# Berlekamp–Massey algorithm

The Berlekamp–Massey algorithm is an algorithm that will find the shortest linear feedback shift register (LFSR) for a given binary output sequence. The algorithm will also find the minimal polynomial of a linearly recurrent sequence in an arbitrary field.[1]

Elwyn Berlekamp invented an algorithm for decoding Bose–Chaudhuri–Hocquenghem (BCH) codes.[2][3] James Massey recognized its application to linear feedback shift registers and simplified the algorithm.[4][5] Massey termed the algorithm the LFSR Synthesis Algorithm (Berlekamp Iterative Algorithm) (Massey 1969, p. 124), but it is now known as the Berlekamp–Massey algorithm.

## Description of algorithm

Berlekamp–Massey algorithm is an alternate method to solve the set of linear equations described in Reed–Solomon Peterson decoder, which can be summarized as:

$S_{i + \nu} + \Lambda_1 S_{i+\nu-1} + \cdots + \Lambda_{\nu-1} S_{i+1} + \Lambda_{\nu} S_i = 0.$

In the code examples below, C(x) is a potential instance of Λ(x). The error locator polynomial C(x) for L errors is defined as:

$C(x) = C_{L} \ x^{L} + C_{L-1} \ x^{L-1} + \cdots + C_2 \ x^2 + C_1 \ x + 1$

or reversed:

$C(x) = 1 + C_1 \ x + C_2 \ x^2 + \cdots + C_{L-1} \ x^{L-1} + C_{L} \ x^{L}.$

The goal of the algorithm is to determine the minimal degree L and C(x) which results in:

$S_{n} + C_1 \ S_{n-1} + \cdots + C_L \ S_{n-L} = 0$

for all syndromes, n = L to (N-1).

Algorithm: C(x) is initialized to 1, L is the current number of assumed errors, and initialized to zero. N is the total number of syndromes. n is used as the main iterator and to index the syndromes from 0 to (N-1). B(x) is a copy of the last C(x) since L was updated and initialized to 1. b is a copy of the last discrepancy d (explained below) since L was updated and initialized to 1. m is the number of iterations since L, B(x), and b were updated and initialized to 1.

Each iteration of the algorithm calculates a discrepancy d. At iteration k this would be:

$d = S_{k} + C_1 \ S_{k-1} + \cdots + C_L \ S_{k-L}.$

If d is zero, the algorithm assumes that C(x) and L are correct for the moment, increments m, and continues.

If d is not zero, the algorithm adjusts C(x) so that a recalculation of d would be zero:

$C(x) = C(x) \ - \ (d / b) \ x^m \ B(x).$

The xm term shifts B(x) so it follows the syndromes corresponding to 'b'. If the previous update of L occurred on iteration j, then m = k - j, and a recalculated discrepancy would be:

$d = S_{k} + C_1 \ S_{k-1} + \cdots - (d/b) (S_{j} + B_1 \ S_{j-1} + \cdots ).$

This would change a recalculated discrepancy to:

$d = d - (d/b)b = d - d = 0. \$

The algorithm also needs to increase L (number of errors) as needed. If L equals the actual number of errors, then during the iteration process, the discrepancies will become zero before n becomes greater than or equal to (2 L). Otherwise L is updated and algorithm will update B(x), b, increase L, and reset m = 1. The L = (n + 1 - L) formula limits L to the number of available syndromes used to calculate discrepancies, and also handles the case where L increases by more than 1.

## The algorithm for the binary field

The following is the Berlekamp–Massey algorithm specialized for the typical binary finite field F2 and GF(2). The field elements are 0 and 1. The field operations + and − are identical and become the exclusive or operation, XOR. The multiplication operator * becomes the logical AND operation. The division operator reduces to the identity operation (i.e., field division is only defined for dividing by 1, and x/1 = x).

1. Let $s_0, s_1, s_2 \cdots s_{n-1}$ be the bits of the stream.
2. Initialise two arrays $b$ and $c$ each of length $n$ to be zeroes, except $b_0 \leftarrow 1, c_0 \leftarrow 1$
3. assign $L \leftarrow 0, m \leftarrow -1$.
4. For $N = 0$ step 1 while $N < n$:
• Let $d$ be $s_N + c_1s_{N-1} + c_2s_{N-2} + \cdots + c_Ls_{N-L}$.
• if $d = 0$, then $c$ is already a polynomial which annihilates the portion of the stream from $N-L$ to $N$.
• else:
• Let $t$ be a copy of $c$.
• Set $c_{N-m} \leftarrow c_{N-m} \oplus b_0, c_{N-m+1} \leftarrow c_{N-m+1} \oplus b_1, \dots$ up to $c_{n-1} \leftarrow c_{n-1} \oplus b_{n-N+m-1}$ (where $\oplus$ is the Exclusive or operator).
• If $L \le \frac{N}{2}$, set $L \leftarrow N+1-L$, set $m \leftarrow N$, and let $b \leftarrow t$; otherwise leave $L$, $m$ and $b$ alone.

At the end of the algorithm, $L$ is the length of the minimal LFSR for the stream, and we have $c_Ls_a + c_{L-1}s_{a+1} + c_{L-2}s_{a+2} + \cdots = 0$ for all $a$.

## Code sample for the binary field in Java

The following code sample is for a binary field.

    public static int runTest(int[] array) {
final int N = array.length;
int[] b = new int[N];
int[] c = new int[N];
int[] t = new int[N];
b[0] = 1;
c[0] = 1;
int l = 0;
int m = -1;
for (int n = 0; n < N; n++) {
int d = 0;
for (int i = 0; i <= l; i++) {
d ^= c[i] * array[n - i];
}
if (d == 1) {
System.arraycopy(c, 0, t, 0, N);
int N_M = n − m;
for (int j = 0; j < N − N_M; j++) {
c[N_M + j] ^= b[j];
}
if (l <= n / 2) {
l = n + 1 - l;
m = n;
System.arraycopy(t, 0, b, 0, N);
}
}
}
return l;
}


## Berlekamp–Massey algorithm for fields

The algorithm from Massey (1969, p. 124).

  polynomial(field K) s(x) = ... /* coeffs are s_j; output sequence as N-1 degree polynomial) */
/* connection polynomial */
polynomial(field K) C(x) = 1;  /* coeffs are c_j */
polynomial(field K) B(x) = 1;
int L = 0;
int m = 1;
field K b = 1;
int n;
for (n = 0; n < N; n++)
{
/* calculate discrepancy */
field K d = s_n + \Sigma_{i=1}^L c_i * s_{n-i};
if (d == 0)
{
/* annihilation continues */
m = m + 1;
}
else if (2 * L <= n)
{
/* temporary copy of C(x) */
polynomial(field K) T(x) = C(x);
C(x) = C(x) - d b^{-1} x^m B(x);
L = n + 1 - L;
B(x) = T(x);
b = d;
m = 1;
}
else
{
C(x) = C(x) - d b^{-1} x^m B(x);
m = m + 1;
}
}
return L;


3. ^ Berlekamp, Elwyn R. (1984) [1968], Algebraic Coding Theory, Laguna Hills, CA: Aegean Park Press, ISBN 0-89412-063-8 Unknown parameter |ed= ignored (help). Previous publisher McGraw-Hill, New York, NY.