Direction runs of a line of mixed left-to-right and right-to-left text, for hosts that shape one direction at a time.

This is the implicit part of the Unicode bidirectional algorithm (UAX #9) for a single line: the paragraph direction from the first strong character, European and Arabic numbers, neutrals between runs, and reordering by level. Explicit embeddings, overrides, isolates and bracket pairs are not handled; text using them lays out as if they were absent.