An element is selected from the sequence and called the pivot. The sequence is then re-arranged so all elements with a value less than the pivot are before the pivot and all elements with a greater value are after it.
The sub-sequences before and after the pivot are then recursively sorted.
Implementation in Redcode:
org qsort
stack equ outer-2
pivot equ outer-1
outer mov p, <stack
mov {p, pivot
inner slt pivot, >p
y djn inner, #LENGTH-1
jmz done, y
mov <p, *p
mov {p, @p
djn inner, y
done mov.a @stack, p
mov.ba p, @stack
mov pivot, >p
qsort mov.ab p, y
sub.b p, y
slt y, #2
jmp outer, <y
mov.f >stack, p
jmn qsort, p
p dat FIRST+LENGTH, FIRST
