Builds a correctly-sized vector for an edge case from n/inc alone, filled
with a sentinel (99) at every position. A routine that only ever touches
vec[i*inc] for i in [0,n) — whether overwriting it or accumulating into
it — leaves every other position exactly as it started, so a uniform
sentinel fill makes "the right positions changed" and "the gap positions
were left alone" both observable, without hand-typing a buffer sized to
match n/inc.
Builds a correctly-sized vector for an edge case from n/inc alone, filled with a sentinel (99) at every position. A routine that only ever touches
vec[i*inc]for i in [0,n) — whether overwriting it or accumulating into it — leaves every other position exactly as it started, so a uniform sentinel fill makes "the right positions changed" and "the gap positions were left alone" both observable, without hand-typing a buffer sized to match n/inc.