WSJT-X/lib/fsk4hf/bitflip128_90.f90

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subroutine bitflip128_90(llr,message77,cw,nharderror)
!
! A hard-decision bit flipping decoder for the (128,90) code.
!
use iso_c_binding, only: c_loc,c_size_t
use crc
integer, parameter:: N=128, K=90, M=N-K
integer*1 cw(N),apmask(N)
integer*1 decoded(K)
integer*1 message77(77)
integer Nm(11,M)
integer Mn(3,N)
integer nrw(M)
integer synd(M)
integer nuns(N)
real zn(N)
real llr(N)
include "ldpc_128_90_reordered_parity.f90"
decoded=0
zn=llr
do iter=0,0
! Check to see if we have a codeword (check before we do any iteration).
cw=0
where( zn .gt. 0. ) cw=1
ncheck=0
nuns=0
do i=1,M
synd(i)=sum(cw(Nm(1:nrw(i),i)))
if( mod(synd(i),2) .ne. 0 ) then
ncheck=ncheck+1
do j=1,nrw(i)
nuns(Nm(j,i))=nuns(Nm(j,i))+1
enddo
endif
enddo
if( ncheck .eq. 0 ) then ! we have a codeword - reorder the columns and return it
decoded=cw(1:K)
call chkcrc13a(decoded,nbadcrc)
if(nbadcrc.eq.0) then
message77=decoded(1:77)
nharderror=count( (2*cw-1)*llr .lt. 0.0 )
return
endif
endif
! flip the sign on the symbols that show up in the largest number
! of un-satisfied parity checks
where( nuns .eq. maxval(nuns) ) zn=-zn
enddo
llr=zn
nharderror=-1
return
end subroutine bitflip128_90