WORK_LOG.md

@@ -39,3 +39,57 @@ The routing method selection logic was updated so perpendicular port-angle cases
 
 mxpic_router/mxpic_router/builder.py
 mxpic_router/tests/test_eda_router_contract.py
+
+
+
+
+
+## 2026-06-04
+
+### Scope
+
+Backend routing spacing behavior and automatic S-bend refinement in `mxpic_router`.
+
+### Summary
+
+Recent work focused on reducing overlap and crossing risks for grouped automatic S-bend connections while keeping the generated GDS impact limited.
+
+### Completed Work
+
+#### 1. S-Bend Spacing Issue Investigation
+
+A route overlap issue was investigated for layouts containing multiple parallel or near-parallel automatic S-bend routes.
+
+The issue was most visible when several component output ports were connected to several port elements. In these cases, the initial straight section of multiple S-bends could overlap or become too close when source and target ports were arranged in parallel.
+
+#### 2. Conservative `Lstart`-Based S-Bend Adjustment
+
+Spacing control was implemented through the native Nazca `sbend_p2p` `Lstart` parameter.
+
+The backend now groups compatible automatic S-bend routes and assigns different `Lstart` values before route generation. This preserves the native S-bend route style and avoids manually constructed detours.
+
+The adjustment applies only to automatic `sbend_p2p` routes. Manual-point routes, U-bends, perpendicular `strt_bend_strt_p2p` routes, and generic bend routes are left unchanged.
+
+#### 3. Bundle-Level Route Grouping and Ordering
+
+Compatible S-bend routes are grouped by routing direction, xsection, width, and radius. The `Lstart` assignment logic was refined for both same-direction fan-out/fan-in routes and symmetric convergence/divergence routes, so outer routes can receive larger offsets when needed.
+
+#### 4. Width-Aware Spacing Step
+
+The base step is now calculated as:
+
+```text
+route width + 10um
+```
+
+For example, a 0.5um-wide route uses a 10.5um `Lstart` step, while a 40um-wide route uses a 50um `Lstart` step.
+
+#### 5. Test Coverage Update
+
+Contract tests were added and updated to cover automatic S-bend `Lstart` assignment, bundle-level grouping behavior, route ordering, width-aware spacing, and preservation of earlier rotation and perpendicular-routing fixes.
+
+### Modified Files
+
+mxpic_router/mxpic_router/builder.py
+mxpic_router/tests/test_eda_router_contract.py
+