/*
Compute series approximations for Transverse Mercator Projection

Copyright (c) Charles Karney (2009-2010) <charles@karney.com> and
licensed under the MIT/X11 License.  For more information, see
http://geographiclib.sourceforge.net/

Reference:

   Charles F. F. Karney,
   Transverse Mercator with an accuracy of a few nanometers,
   J. Geodesy 85(8), 475-485 (Aug. 2011).
   DOI 10.1007/s00190-011-0445-3
   preprint http://arxiv.org/abs/1002.1417
   resource page http://geographiclib.sf.net/tm.html

Compute coefficient for forward and inverse trigonometric series for
conversion from conformal latitude to rectifying latitude.  This prints
out assignments which with minor editing are suitable for insertion into
C++ code.  (N.B. n^3 in the output means n*n*n; 3/5 means 0.6.)

To run, start maxima and enter

    writefile("tmseries.out")$
    load("tmseries.mac")$
    closefile()$

With maxpow = 6, the output (after about 5 secs) is

A=a/(n+1)*(1+n^2/4+n^4/64+n^6/256);
alpha[1]=n/2-2*n^2/3+5*n^3/16+41*n^4/180-127*n^5/288+7891*n^6/37800;
alpha[2]=13*n^2/48-3*n^3/5+557*n^4/1440+281*n^5/630-1983433*n^6/1935360;
alpha[3]=61*n^3/240-103*n^4/140+15061*n^5/26880+167603*n^6/181440;
alpha[4]=49561*n^4/161280-179*n^5/168+6601661*n^6/7257600;
alpha[5]=34729*n^5/80640-3418889*n^6/1995840;
alpha[6]=+212378941*n^6/319334400;
beta[1]=n/2-2*n^2/3+37*n^3/96-n^4/360-81*n^5/512+96199*n^6/604800;
beta[2]=n^2/48+n^3/15-437*n^4/1440+46*n^5/105-1118711*n^6/3870720;
beta[3]=17*n^3/480-37*n^4/840-209*n^5/4480+5569*n^6/90720;
beta[4]=4397*n^4/161280-11*n^5/504-830251*n^6/7257600;
beta[5]=4583*n^5/161280-108847*n^6/3991680;
beta[6]=+20648693*n^6/638668800;

Notation of output matches that of

 L. Krueger,
 Konforme Abbildung des Erdellipsoids in der Ebene
 Royal Prussian Geodetic Institute, New Series 52, 172 pp. (1912).

with gamma replaced by alpha.

Alter maxpow to generate more or less terms for the series
approximations to the forward and reverse projections.  This has been
tested out to maxpow = 30; but this takes a long time (see below).

*/

/* Timing:
  maxpow  time
    4      2s
    6      5s
    8     11s
   10     24s
   12     52s
   20    813s = 14m
   30  13535s = 226m
*/

maxpow:6$ /* Max power for forward and reverse projections */
/* Notation

   e = eccentricity
   e2 = e^2 = f*(2-f)
   n = third flattening = f/(2-f)
   phi = (complex) geodetic latitude
   zetap = Gauss-Schreiber TM = complex conformal latitude
   psi = Mercator = complex isometric latitude
   zeta = scaled UTM projection = complex rectifying latitude
*/

taylordepth:6$
triginverses:'all$
/*
    revert
       var2 = expr(var1) = series in eps
    to
       var1 = revertexpr(var2) = series in eps

Require that expr(var1) = var1 to order eps^0.  This throws in a
trigreduce to convert to multiple angle trig functions.
*/
reverta(expr,var1,var2,eps,pow):=block([tauacc:1,sigacc:0,dsig],
  dsig:ratdisrep(taylor(expr-var1,eps,0,pow)),
  dsig:subst([var1=var2],dsig),
  for n:1 thru pow do (tauacc:trigreduce(ratdisrep(taylor(
    -dsig*tauacc/n,eps,0,pow))),
    sigacc:sigacc+expand(diff(tauacc,var2,n-1))),
  var2+sigacc)$


/* Expansion for atan(x+eps) for small eps.  Equivalent to
   taylor(atan(x + eps), eps, 0, maxpow) but tidied up a bit.
 */
atanexp(x,eps):=''(ratdisrep(taylor(atan(x+eps),eps,0,maxpow)))$

/* Convert from n to e^2 */
e2:4*n/(1+n)^2$

/* zetap in terms of phi.  The expansion of
   atan(sinh( asinh(tan(phi)) + e * atanh(e * sin(phi)) ))
*/
zetap_phi:block([psiv,tanzetap,zetapv,qq,e],
    /* Here qq = atanh(sin(phi)) = asinh(tan(phi)) */
    psiv:qq-e*atanh(e*tanh(qq)),
    psiv:subst([e=sqrt(e2),qq=atanh(sin(phi))],
      ratdisrep(taylor(psiv,e,0,2*maxpow)))
    +asinh(sin(phi)/cos(phi))-atanh(sin(phi)),
    tanzetap:subst([abs(cos(phi))=cos(phi),sqrt(sin(phi)^2+cos(phi)^2)=1],
      ratdisrep(taylor(sinh(psiv),n,0,maxpow)))+tan(phi)-sin(phi)/cos(phi),
    zetapv:atanexp(tan(phi),tanzetap-tan(phi)),
    zetapv:subst([cos(phi)=sqrt(1-sin(phi)^2),
      tan(phi)=sin(phi)/sqrt(1-sin(phi)^2)],
      (zetapv-phi)/cos(phi))*cos(phi)+phi,
    zetapv:ratdisrep(taylor(zetapv,n,0,maxpow)),
    expand(trigreduce(zetapv)))$

