-- $Id: xss_distance_01.lua,v 1.5 1998/04/29 21:17:23 dj Exp dj $
--
--  xss_distance: returns the distance from an XSS source to CAP
--  datum A in mm.  upon error it calls error() with an appropriate
--  message.
--
--  Parameters:
--
--    For EIPS, HIREFS, and PIGS sources call the function as either
--         d = xss_distance( 'EIPS' )
--         d = xss_distance( 'PIGS' )
--    or
--         d = xss_distance( 'HIREFS' )
--
--    For the DCM source, if you know the glancing angle,
--    call it as
--         d = xss_distance( 'DCM', ga )
--
--    If you don't know the glancing angle, you'll need to specify
--    the crystal and either the requested energy or wavelength.
--    The crystal is one of 'TAP', 'Ge', or 'Al'. The energy is
--    in keV, and the wavelength is in Angstroms.  
--
--         d = xss_distance( 'DCM', crystal, 'energy', energy )
--         d = xss_distance( 'DCM', crystal, 'wavelength', wavelength )
--
--
--  All constants and formulae are taken from 
--	 http://wwwastro.msfc.nasa.gov/xray/xraycal/distance.htm
--  on Wed Apr 29 15:28:49 EDT 1998. It is archived in distance.htm
--
--  Author: D. Jerius

function xss_distance( source, ... )

  local distance, func
  local EIPS_distance, PIGS_distance, DCM_45_distance
  local spacing, node2datumA

  -- distances to HRMA node [m]
  EIPS_distance = 527.522
  PIGS_distance = 527.895
  HIREFS_distance = 527.414
  DCM_45_distance = 528.439

  -- crystal lattice spacing [Angstrom]
  spacing = { TAP = 12.8784, Ge = 3.26643, Si = 1.35776 }

  -- distance from HRMA node to CAP datum A [m]
  node2datumA = -0.018

  func = 'xss_distance: '

  if ( source == 'EIPS' ) then
    distance = EIPS_distance
  
  elseif ( source == 'PIGS' ) then
    distance = PIGS_distance

  elseif ( source == 'HIREFS' ) then
    distance = HIREFS_distance

  elseif ( source == 'DCM' ) then
    local ga

    -- figure out how this is being called

    -- just the glancing angle
    if ( 1 == arg.n and 'number' == type(arg[1]) ) then
      ga = arg[1]

    -- must be specifying a crystal
    elseif ( nil ~= spacing[arg[1]] ) then
   
      local wavelength
      local unit = arg[2]
      local value = arg[3]
      local crystal = arg[1]

      -- energy or wavelength?

      if ( 'energy' == unit ) then
        local hc = 12.3980   -- planck's constant * speed of light in keV Angstroms
	if ( 'number' ~= type( value ) ) then
	  error( func .. 'illegal value for energy: ' .. value )
        end
        wavelength = hc / value

      elseif (  'wavelength' == unit ) then
	if ( 'number' ~= type( value ) ) then
	  error( func .. 'illegal value for wavelength: ' .. value )
        end
	wavelength = value

      else
        error ( func .. 'unknown DCM unit: ' .. unit )
      end

      ga = asin( wavelength / spacing[crystal] / 2 )
 
    else
      error( func .. "unknown DCM option: " .. arg[1])
    end

    distance  = DCM_45_distance + 0.030 * ( 1/sin(2 * ga) - cos(2 * ga)/sin(2 * ga) - 1 )

  end

  return 1000 * ( distance + node2datumA )

end


$if nil

print ( xss_distance( 'EIPS' ) )
print ( xss_distance( 'PIGS' ) )

print ( xss_distance( 'DCM', 'TAP', 'energy', 2.315 ) )
print ( xss_distance( 'DCM', 'Ge',  'energy', 9.128 ) )
print ( xss_distance( 'DCM', 'Si',  'energy', 21.959 ) )

print ( xss_distance( 'DCM', 'TAP', 'energy', 0.819 ) )
print ( xss_distance( 'DCM', 'Ge',  'energy', 3.229 ) )
print ( xss_distance( 'DCM', 'Si',  'energy', 7.768 ) )

print ( xss_distance( 'DCM', 12 ) )
print ( xss_distance( 'DCM', 'foo' ) )
print ( xss_distance( 'DCM', 'Ge', 'foo' ) )
print ( xss_distance( 'DCM', 'Ge', 'energy', 'foo' ) )
print ( xss_distance( 'DCM', 'Ge', 'wavelength', 'foo' ) )

$end