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## Side by Side Diff: Modules/_decimal/literature/bignum.txt

Issue 7652: Merge C version of decimal into py3k.
Patch Set: Created 7 years, 7 months ago
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2
3 Bignum support (Fast Number Theoretic Transform or FNT):
4 ========================================================
5
6 Bignum arithmetic in libmpdec uses the scheme for fast convolution
7 of integer sequences from:
8
9 J. M. Pollard: The fast Fourier transform in a finite field
10 http://www.ams.org/journals/mcom/1971-25-114/S0025-5718-1971-0301966-0/home.html
11
12
13 The transform in a finite field can be used for convolution in the same
14 way as the Fourier Transform. The main advantages of the Number Theoretic
15 Transform are that it is both exact and very memory efficient.
16
17
18 Convolution in pseudo-code:
19 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
20
21 fnt_convolute(a, b):
22 x = fnt(a) # forward transform of a
23 y = fnt(b) # forward transform of b
24 z = pairwise multiply x[i] and y[i]
25 result = inv_fnt(z) # backward transform of z.
26
27
28 Extending the maximum transform length (Chinese Remainder Theorem):
29 -------------------------------------------------------------------
30
31 The maximum transform length is quite limited when using a single
32 prime field. However, it is possible to use multiple primes and
33 recover the result using the Chinese Remainder Theorem.
34
35
36 Multiplication in pseudo-code:
37 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
38
39 _mpd_fntmul(u, v):
40 c1 = fnt_convolute(u, v, P1) # convolute modulo prime1
41 c2 = fnt_convolute(u, v, P2) # convolute modulo prime2
42 c3 = fnt_convolute(u, v, P3) # convolute modulo prime3
43 result = crt3(c1, c2, c3) # Chinese Remainder Theorem
44
45
46 Optimized transform functions:
47 ------------------------------
48
49 There are three different fnt() functions:
50
51 std_fnt: "standard" decimation in frequency transform for array lengths
52 of 2**n. Performs well up to 1024 words.
53
54 sixstep: Cache-friendly algorithm for array lengths of 2**n. Outperforms
55 std_fnt for large arrays.
56
57 fourstep: Algorithm for array lengths of 3 * 2**n. Also cache friendly
58 in large parts.
59
60
61 List of bignum-only files:
62 --------------------------
63
64 Functions from these files are only used in _mpd_fntmul().
65
66 umodarith.h -> fast low level routines for unsigned modular arithmetic
67 numbertheory.c -> routines for setting up the FNT
68 difradix2.c -> decimation in frequency transform, used as the
69 "base case" by the following three files:
70
71 fnt.c -> standard transform for smaller arrays
72 sixstep.c -> transform large arrays of length 2**n
73 fourstep.c -> transform arrays of length 3 * 2**n
74
75 convolute.c -> do the actual fast convolution, using one of
76 the three transform functions.
77 transpose.c -> transpositions needed for the sixstep algorithm.
78 crt.c -> Chinese Remainder Theorem: use information from three
79 transforms modulo three different primes to get the
80 final result.
81
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