3.1 KiB
Final-Pass Emergency Stretch
Superseded note: the fixed-dimension emergency-stretch decision remains
current, but the historical hard/atomic fixed-cost exception below does not.
The accepted TeX active-path semantics are recorded in
20260802-final-pass-active-path-preservation.md.
Context
The narrow showcase still produced isolated CJK source lines after the policy work split automatic inline code from explicit hard atoms. That made the earlier forbidden-run explanation too narrow: the visible failure was not owned by unit suffix policy, buffer projection, or a screenshot-specific CJK penalty. It was the final K-P pass assigning ordinary underfull candidates to a fixed-cost emergency path outside the normal badness and demerits model.
Decision
Keep the strict pass unchanged. When strict K-P cannot reach the paragraph end, rerun with a finite background emergency stretch available to ordinary underfull candidates. Those candidates still compute adjustment ratio, badness, fitness, and demerits, so the global DP chooses between them by the same model as ordinary K-P.
Task035 tried making that stretch scale with the line/candidate width. Full
regression falsified that detail. Task036 must use a TeX-style fixed
dimension instead: ekp-emergency-stretch-pixel is nil for an automatic
value around three display-font M widths, or a non-negative integer for a
fixed pixel value.
Keep the artificial fixed-cost emergency transition only for a truly overfull first permitted hard or atomic run. That case has no ordinary candidate to score. The Elisp 1D path, Elisp looseness/parshape path, and C engine use the same rule.
Why the old fallback was wrong
The earlier repair treated the symptom as a forbidden-run boundary problem and remembered a fuller prefix. That explained one rigid-atom screenshot, but it left the deeper model split in place: some ordinary underfull lines were still not compared by normal badness, fitness, and demerits. At narrow widths that flattened costs enough for one-glyph lines to remain competitive.
The accepted direction matches TeX's emergency-pass shape more closely: emergency stretch is background flexibility for badness calculation, not a separate shortcut that bypasses the line-quality model. It must be a fixed dimension for the paragraph context, not a value derived from each line measure.
Test lesson
The first 84px/168px CJK oracle was invalid because it assumed two CJK glyphs always fit when the width is twice one glyph. It ignored glue, kinsoku, and attached punctuation. The replacement tests use the real mixed showcase text and reject isolated CJK source lines through the core, public buffer path, and GUI oracle.
Consequences
There is no CJK-orphan penalty, no number-unit special case, and no renderer compensation. Policy compilation still decides which gaps are legal before DP. Core K-P owns the final line choices, the renderer must agree with the chosen fixed stretch dimension, and both engines must stay in parity whenever emergency stretch behavior changes. The 15-field C paragraph ABI remains a constraint for task036.