Forney algorithm

In coding theory, the Forney algorithm calculates the error values at known error locations. It is used as one of the steps in decoding BCH codes and Reed-Solomon codes. George David Forney Jr. developed the algorithm in 1965.

Procedure

Code words look like polynomials. By design, the generator polynomial has consecutive roots α^c, α^c+1,..., α^c+''d−2.
Syndromes
Error location polynomial
The zeros of Λ are X''₁⁻¹,..., X_ν⁻¹. The zeros are the reciprocals of the error locations.
Once the error locations are known, the next step is to determine the error values at those locations. The error values are then used to correct the received values at those locations to recover the original codeword.
In the more general case, the error weights can be determined by solving the linear system
However, there is a more efficient method known as the Forney algorithm, which is based on Lagrange interpolation. First calculate the error evaluator polynomial
Where is the partial syndrome polynomial:
Then evaluate the error values:
The value is often called the "first consecutive root" or "fcr". Some codes select, so the expression simplifies to:

Formal derivative

Λ' is the formal derivative of the error locator polynomial Λ:
In the above expression, note that i is an integer, and λ_i would be an element of the finite field. The operator ⋅ represents ordinary multiplication which is the same as the finite field's multiplication operator, i.e.
For instance, in characteristic 2, according as i is even or odd.

Derivation

Lagrange interpolation
gives a derivation of the Forney algorithm.

Erasures

Define the erasure locator polynomial
Where the erasure locations are given by j_i. Apply the procedure described above, substituting Γ for Λ.
If both errors and erasures are present, use the error-and-erasure locator polynomial