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# Copyright(c) 1986 Association of Universities for Research in Astronomy Inc.
define LOGPTR 32 # log2(maxpts) (4e9)
# QSORT -- General quicksort for arbitrary objects. X is an integer array
# indexing the array to be sorted. The user supplied COMPARE function is used
# to compare objects indexed by X:
#
# -1,0,1 = compare (x1, x2)
#
# where the value returned by COMPARE has the following significance:
#
# -1 obj[x1] < obj[x2]
# 0 obj[x1] == obj[x2]
# 1 obj[x1] > obj[x2]
#
# QSORT reorders the elements of the X array, which must be of type integer.
# **NOTE** - See also gqsort.x, a more recent version of this routine.
procedure qsort (x, nelem, compare)
int x[ARB] # array to be sorted
int nelem # number of elements in array
extern compare() # function to be called to compare elements
int i, j, k, lv[LOGPTR], p, pivot, uv[LOGPTR], temp
define swap {temp=$1;$1=$2;$2=temp}
int compare()
begin
lv[1] = 1
uv[1] = nelem
p = 1
while (p > 0) {
if (lv[p] >= uv[p]) # only one elem in this subset
p = p - 1 # pop stack
else {
# Dummy loop to trigger the optimizer.
do p = p, ARB {
i = lv[p] - 1
j = uv[p]
# Select as the pivot the element at the center of the
# subfile, to avoid quadratic behavior on an already
# sorted list.
k = (lv[p] + uv[p]) / 2
swap (x[j], x[k])
pivot = x[j] # pivot line
while (i < j) {
for (i=i+1; compare (x[i], pivot) < 0; i=i+1)
;
for (j=j-1; j > i; j=j-1)
if (compare (x[j], pivot) <= 0)
break
if (i < j) # out of order pair
swap (x[i], x[j]) # interchange elements
}
j = uv[p] # move pivot to position i
swap (x[i], x[j]) # interchange elements
if (i-lv[p] < uv[p] - i) { # stack so shorter done first
lv[p+1] = lv[p]
uv[p+1] = i - 1
lv[p] = i + 1
} else {
lv[p+1] = i + 1
uv[p+1] = uv[p]
uv[p] = i - 1
}
break
}
p = p + 1 # push onto stack
}
}
end
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