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PlmBar.f95
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PlmBar.f95
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subroutine PlmBar(p, lmax, z, csphase, cnorm, exitstatus)
!------------------------------------------------------------------------------
!
! This function evalutates all of the normalized associated Legendre
! functions up to degree lmax. The functions are initially scaled by
! 10^280 sin^m in order to minimize the effects of underflow at large m
! near the poles (see Holmes and Featherstone 2002, J. Geodesy, 76, 279-299).
! On a macOS system with a maximum allowable double precision value of
! 2.225073858507203E-308 the scaled portion of the algorithm will not overflow
! for degrees less than or equal to 2800.
!
! For each value of m, the rescaling factor is computed as
! rescalem=rescalem*sin(theta), with the intial value of rescalem being equal
! to 1/scalef (which is here equal to 10^280). This will gradually reduce this
! huge number to a tiny number, and will ultimately underflow. In order to
! prevent this underflow, when rescalem becomes less than 10^(-280), the
! subsequent rescaling factors of sin(theta) will be directly applied to Plm,
! and then this number will be multipled by the old value of rescalem.
!
! Temporary variables are saved in an allocated array. In order to explicitly
! deallocate this memory, call this routine with a spherical harmonic degree
! of -1.
!
! Calling Parameters
!
! OUT
! p A vector of all associated Legendgre polynomials.
! evaluated at z up to lmax. The length must by greater
! or equal to (lmax+1)*(lmax+2)/2.
!
! OPTIONAL (IN)
! csphase 1: Do not include the phase factor of (-1)^m (default).
! -1: Apply the phase factor of (-1)^m.
! cnorm 0: Use real normalization.
! 1: Use complex normalization.
!
! IN
! lmax Maximum spherical harmonic degree to compute.
! z cos(colatitude) or sin(latitude).
!
! OPTIONAL (OUT)
! exitstatus If present, instead of executing a STOP when an error
! is encountered, the variable exitstatus will be
! returned describing the error.
! 0 = No errors;
! 1 = Improper dimensions of input array;
! 2 = Improper bounds for input variable;
! 3 = Error allocating memory;
! 4 = File IO error.
!
! Notes:
!
! 1. The employed normalization is the "geophysical convention." The integral
! of (PlmBar*cos(m theta))**2 or (PlmBar*sin (m theta))**2 over all
! space is 4 pi.
! 2. The integral of PlmBar**2 over (-1,1) is 2 * (2 - delta(0,m)).
! If CNORM=1, then this is equal to 2.
! 3. The index of the array p corresponds to l*(l+1)/2 + m + 1. As such
! the array p should be dimensioned as (lmax+1)*(lmax+2)/2 in the
! calling routine.
! 4. The default is to exclude the Condon-Shortley phase of (-1)^m.
!
! Copyright (c) 2005-2019, SHTOOLS
! All rights reserved.
!
!------------------------------------------------------------------------------
use ftypes
implicit none
integer(int32), intent(in) :: lmax
real(dp), intent(out) :: p(:)
real(dp), intent(in) :: z
integer(int32), intent(in), optional :: csphase, cnorm
integer(int32), intent(out), optional :: exitstatus
real(dp) :: pmm, rescalem, u, scalef
real(dp), save, allocatable :: f1(:), f2(:), sqr(:)
integer(int32) :: k, kstart, m, l, astat(3)
integer(int32), save :: lmax_old = 0
integer(int32) :: phase
!$OMP threadprivate(f1, f2, sqr, lmax_old)
if (present(exitstatus)) exitstatus = 0
if (lmax == -1) then
if (allocated (sqr)) deallocate (sqr)
if (allocated (f1)) deallocate (f1)
if (allocated (f2)) deallocate (f2)
lmax_old = 0
return
end if
if (size(p) < (lmax+1)*(lmax+2)/2) then
print*, "Error --- PlmBar"
print*, "P must be dimensioned as (LMAX+1)*(LMAX+2)/2 " // &
"where LMAX is ", lmax
print*, "Input array is dimensioned ", size(p)
if (present(exitstatus)) then
exitstatus = 1
return
else
stop
end if
else if (lmax < 0) then
print*, "Error --- PlmBar"
print*, "LMAX must be greater than or equal to 0."
print*, "Input value is ", lmax
if (present(exitstatus)) then
exitstatus = 2
return
else
stop
end if
else if(abs(z) > 1.0_dp) then
print*, "Error --- PlmBar"
print*, "ABS(Z) must be less than or equal to 1."
