;+
; ROUTINE:  findex
;
; PURPOSE:  Compute "floating point index" into a table using binary
;           search.  The resulting output may be used with INTERPOLATE.
;
; USAGE:    result = findex(u,v)
;
; INPUT:    
;   u       a monotonically increasing or decreasing 1-D grid
;   v       a scalar, or array of values
;
; OUTPUT:
;   result  Floating point index. Integer part of RESULT(i) gives
;           the index into to U such that V(i) is between
;           U(RESULT(i)) and U(RESULT(i)+1).  The fractional part
;           is the weighting factor
;
;                          V(i)-U(RESULT(i))
;                          ---------------------
;                     U(RESULT(i)+1)-U(RESULT(i))
;
;
; DISCUSSION: 
;           This routine is used to expedite one dimensional
;           interpolation on irregular 1-d grids.  Using this routine
;           with INTERPOLATE is much faster then IDL's INTERPOL
;           procedure because it uses a binary instead of linear
;           search algorithm.  The speedup is even more dramatic when
;           the same independent variable (V) and grid (U) are used
;           for several dependent variable interpolations.
;
;  
; EXAMPLE:  
;
;; In this example I found the FINDEX + INTERPOLATE combination
;; to be about 60 times faster then INTERPOL.
;
;  u=randomu(iseed,200000) & u=u(SORT(u))
;  v=randomu(iseed,10)     & v=v(SORT(v))
;  y=randomu(iseed,200000) & y=y(SORT(y))
;
;  t=systime(1) & y1=interpolate(y,findex(u,v)) & print,systime(1)-t
;  t=systime(1) & y2=interpol(y,u,v)            & print,systime(1)-t
;  print,f='(3(a,10f7.4/))','findex:   ',y1,'interpol: ',y2,'diff:     ',y1-y2
;
; AUTHOR:   Paul Ricchiazzi                        21 Feb 97
;           Institute for Computational Earth System Science
;           University of California, Santa Barbara
;           paul@icess.ucsb.edu
;
; REVISIONS:
;
; LICENCE: (note added by Sylwester Arabas)
;  This code was posted by Paul Ricchiazzi at the comp.land.idl-pvwave 
;  newsgroup on Feb 21 1997. The post mentions 'donating this procedure
;  to the IDL community' - it is thus regarded as 'public domain' licence
;  compatible with GNU GPL.
;
;-
;
function FINDEX, u, v, test=test
;
nu=N_ELEMENTS(u)
nv=N_ELEMENTS(v)
;
us=u-SHIFT(u,+1)
us=us(1:*)
umx=max(us,min=umn)
if umx gt 0 and umn lt 0 then MESSAGE,'u must be monotonic'
if umx gt 0 then inc=1 else inc=0
;
maxcomp=FIX(ALOG(FLOAT(nu))/ALOG(2.)+.5) 
;
; maxcomp = maximum number of binary search iteratios
;
jlim=LONARR(2,nv)
jlim(0,*)=0                     ; array of lower limits
jlim(1,*)=nu-1                  ; array of upper limits
;
iter=0
repeat begin
   jj=(jlim(0,*)+jlim(1,*))/2
   ii=WHERE(v ge u(jj),n) & if n gt 0 then jlim(1-inc,ii)=jj(ii)
   ii=WHERE(v lt u(jj),n) & if n gt 0 then jlim(inc,ii)=jj(ii)
   jdif=max(jlim(1,*)-jlim(0,*))
   if iter gt maxcomp then begin
      print, maxcomp, iter, jdif
      MESSAGE, 'binary search failed'
   endif
   iter=iter+1
endrep until jdif eq 1 
;
w=v-v
w(*)=(v-u(jlim(0,*)))/(u(jlim(0,*)+1)-u(jlim(0,*)))+jlim(0,*)
;
if KEYWORD_SET(test) then STOP
;
return, w
;
end
