(gmp.info)Basecase Division


Basecase Division
-----------------

   Basecase NxM division is like long division done by hand, but in base
2^mp_bits_per_limb.  See Knuth section 4.3.1 algorithm D, and
`mpn/generic/sb_divrem_mn.c'.

   Briefly stated, while the dividend remains larger than the divisor,
a high quotient limb is formed and the Nx1 product q*d subtracted at
the top end of the dividend.  With a normalized divisor (most
significant bit set), each quotient limb can be formed with a 2x1
division and a 1x1 multiplication plus some subtractions.  The 2x1
division is by the high limb of the divisor and is done either with a
hardware divide or a multiply by inverse (the same as in Note: Single
Limb Division) whichever is faster.  Such a quotient is sometimes one
too big, requiring an addback of the divisor, but that happens rarely.

   With Q=N-M being the number of quotient limbs, this is an O(Q*M)
algorithm and will run at a speed similar to a basecase QxM
multiplication, differing in fact only in the extra multiply and divide
for each of the Q quotient limbs.


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