The question--What use are one-bit counts for a bit string?--would occur only
to someone who was unfamiliar with bit maps and their uses.
These uses abound. As a concrete example consider what I call Marked Days
Tables (MDTs). ("I mark this day with a white stone", Lewis Carroll.) They
contain a bit for each day in a sequence of one or, usually, more years, the
bits associated with marked days having the value 1 and those associated with
unmarked days having the value 0. Such questions as
o What is the date d of the i-th unmarked day following the date D?
o How many unmarked days are there between the date d and the date D?
o Given two such MDTs what is the date d of the i-th day unmarked in both
following the date D?
then abound. Chugging along a bit string masks in hand counting 0s or 1s is
one way to answer them, but it is not an appealing one. (For table
construction there is even a need for assembly-time analogues of the
execution-time facilities that are the subject of this thread.)
I can feel some sympathy for sturdy, severely practical types who do not want
to be entrapped into thinking about anything that is not immediately useful;
but those anxieties are misplaced here.
John Gilmore Ashland, MA 01721-1817 USA
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