maple implementation of Berlekamp-Massey algorithm
\\PMlinkescapetext{># Maple code for the Berlekamp-Massey algorithm
# Adapted from www.cs.wisc.edu/~cs435-1/bermas.m# Transliteration of# Massey, "Shift-Register Synthesis and BCH Decoding,"# IEEE Trans. Inform. Theory, 15(1):122-127, 1969.# Input: P, either 0 or a prime# If P>0 then we work over the field K = Z/Z[P] (mod P)# else we work over the field K = Q (rationals)# N, a positive integer# s, a list of >= 2*N terms in K# x, a formal variable# Returns: Unique monic annihilator of minimum degree, over K[x]. BM := proc(s, N, P, x) local C,B,T,L,k,i,n,d,b,safemod; ASSERT(nops(s) = 2*N); safemod := (exp, P) -> `if`(P=0, exp, exp mod P); B := 1; C := 1; L := 0; k := 1; b := 1; for n from 0 to 2*N-1 do d := s[n+1]; for i from 1 to L do d := safemod(d + coeff(C,x^i)*s[n-i+1], P); od; if d=0 then k := k+1 fi; if (d <> 0 and 2*L > n) then C := safemod(expand(C - d*x^k*B/b), P); k := k+1; fi; if (d <> 0 and 2*L <= n) then T := C; C := safemod(expand(C - d*x^k*B/b), P); B := T; L := n+1-L; k := 1; b := d; fi; od; return C; end:}The following test demonstrates usage and verifies that this works:
\\PMlinkescapetext{> P := 103: d := 4: num := 21+83*x+90*x^2+4*x^3: # degree < d den := 1+11*x+23*x^2+58*x^3+69*x^4: # monic, degree <= d f := series(num/den, x=0, 2*d) mod P: s := [seq(coeff(f, x, i), i=0..2*d-1)]: BM(s, d, P, x);}The annihilator is the same as denominator, as we expect.