print*, "Input value is ", z
if (present(exitstatus)) then
exitstatus = 2
return
else
stop
end if
end if
if (present(csphase)) then
if (csphase == -1) then
phase = -1
else if (csphase == 1) then
phase = 1
else
print*, "Error --- PlmBar"
print*, "CSPHASE must be 1 (exclude) or -1 (include)."
print*, "Input value is ", csphase
if (present(exitstatus)) then
exitstatus = 2
return
else
stop
end if
end if
else
phase = 1
end if
scalef = 1.0e-280_dp
if (lmax > lmax_old) then
if (allocated (sqr)) deallocate (sqr)
if (allocated (f1)) deallocate (f1)
if (allocated (f2)) deallocate (f2)
allocate (sqr(2*lmax+1), stat=astat(1))
allocate (f1((lmax+1)*(lmax+2)/2), stat=astat(2))
allocate (f2((lmax+1)*(lmax+2)/2), stat=astat(3))
if (astat(1) /= 0 .or. astat(2) /= 0 .or. astat(3) /= 0) then
print*, "Error --- PlmBar"
print*, "Problem allocating arrays SQR, F1 and F2", astat(1), &
astat(2), astat(3)
if (present(exitstatus)) then
exitstatus = 3
return
else
stop
end if
end if
!----------------------------------------------------------------------
!
! Precompute square roots of integers that are used several times.
!
!----------------------------------------------------------------------
do l = 1, 2 * lmax+1
sqr(l) = sqrt(dble(l))
end do
!----------------------------------------------------------------------
!
! Precompute multiplicative factors used in recursion relationships
! Plmbar(l,m) = x*f1(l,m)*Plmbar(l-1,m) - Plmbar(l-2,m)*f2(l,m)
! k = l*(l+1)/2 + m + 1
! Note that prefactors are not used for the case when m=l and m=l-1,
! as a different recursion is used for these two values.
!
!----------------------------------------------------------------------
k = 3
do l = 2, lmax, 1
k = k + 1
f1(k) = sqr(2*l-1) * sqr(2*l+1) / dble(l)
f2(k) = dble(l-1) * sqr(2*l+1) / sqr(2*l-3) / dble(l)
do m = 1, l-2
k = k + 1
f1(k) = sqr(2*l+1) * sqr(2*l-1) / sqr(l+m) / sqr(l-m)
f2(k) = sqr(2*l+1) * sqr(l-m-1) * sqr(l+m-1) &
/ sqr(2*l-3) / sqr(l+m) / sqr(l-m)
end do
k = k + 2
end do
lmax_old = lmax
end if
!--------------------------------------------------------------------------
!
! Calculate P(l,0). These are not scaled.
!
!--------------------------------------------------------------------------
u = sqrt((1.0_dp - z) * (1.0_dp + z)) ! sin(theta)
p(1) = 1.0_dp
if (lmax == 0) return
p(2) = sqr(3) * z
k = 2
do l = 2, lmax, 1
k = k + l
p(k) = f1(k) * z * p(k-l) - f2(k) * p(k-2*l+1)
end do
!--------------------------------------------------------------------------
!
! Calculate P(m,m), P(m+1,m), and P(l,m)
!
!--------------------------------------------------------------------------
if (present(cnorm)) then
if (cnorm == 1) then
pmm = scalef
else
pmm = sqr(2) * scalef
end if
else
pmm = sqr(2) * scalef
end if
rescalem = 1.0_dp / scalef
kstart = 1
do m = 1, lmax - 1, 1
rescalem = rescalem * u
! Calculate P(m,m)
kstart = kstart + m + 1
pmm = phase * pmm * sqr(2*m+1) / sqr(2*m)
p(kstart) = pmm
! Calculate P(m+1,m)
k = kstart + m + 1
p(k) = z * sqr(2*m+3) * pmm
! Calculate P(l,m)
do l = m + 2, lmax, 1
k = k + l
p(k) = z * f1(k) * p(k-l) - f2(k) * p(k-2*l+1)
p(k-2*l+1) = p(k-2*l+1) * rescalem
end do
p(k) = p(k) * rescalem
p(k-lmax) = p(k-lmax) * rescalem
end do
! Calculate P(lmax,lmax)
rescalem = rescalem * u
kstart = kstart + m + 1
p(kstart) = phase * pmm * sqr(2*lmax+1) / sqr(2*lmax) * rescalem
end subroutine PlmBar