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(Not necessarily expecting a merge) Unbiased Chi2 sampling in PURR #346

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(Not necessarily expecting a merge) Unbiased Chi2 sampling in PURR #346

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58 changes: 28 additions & 30 deletions src/purr.f90
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Original file line number Diff line number Diff line change
Expand Up @@ -1698,31 +1698,8 @@ subroutine ladr2(nseqz,elow,ehigh,nr,er,gnr,gfr,ggr,gxr,gt)
! internals
integer::nr0,ir,idone,ndf,i,n
real(kr)::dcon
real(kr),dimension(20,4),parameter::chisq=reshape((/&
1.31003e-3_kr,9.19501e-3_kr,.0250905e0_kr,.049254e0_kr,&
.0820892e0_kr,.124169e0_kr,.176268e0_kr,.239417e0_kr,&
.314977e0_kr,.404749e0_kr,.511145e0_kr,.637461e0_kr,&
.788315e0_kr,.970419e0_kr,1.194e0_kr,1.47573e0_kr,&
1.84547e0_kr,2.36522e0_kr,3.20371e0_kr,5.58201e0_kr,&
.0508548e0_kr,.156167e0_kr,.267335e0_kr,.38505e0_kr,&
.510131e0_kr,.643564e0_kr,.786543e0_kr,.940541e0_kr,&
1.1074e0_kr,1.28947e0_kr,1.48981e0_kr,1.71249e0_kr,&
1.96314e0_kr,2.24984e0_kr,2.58473e0_kr,2.98744e0_kr,&
3.49278e0_kr,4.17238e0_kr,5.21888e0_kr,7.99146e0_kr,&
.206832e0_kr,.470719e0_kr,.691933e0_kr,.901674e0_kr,&
1.10868e0_kr,1.31765e0_kr,1.53193e0_kr,1.75444e0_kr,&
1.98812e0_kr,2.23621e0_kr,2.50257e0_kr,2.79213e0_kr,&
3.11143e0_kr,3.46967e0_kr,3.88053e0_kr,4.36586e0_kr,&
4.96417e0_kr,5.75423e0_kr,6.94646e0_kr,10.0048e0_kr,&
.459462e0_kr,.893735e0_kr,1.21753e0_kr,1.50872e0_kr,&
1.78605e0_kr,2.05854e0_kr,2.33194e0_kr,2.61069e0_kr,&
2.89878e0_kr,3.20032e0_kr,3.51995e0_kr,3.86331e0_kr,&
4.23776e0_kr,4.65345e0_kr,5.12533e0_kr,5.67712e0_kr,&
6.35044e0_kr,7.22996e0_kr,8.541e0_kr,11.8359e0_kr/),(/20,4/))
real(kr),parameter::start=19.9999e0_kr
real(kr),parameter::zero=0.e0_kr
real(kr)::rnsum
rnsum=0

!--select resonance parameters
!--until resonance energy is above ehi
Expand Down Expand Up @@ -1753,16 +1730,13 @@ subroutine ladr2(nseqz,elow,ehigh,nr,er,gnr,gfr,ggr,gxr,gt)
! choose neutron width for ndn degrees of freedom
ndf=ndfn(i)
if (ndf.gt.0) gn=gn/ndf
n=int(1+start*rann(kk))
rnsum=rnsum+n
xr=chisq(n,ndf)
xr=chisq(kk,ndf)
gn=gn*xr
! choose fission width for ndf degrees of freedom
if (cgf(i).ne.zero.and.ndff(i).ne.0) then
ndf=ndff(i)
if (ndf.gt.0) gf=gf/ndf
n=int(1+start*rann(kk))
xr=chisq(n,ndf)
xr=chisq(kk,ndf)
gf=gf*xr
endif
! choose competitive width for ndx degrees of freedom
Expand All @@ -1771,8 +1745,7 @@ subroutine ladr2(nseqz,elow,ehigh,nr,er,gnr,gfr,ggr,gxr,gt)
if (ndf.gt.0) gx=gx/ndf
dn=20
dn=dn-dn/10000
n=int(1+dn*rann(kk))
xr=chisq(n,ndf)
xr=chisq(kk,ndf)
gx=gx*xr
endif
gt(ir)=gn+gf+gg+gx
Expand Down Expand Up @@ -2891,4 +2864,29 @@ real(kr) function rann(idum)
return
end function rann

! generates one sample from a standard normal distribution
! note that we only use the first from the Box-Muller because it
! seems there is potentially a little correlation between two
! consecutive samples from the PURR RNG?
real(kr) function gauss(idum)
integer,intent(inout)::idum
real(kr)::u1,u2,z1,z2
real(kr), parameter :: pi = 3.141592653589793
u1=rann(idum)
u2=rann(idum)
gauss = sqrt(-2.0_kr * log(u1)) * cos(2.0_kr * pi * u2)
end function

real(kr) function chisq(idum, dof)
integer,intent(inout)::idum
integer,intent(in)::dof
integer::j
real(kr)::z
chisq=0.0_kr
do j=1,dof
z=gauss(idum)
chisq=chisq + z*z
enddo
end function chisq

end module purm