The two halves, and the two parts one of them does not need

The two halves, and the two parts one of them does not need

A bidirectional index over 8,192 characters, sampling every 32 rows, drawn as the five parts each half is made of. The reverse half exists to be counted in: an extension asks it for ranks and never for a position, because its rows are positions in a text written backwards and the occurrences a search reports come out of the forward interval. So the marks and the sampled positions — the pale rows — can go, and what is left still answers every question anything asks it. That is 11,791 bits of 70,610, or 16.7% of the whole structure.

Drawn at 700 × 362, wide on the page. Everything above is what half-index returns with no arguments; the caption is the generator's own, computed from the numbers in the drawing rather than written beside it.

4 essays call half-index. The drawing above is what it returns with no arguments at all; every call below passes it something, because a placement that passes nothing draws whichever member of the family the generator happens to default to rather than the one its essay argues about — which is what optcheck and figfill exist to catch.