#======================================================================

#                    . 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;