FA-80430 / Bidirectional text layout / Member archive

Neutral run resolution: start-of-sequence context · case 05

Leading neutrals ignore the sequence start type.

Member previewVariant 5 · 3 implementations · 8 checks per implementation

Case contract

Input [resolved weak classes, sos, eos, embedding level]. A maximal run of neutrals (ON, WS, B, S) takes the direction of its surrounding strong types when both sides agree (EN and AN count as R; sos/eos at the edges); otherwise the embedding direction (L for even level, R for odd). Return classes.

Why this case matters

Mixed right-to-left and left-to-right text must resolve levels and visual order exactly, or words, numbers and carets land in the wrong place.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
regression: start-of-sequence context	`["L", "L", "L", "R", "L", "R", "AN", "R"]`	`["R", "R", "R", "R", "L", "R", "AN", "R"]`	Failed

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