In the context the order of iterators is that the returned value is
always positive

the completion function gets int start and int end representing
boundaries of the completed text. The value is provided offset so there
should not be any need for negative values and I believe we may promote
that by internaly treating the values as unsigneds everywhere.

plus use constexpr to define the compile time constant

size_t should be use for container indexes and container related
distances

some casts have to be reinterpret_casts because of type incompatibility
between unsigned char * and char *

ints unnecessarily reduce range of values for indexes

remove not needed cast

Map keys are exactly corresponding to vector indexes in the algorithm.