From c43e58792cb87be889e4bdc032bc015a61ac57e9 Mon Sep 17 00:00:00 2001
From: Steven Franke <s.j.franke@icloud.com>
Date: Sun, 26 Apr 2020 14:45:39 -0500
Subject: [PATCH] Work-in-progress hybrid bp/osd decoder for FT8.

---
 lib/ft8/decode174_91.f90 | 130 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 130 insertions(+)
 create mode 100644 lib/ft8/decode174_91.f90

diff --git a/lib/ft8/decode174_91.f90 b/lib/ft8/decode174_91.f90
new file mode 100644
index 000000000..9538141d2
--- /dev/null
+++ b/lib/ft8/decode174_91.f90
@@ -0,0 +1,130 @@
+subroutine decode174_91(llr,apmask,maxiterations,message77,cw,nharderror,iter,ncheck,dmin,isuper)
+!
+! A hybrid bp/osd decoder for the (174,91) code.
+!
+use iso_c_binding, only: c_loc,c_size_t
+use crc
+integer, parameter:: N=174, K=91, M=N-K
+integer*1 cw(N),apmask(N)
+integer*1 decoded(K)
+integer*1 message77(77)
+integer nrw(M),ncw
+integer Nm(7,M)   
+integer Mn(3,N)  ! 3 checks per bit
+integer synd(M)
+real tov(3,N)
+real toc(7,M)
+real tanhtoc(7,M)
+real zn(N),zsum(N)
+real llr(N)
+real Tmn
+
+include "ldpc_174_91_c_reordered_parity.f90"
+
+decoded=0
+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
+
+maxsuper=5
+maxiterations=1
+
+zsum=0.0
+do isuper=1,maxsuper
+
+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
+! 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
+!   if( mod(synd(i),2) .ne. 0 ) write(*,*) 'check ',i,' unsatisfied'
+  enddo
+! write(*,*) 'number of unsatisfied parity checks ',ncheck
+  if( ncheck .eq. 0 ) then ! we have a codeword - if crc is good, return it
+    decoded=cw(1:K)
+    call chkcrc14a(decoded,nbadcrc)
+    nharderror=count( (2*cw-1)*llr .lt. 0.0 )
+    if(nbadcrc.eq.0) then
+      message77=decoded(1:77)
+dmin=0.0
+      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
+
+      return
+    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(1:7,i)=tanh(-toc(1:7,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
+  ndeep=1
+llr=zsum
+  call osd174_91(llr,apmask,ndeep,message77,cw,nharderror,dmin)
+  if(nharderror.gt.0) then
+    return
+  endif
+enddo   ! super iterations
+nharderror=-1
+return
+end subroutine decode174_91