#======================================================================
#                    . I N T E R P . L I N E A R 
#                    doc: Wed Nov 22 21:01:09 2000
#                    dlm: Fri Sep 23 16:32:20 2011
#                    (c) 2000 A.M. Thurnherr
#                    uE-Info: 62 0 NIL 0 0 72 2 2 4 NIL ofnI
#======================================================================

# linear interpolation

# HISTORY:
# 	Nov 22, 2000: - created
#	Jul 28, 2006: - Version 3.3 [HISTORY]
#				  - added xf to ISInit() args
#	Aug 22, 2006: - modified to allow use with [match]
#	Aug  5, 2008: - added idr param to IS_init()
#	Sep 23, 2011: - added support for xfnr==-1 (%RECNO)

# NOTES:
#	- the [.interp.*] routines assume that x increases strictly monotonically;
#	  for utilities dealing with non-monotonic data, use [.nminterp.*] instead
#	- the [.interp.*] routines are written to work on multiple buffers,
#	  rather than just @ants_; this implies that data created by IS_init()
#	  must be stored separately for each buffer

#++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
#
# THE FOLLOWING VARIABLES MUST BE SET GLOBALLY (i.e. during loading)
#
#	$IS_opts			string of allowed options
#	$IS_optsUsage		usage information string for options
#
#++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

$IS_opts = "";
$IS_optsUsage = "";

#++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
#
# &IS_usage() 	mangle parameters (options, really)
#
#++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

sub IS_usage() {}

#+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
#
# &IS_init(br,f,xf)			init interpolation of field f
#		br					data buffer reference
#		idr					init-data reference
#       f					field number
#		xf					x field number or -1 for %RECNO
#       <ret val>           none
#
#++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

sub IS_init($$$$) {}

#+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
#
# &IS_interpolate(bR,idR,xf,xv,xi,f)	interpolate field f
#		bR							data buffer reference
#		idR							init-data reference
#		xf							x field or -1 for %RECNO
#		xv							x value
#		xi							index of last record with x-value <= x
#		f							field number to interpolate
#		<ret val>					interpolated value
#
# NB:
#	- handle f == xf
#	- return NaN if any of the y values required is NaN
#
#++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

sub IS_interpolate($$$$$$)
{
	my($bR,$idR,$xf,$xv,$xi,$f) = @_;
	return $xv if ($xf == $f);
	return nan unless (numberp($bR->[$xi][$f]) && numberp($bR->[$xi+1][$f]));

	my($sc) = ($xf < 0)
			? ($xv - $bR->[$xi][$xf]) / ($bR->[$xi+1][$xf] - $bR->[$xi][$xf]);
			: ($xv - $bR->[$xi][$xf]) / ($bR->[$xi+1][$xf] - $bR->[$xi][$xf]);
	return $bR->[$xi][$f] + $sc * ($bR->[$xi+1][$f] - $bR->[$xi][$f]);
}

#----------------------------------------------------------------------

1;