A bitwhich object represents a boolean filter like a bit object (NAs are not allowed)
but uses a sparse representation suitable for very skewed (asymmetric) selections.
Three extreme cases are represented with logical values, no length via logical(),
all TRUE with TRUE and all FALSE with FALSE. All other selections are represented with
positive or negative integers, whatever is shorter.
This needs less RAM compared to logical (and often less than bit or which).
Logical operations are fast if the selection is asymetric (only few or almost all selected).

an object of class 'bitwhich' carrying two attributes

maxindex

see above

poslength

see above

Arguments

maxindex

length of the vector

x

Information about which positions are FALSE or TRUE: either logical() or TRUE or FALSE or a integer vector of positive or of negative subscripts.

xempty

what to assume about parameter x if x=integer(0), typically TRUE or FALSE.

poslength

tuning: poslength is calculated automatically, you can give poslength explicitely, in this case it must be correct and x must be sorted and not have duplicates.

is.unsorted

tuning: FALSE implies that x is already sorted and sorting is skipped