FA-80772 / Bidirectional text layout / Member archive

Isolating run sequences: chained sequence end · case 02

eos of an isolate-spanning sequence is taken from the middle of the text.

Member previewVariant 2 · 3 implementations · 8 checks per implementation

Case contract

Input [classes, levels, paragraph level]. Split into maximal level runs; chain a run ending in a matched isolate initiator (LRI/RLI/FSI) with the run starting at its matching PDI (stack matching; unmatched PDI ignored). For each sequence sos = parity of max(level of first char, level of preceding char or paragraph level); eos = parity of max(level of last char, level of following char or paragraph level), using the paragraph level when the last char is an isolate initiator. Return [[indices, sos, eos]].

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: chained sequence end	`[[[0, 1, 2, 5, 6, 7, 8], "R", "R"], [[3, 4], "R", "R"], [[9], "L", "L"], [[10, 11, 12], "L", "R"]]`	`[[[0, 1, 2, 5, 6, 7, 8], "R", "L"], [[3, 4], "R", "R"], [[9], "L", "L"], [[10, 11, 12], "L", "R"]]`	Failed

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