The names _shiftdown and _shiftup are misleading and they should be exchanged. While building a min-heap, each element is compared with its child elements so to bubble it down, not 'up'.

The names should not be changed, because they're essentially arbitrary, with no compelling reason to favor one over the other - and there's no consistency about this in the literature either.

For example, when building a min-heap, the vast majority of elements that move go up in the heap, not down. It's only the single element at the top of the current sub-heap that moves down.

Which is the heart of the inherent ambiguity: something can't "go up" without something else "going down". The current names reflect the directions of the bulk of the data that moves (_siftup() generally moves children up to the their parent positions, and _siftdown() the opposite).