subroutine binsrt(x,y,rw,npd,ind,rind,tx,ty,ilst,nerror)
# Sort the data points into bins.
# Called by master.
implicit double precision(a-h,o-z)
dimension x(npd), y(npd), tx(npd), ty(npd)
integer rind(npd)
dimension ind(npd), ilst(npd)
dimension rw(4)
nerror = -1
kdiv = int(1+dble(npd)**0.25) # Round high.
xkdiv = dble(kdiv)
# Dig out the corners of the rectangular window.
xmin = rw(1)
xmax = rw(2)
ymin = rw(3)
ymax = rw(4)
w = xmax-xmin
h = ymax-ymin
# Number of bins is to be approx. sqrt(npd); thus number of subdivisions
# on each side of rectangle is approx. npd**(1/4).
dw = w/xkdiv
dh = h/xkdiv
# The width of each bin is dw; the height is dh. We shall move across
# the rectangle from left to right, then up, then back from right to
# left, then up, .... Note that kx counts the divisions from the left,
# ky counts the divisions from the bottom; kx is incremented by ink, which
# is +/- 1 and switches sign when ky is incremented; ky is always
# incremented by 1.
kx = 1
ky = 1
ink = 1
k = 0
do i = 1,npd { ilst(i) = 0 } # Keeps a list of those points already added
while(ky<=kdiv) { # to the new list.
do i = 1,npd {
if(ilst(i)==1) next # The i-th point has already been added
# to the new list.
# If the i-th point is in the current bin, add it to the list.
xt = x(i)
yt = y(i)
ix = int(1+(xt-xmin)/dw)
if(ix>kdiv) ix = kdiv
jy = int(1+(yt-ymin)/dh)
if(jy>kdiv) jy = kdiv
if(ix==kx&jy==ky) {
k = k+1
ind(i) = k # Index i is the pos'n. of (x,y) in the
rind(k) = i # old list; k is its pos'n. in the new one.
tx(k) = xt
ty(k) = yt
ilst(i) = 1 # Cross the i-th point off the old list.
}
}
# Move to the next bin.
kc = kx+ink
if((1<=kc)&(kc<=kdiv)) kx = kc
else {
ky = ky+1
ink = -ink
}
}
# Check that all points from old list have been added to the new,
# with no spurious additions.
if(k!=npd) {
nerror = 2
return
}
# Copy the new sorted vector back on top of the old ones.
do i = 1,npd {
x(i) = tx(i)
y(i) = ty(i)
}
return
end
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