lastPosBlockOffset

file offset for the last position in the last block, if there are more than positions; otherwise -1

One might think to use total term frequency to track how many positions are left to read as we decode the blocks, and decode the last block differently when num_left_positions < BLOCK_SIZE. Unfortunately this won't work since the tracking will be messed up when we skip blocks as the skipper will only tell us new position offset (start of block) and number of positions to skip for that block, without telling us how many positions it has skipped.