DST: bitmap edge cases — large offsets and SETBIT-then-GETBIT sequences

[nerdsane]

Context

Found during DST methodology review of bitmap implementation (commit e9be18b).

Problems

1. Offset range too narrow: DST generates offsets 0-2047 but never tests near MAX_BIT_OFFSET (4,294,967,296). Should occasionally test boundary offsets and large allocations (~125KB for offset 1,000,000).

2. No explicit read-after-write test: No SETBIT-then-GETBIT sequence on the same key+offset. Relies on random interleaving which is rare at 5% probability.

3. Bitmap in final else clause: Fragile if new string ops are added above — bitmap coverage could silently drop to zero.

Suggested Fix

• 5% of bitmap ops: use rng.gen_range(0, MAX_BIT_OFFSET + 100) for boundary testing
• After each SETBIT, do a verification GETBIT on the same key+offset
• Consider a named constant for the bitmap threshold instead of relying on else

Severity

Low — coverage gap, not a correctness bug.

Found By

DST review agent (Issues 3, 5, 6)