS** and **Bellman-Ford** are now available in the C++ backend, enabling faster traversal and shortest-path computations, especially on large or dense graphs. |
| 15 | + |
| 16 | +--- |
| 17 | + |
| 18 | +### 2. Depth-First Search (DFS) — Iterative, Flexible, and Fast |
| 19 | + |
| 20 | +The C++ DFS implementation mirrors the behavior of the existing Python version while significantly improving runtime performance. |
| 21 | +Key highlights include: |
| 22 | + |
| 23 | +- **Iterative stack-based design** — avoids recursion depth limits |
| 24 | +- **Support for both graph types** — adjacency list and adjacency matrix |
| 25 | +- **Custom callback operations** — users can attach operations (like node labeling or early stopping) via function arguments |
| 26 | +- **Full compatibility** — results and traversal order match the Python backend for consistent testing |
| 27 | + |
| 28 | +This design not only improves efficiency but also provides a framework for extending graph traversal algorithms with user-defined logic, something that will integrate well with future backend expansion (e.g., parallel traversal in LLVM). |
| 29 | + |
| 30 | +--- |
| 31 | + |
| 32 | +### 3. Bellman-Ford Algorithm — Optimized with SPFA |
| 33 | + |
| 34 | +The Bellman-Ford shortest path algorithm, implemented as `shortest_paths_bellman_ford_adjacency_list`, was enhanced with the **Shortest Path Faster Algorithm (SPFA)** optimization. |
| 35 | +This approach drastically reduces redundant relaxations on sparse graphs while preserving correctness on graphs with negative weights. |
| 36 | + |
| 37 | +Key improvements include: |
| 38 | + |
| 39 | +- **SPFA optimization** for improved average-case performance |
| 40 | +- **Negative weight and cycle detection** |
| 41 | +- **Consistent output format** — returns distances and predecessors identical to Python’s implementation |
| 42 | +- **Optional target node parameter** for partial shortest-path computation |
| 43 | + |
| 44 | +These features make the C++ backend robust for a wide range of applications from general-purpose graph analytics to low-level algorithm benchmarking. |
| 45 | + |
| 46 | +--- |
| 47 | + |
| 48 | +### 4. Testing and Validation |
| 49 | + |
| 50 | +This update includes extensive test coverage to ensure correctness and cross-backend consistency. |
| 51 | +Highlights: |
| 52 | + |
| 53 | +- Added dedicated C++ backend tests for DFS and Bellman-Ford |
| 54 | +- Verified behavior for graphs with both positive and negative weights |
| 55 | +- Included cycle detection validation in Bellman-Ford |
| 56 | +- Confirmed callback handling in DFS for custom operations |
| 57 | + |
| 58 | +All tests across Python and C++ backends pass successfully, ensuring stable integration with the existing codebase. |
| 59 | + |
| 60 | +--- |
| 61 | + |
| 62 | +### 5. The Road Ahead |
| 63 | + |
| 64 | +With DFS and Bellman-Ford now integrated, the focus shifts toward: |
| 65 | + |
| 66 | +- **Adding more C++ backend algorithms** — including SCC’s and max flow algorithms |
| 67 | +- **Backend benchmarking suite** — to quantify speedups across graph sizes and structures |
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