WSJT-X/lib/fst4/decode240_74.f90

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subroutine decode240_74(llr,Keff,maxosd,norder,apmask,message74,cw,ntype,nharderror,dmin)
!
! A hybrid bp/osd decoder for the (240,74) code.
!
! maxosd<0: do bp only
! maxosd=0: do bp and then call osd once with channel llrs
! maxosd>1: do bp and then call osd maxosd times with saved bp outputs
! norder : osd decoding depth
!
integer, parameter:: N=240, K=74, M=N-K
integer*1 cw(N),apmask(N)
integer*1 nxor(N),hdec(N)
integer*1 message74(74),m74(74)
integer nrw(M),ncw
integer Nm(5,M)
integer Mn(3,N) ! 3 checks per bit
integer synd(M)
real tov(3,N)
real toc(5,M)
real tanhtoc(5,M)
real zn(N),zsum(N),zsave(N,3)
real llr(N)
real Tmn
include "ldpc_240_74_parity.f90"
maxiterations=30
nosd=0
if(maxosd.gt.3) maxosd=3
if(maxosd.eq.0) then ! osd with channel llrs
nosd=1
zsave(:,1)=llr
elseif(maxosd.gt.0) then !
nosd=maxosd
elseif(maxosd.lt.0) then ! just bp
nosd=0
endif
toc=0
tov=0
tanhtoc=0
! initialize messages to checks
do j=1,M
do i=1,nrw(j)
toc(i,j)=llr((Nm(i,j)))
enddo
enddo
ncnt=0
nclast=0
zsum=0.0
do iter=0,maxiterations
! Update bit log likelihood ratios (tov=0 in iteration 0).
do i=1,N
if( apmask(i) .ne. 1 ) then
zn(i)=llr(i)+sum(tov(1:ncw,i))
else
zn(i)=llr(i)
endif
enddo
zsum=zsum+zn
if(iter.gt.0 .and. iter.le.maxosd) then
zsave(:,iter)=zsum
endif
! Check to see if we have a codeword (check before we do any iteration).
cw=0
where( zn .gt. 0. ) cw=1
ncheck=0
do i=1,M
synd(i)=sum(cw(Nm(1:nrw(i),i)))
if( mod(synd(i),2) .ne. 0 ) ncheck=ncheck+1
enddo
if( ncheck .eq. 0 ) then ! we have a codeword - if crc is good, return it
m74=0
m74(1:74)=cw(1:74)
call get_crc24(m74,74,nbadcrc)
if(nbadcrc.eq.0) then
message74=cw(1:74)
hdec=0
where(llr .ge. 0) hdec=1
nxor=ieor(hdec,cw)
nharderror=sum(nxor)
dmin=sum(nxor*abs(llr))
ntype=1
return
endif
endif
if( iter.gt.0 ) then ! this code block implements an early stopping criterion
! if( iter.gt.10000 ) then ! this code block implements an early stopping criterion
nd=ncheck-nclast
if( nd .lt. 0 ) then ! # of unsatisfied parity checks decreased
ncnt=0 ! reset counter
else
ncnt=ncnt+1
endif
! write(*,*) iter,ncheck,nd,ncnt
if( ncnt .ge. 5 .and. iter .ge. 10 .and. ncheck .gt. 15) then
nharderror=-1
exit
endif
endif
nclast=ncheck
! Send messages from bits to check nodes
do j=1,M
do i=1,nrw(j)
ibj=Nm(i,j)
toc(i,j)=zn(ibj)
do kk=1,ncw ! subtract off what the bit had received from the check
if( Mn(kk,ibj) .eq. j ) then
toc(i,j)=toc(i,j)-tov(kk,ibj)
endif
enddo
enddo
enddo
! send messages from check nodes to variable nodes
do i=1,M
tanhtoc(1:5,i)=tanh(-toc(1:5,i)/2)
enddo
do j=1,N
do i=1,ncw
ichk=Mn(i,j) ! Mn(:,j) are the checks that include bit j
Tmn=product(tanhtoc(1:nrw(ichk),ichk),mask=Nm(1:nrw(ichk),ichk).ne.j)
call platanh(-Tmn,y)
! y=atanh(-Tmn)
tov(i,j)=2*y
enddo
enddo
enddo ! bp iterations
do i=1,nosd
zn=zsave(:,i)
call osd240_74(zn,Keff,apmask,norder,message74,cw,nharderror,dminosd)
if(nharderror.gt.0) then
hdec=0
where(llr .ge. 0) hdec=1
nxor=ieor(hdec,cw)
dmin=sum(nxor*abs(llr))
ntype=2
return
endif
enddo
ntype=0
nharderror=-1
dminosd=0.0
return
end subroutine decode240_74