inire/DOCS.md

# Inire Configuration & API Documentation

This document describes the user-tunable parameters for the `inire` auto-router.

## 1. AStarRouter Parameters

The `AStarRouter` is the core pathfinding engine. It can be configured directly through its constructor.

| Parameter | Type | Default | Description |
| :--- | :--- | :--- | :--- |
| `node_limit` | `int` | 1,000,000 | Maximum number of states to explore per net. Increase for very complex paths. |
| `straight_lengths` | `list[float]` | `[1.0, 5.0, 25.0]` | Discrete step sizes for straight waveguides (µm). Larger steps speed up search in open space. |
| `bend_radii` | `list[float]` | `[10.0]` | Available radii for 90-degree turns (µm). Multiple values allow the router to pick the best fit. |
| `sbend_offsets` | `list[float]` | `[-5, -2, 2, 5]` | Lateral offsets for parametric S-bends (µm). |
| `sbend_radii` | `list[float]` | `[10.0]` | Available radii for S-bends (µm). |
| `snap_to_target_dist`| `float` | 20.0 | Distance (µm) at which the router attempts an exact bridge to the target port. |
| `bend_penalty` | `float` | 50.0 | Flat cost added for every 90-degree bend. Higher values favor straight lines. |
| `sbend_penalty` | `float` | 100.0 | Flat cost added for every S-bend. Usually higher than `bend_penalty`. |
| `bend_collision_type`| `str` | `"arc"` | Collision model for bends: `"arc"`, `"bbox"`, or `"clipped_bbox"`. |
| `bend_clip_margin` | `float` | 10.0 | Margin (µm) for the `"clipped_bbox"` collision model. |

### Bend Collision Models
*   `"arc"`: High-fidelity model following the exact curved waveguide geometry.
*   `"bbox"`: Conservative model using the axis-aligned bounding box of the bend. Fast but blocks more space.
*   `"clipped_bbox"`: A middle ground that uses the bounding box but clips corners that are far from the waveguide.

---

## 2. CostEvaluator Parameters

The `CostEvaluator` defines the "goodness" of a path.

| Parameter | Type | Default | Description |
| :--- | :--- | :--- | :--- |
| `unit_length_cost` | `float` | 1.0 | Cost per µm of wire length. |
| `greedy_h_weight` | `float` | 1.1 | Heuristic weight. `1.0` is optimal; higher values (e.g., `1.5`) are faster but may produce longer paths. |
| `congestion_penalty`| `float` | 10,000.0 | Multiplier for overlaps in the multi-net Negotiated Congestion loop. |

---

## 3. PathFinder Parameters

The `PathFinder` orchestrates multi-net routing using the Negotiated Congestion algorithm.

| Parameter | Type | Default | Description |
| :--- | :--- | :--- | :--- |
| `max_iterations` | `int` | 10 | Maximum number of rip-up and reroute iterations to resolve congestion. |
| `base_congestion_penalty` | `float` | 100.0 | Starting penalty for overlaps. This value is multiplied by `1.5` each iteration if congestion persists. |

---

## 4. CollisionEngine Parameters

| Parameter | Type | Default | Description |
| :--- | :--- | :--- | :--- |
| `clearance` | `float` | (Required) | Minimum required distance between any two waveguides or obstacles (µm). |
| `safety_zone_radius`| `float` | 0.0021 | Radius (µm) around ports where collisions are ignored to allow PDK boundary incidence. |

---

## 4. Physical Units & Precision
-   **Coordinates**: Micrometers (µm).
-   **Grid Snapping**: The router internally operates on a **1nm** grid for final ports and a **1µm** lattice for expansion moves.
-   **Search Space**: Assumptions are optimized for design areas up to **20mm x 20mm**.

---

## 5. Best Practices & Tuning Advice

### Speed vs. Optimality
The `greedy_h_weight` is your primary lever for search performance.
-   **`1.0`**: Dijkstra-like behavior. Guarantees the shortest path but is very slow.
-   **`1.1` to `1.2`**: Recommended range. Balances wire length with fast convergence.
-   **`> 1.5`**: Extremely fast "greedy" search. May produce zig-zags or suboptimal detours.

### Avoiding "Zig-Zags"
If the router produces many small bends instead of a long straight line:
1.  Increase `bend_penalty` (e.g., set to `100.0` or higher).
2.  Ensure `straight_lengths` includes larger values like `25.0` or `100.0`.
3.  Decrease `greedy_h_weight` closer to `1.0`.

### Handling Congestion
In multi-net designs, if nets are overlapping:
1.  Increase `congestion_penalty` in `CostEvaluator`.
2.  Increase `max_iterations` in `PathFinder`.
3.  If a solution is still not found, check if the `clearance` is physically possible given the design's narrowest bottlenecks.

### S-Bend Usage
Parametric S-bends are triggered by the `sbend_offsets` list. If you need a specific lateral shift (e.g., 5.86µm for a 45° switchover), add it to `sbend_offsets`. The router will only use an S-bend if it can reach a state that is exactly on the lattice or the target.