ekp/postmortem/20260728-core-rule-ownership.md

Centralize Rules, Not File Shapes

Context

ekp-para necessarily carries the width-independent data shared by Elisp DP, the C boundary, and rendering. Splitting the struct or ekp.el would mostly add declarations and navigation. Two actual rules, however, were duplicated: edge-space exclusion appeared in four hot paths, and the lossless marker vocabulary appeared in renderer and region integration.

Cache identity and ideal-line measurement already had direct owners and brute-force regressions from earlier work.

Decision

Extract one allocation-free inline function for edge-space exclusion and one constant for layout marker properties. Reuse them at every existing call site and test each rule directly. Keep the coherent paragraph struct and workflows in place.

Alternatives

• Splitting ekp.el by “core,” “common,” or “helpers” was rejected because it moves code without moving a stable responsibility.
• Returning a new line-metrics object per DP candidate was rejected because it adds hot-loop allocation to remove textual duplication.
• Leaving the formulas inline was rejected because four copies had already crossed DP/reconstruction/render ownership boundaries.

Consequences

The most drift-prone edge rule and marker vocabulary now change in one place. Cache keys, line metrics, and marker inversion each have direct tests. Large workflows remain visible instead of being hidden behind pass-through helpers.

Rollback

Inline the function and constant if profiling or compatibility evidence requires it, while retaining the independent rule tests. Do not replace them with a broader abstraction unless it removes additional real duplication.