fd1210e557
Complete task002 through task016 across cache correctness, editor lifecycle, C boundaries, build and release governance, dictionary provenance, performance, interactive workflows, tests, documentation, and final cleanup.
67 lines
2.5 KiB
Markdown
67 lines
2.5 KiB
Markdown
# Complete DP Cache Signatures Instead of Invalidation Watchers
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## Context
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`ekp-para` caches width-dependent DP and rendered results. The old key was
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only the line width, with a special `(width . looseness)` cons when looseness
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was non-zero. Two failures followed:
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- changing a cost parameter reused a result computed under the old value;
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- the cons key never hit an `eql` hash table when reconstructed.
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The DP reads six cost parameters at runtime, so paragraph identity alone
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cannot make those cached results valid.
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## Decision
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Use one flat signature containing line width, looseness, and all six runtime
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cost parameters. Compare keys with structural `equal`.
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Paragraph-construction inputs remain in `ekp--para-key`; DP-only inputs
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remain in `ekp--dp-key`. No cache-clearing side effect crosses that boundary.
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## Alternatives Rejected
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### Variable watchers that clear caches
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Watchers would spread ownership across seven variables and discard
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width-independent paragraph work. Forgetting the next parameter would
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silently recreate the bug.
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### An integer hash of all parameters
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A hash collision would become a correctness failure. Structural keys make
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collisions harmless because equality is still checked.
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### A memoized global signature object
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It saves small list allocations but adds mutable global state and comparison
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logic. A 100,000-hit microbenchmark measured approximately 0.37 seconds for
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the complete key versus 0.17 seconds for the old scalar key: about two
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microseconds per lookup, below the cost threshold that would justify another
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state owner.
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## Consequences
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- Algorithm changes take effect on the next call without manual flushing.
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- Returning to a previous parameter set can reuse its prior cached result.
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- Non-zero looseness now receives real cache hits.
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- Cache lookup performs structural hashing over eight small scalar values.
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- Each distinct parameter set can retain its own result in the paragraph
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cache; normal interactive configuration has low cardinality, but a caller
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that sweeps parameters should clear caches between experiments.
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- No public API, C ABI, saved-file format, or paragraph representation
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changed.
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## Verification
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- The old model failed both the parameter-isolation and identical-signature
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tests.
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- The new model passes all six parameter cases and the cache-identity check.
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- Full ERT, C/Elisp fuzz, warning-clean byte compilation, and checkdoc pass.
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## Rollback
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Revert the key/table change and these regression tests. No migration or data
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cleanup is necessary because caches are process-local.
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