--- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/Modules/_decimal/libmpdec/literature/bignum.txt Sat Mar 10 18:12:20 2012 +0100 @@ -0,0 +1,83 @@ + + +Bignum support (Fast Number Theoretic Transform or FNT): +======================================================== + +Bignum arithmetic in libmpdec uses the scheme for fast convolution +of integer sequences from: + +J. M. Pollard: The fast Fourier transform in a finite field +http://www.ams.org/journals/mcom/1971-25-114/S0025-5718-1971-0301966-0/home.html + + +The transform in a finite field can be used for convolution in the same +way as the Fourier Transform. The main advantages of the Number Theoretic +Transform are that it is both exact and very memory efficient. + + +Convolution in pseudo-code: +~~~~~~~~~~~~~~~~~~~~~~~~~~~ + + fnt_convolute(a, b): + x = fnt(a) # forward transform of a + y = fnt(b) # forward transform of b + z = pairwise multiply x[i] and y[i] + result = inv_fnt(z) # backward transform of z. + + +Extending the maximum transform length (Chinese Remainder Theorem): +------------------------------------------------------------------- + +The maximum transform length is quite limited when using a single +prime field. However, it is possible to use multiple primes and +recover the result using the Chinese Remainder Theorem. + + +Multiplication in pseudo-code: +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + + _mpd_fntmul(u, v): + c1 = fnt_convolute(u, v, P1) # convolute modulo prime1 + c2 = fnt_convolute(u, v, P2) # convolute modulo prime2 + c3 = fnt_convolute(u, v, P3) # convolute modulo prime3 + result = crt3(c1, c2, c3) # Chinese Remainder Theorem + + +Optimized transform functions: +------------------------------ + +There are three different fnt() functions: + + std_fnt: "standard" decimation in frequency transform for array lengths + of 2**n. Performs well up to 1024 words. + + sixstep: Cache-friendly algorithm for array lengths of 2**n. Outperforms + std_fnt for large arrays. + + fourstep: Algorithm for array lengths of 3 * 2**n. Also cache friendly + in large parts. + + +List of bignum-only files: +-------------------------- + +Functions from these files are only used in _mpd_fntmul(). + + umodarith.h -> fast low level routines for unsigned modular arithmetic + numbertheory.c -> routines for setting up the FNT + difradix2.c -> decimation in frequency transform, used as the + "base case" by the following three files: + + fnt.c -> standard transform for smaller arrays + sixstep.c -> transform large arrays of length 2**n + fourstep.c -> transform arrays of length 3 * 2**n + + convolute.c -> do the actual fast convolution, using one of + the three transform functions. + transpose.c -> transpositions needed for the sixstep algorithm. + crt.c -> Chinese Remainder Theorem: use information from three + transforms modulo three different primes to get the + final result. + + +