/* phi in terms of zetap */
phi_zetap:reverta(zetap_phi,phi,zetap,n,maxpow)$

/* Mean radius of meridian */
a1:expand(integrate(
      ratdisrep(taylor((1+n)*(1-e2)/(1-e2*sin(phi)^2)^(3/2),
          n,0,maxpow)),
      phi, 0, %pi/2)/(%pi/2))/(1+n)$

/* zeta in terms of phi.  The expansion of
   zeta = pi/(2*a1) * int( (1-e^2)/(1-e^2*sin(phi)^2)^(3/2) )
*/
zeta_phi:block([zetav],
    zetav:integrate(trigreduce(ratdisrep(taylor(
            (1-e2)/(1-e2*sin(phi)^2)^(3/2)/a1,
            n,0,maxpow))),phi),
    expand(zetav))$

/* phi in terms of zeta */
phi_zeta:reverta(zeta_phi,phi,zeta,n,maxpow)$

/* zeta in terms of zetap */
/* This is slow.  The next version speeds it up a little.
zeta_zetap:expand(trigreduce(ratdisrep(
        taylor(subst([phi=phi_zetap],zeta_phi),n,0,maxpow))))$
*/
zeta_zetap:block([phiv:phi_zetap,zetav:expand(zeta_phi),acc:0],
    for i:0 thru maxpow do (
      phiv:ratdisrep(taylor(phiv,n,0,maxpow-i)),
      acc:acc + expand(n^i * trigreduce(ratdisrep(taylor(
              subst([phi=phiv],coeff(zetav,n,i)),n,0,maxpow-i))))),
    acc)$

/* zetap in terms of zeta */
/* This is slow.  The next version speeds it up a little.
zetap_zeta:expand(trigreduce(ratdisrep(
        taylor(subst([phi=phi_zeta],zetap_phi),n,0,maxpow))))$
*/
zetap_zeta:block([phiv:phi_zeta,zetapv:expand(zetap_phi),acc:0],
    for i:0 thru maxpow do (
      phiv:ratdisrep(taylor(phiv,n,0,maxpow-i)),
      acc:acc + expand(n^i * trigreduce(ratdisrep(taylor(
              subst([phi=phiv],coeff(zetapv,n,i)),n,0,maxpow-i))))),
    acc)$

printa1():=block([],
  print(concat("A=",string(a/(n+1)),"*(",
      string(taylor(a1*(1+n),n,0,maxpow)),");")),
  0)$
printtxf():=block([alpha:zeta_zetap,t],
  for i:1 thru maxpow do (t:coeff(alpha,sin(2*i*zetap)),
    print(concat("alpha[",i,"]=",string(taylor(t,n,0,maxpow)),";")),
    alpha:alpha-expand(t*sin(2*i*zetap))),
/* should return zero */
  alpha:alpha-zetap)$
printtxr():=block([beta:zetap_zeta,t],
  for i:1 thru maxpow do (t:coeff(beta,sin(2*i*zeta)),
    print(concat("beta[",i,"]=",string(taylor(-t,n,0,maxpow)),";")),
    beta:beta-expand(t*sin(2*i*zeta))),
/* should return zero */
  beta:beta-zeta)$

printseries():=[printa1(),printtxf(),printtxr()]$

/* Define array functions which are be read by tm.mac */
defarrayfuns():=block([aa:a1*(1+n),alpha:zeta_zetap,beta:zetap_zeta,t],
  for i:1 thru maxpow do (
    define(a1_a[i](n),ratdisrep(taylor(aa,n,0,i))/(1+n)),
    t:expand(ratdisrep(taylor(alpha,n,0,i))),
    define(zeta_a[i](zp,n),
      zp+sum(coeff(t,sin(2*k*zetap))*sin(2*k*zp),k,1,i)),
    t:diff(t,zetap),
    define(zeta_d[i](zp,n),
      1+sum(coeff(t,cos(2*k*zetap))*cos(2*k*zp),k,1,i)),
    t:expand(ratdisrep(taylor(beta,n,0,i))),
    define(zetap_a[i](z,n),
      z+sum(coeff(t,sin(2*k*zeta))*sin(2*k*z),k,1,i)),
    t:diff(t,zeta),
    define(zetap_d[i](z,n),
      1+sum(coeff(t,cos(2*k*zeta))*cos(2*k*z),k,1,i))))$

printseries()$
/*
defarrayfuns()$
save("tmseries.lsp",maxpow,arrays)$
*/
