Wednesday, 27 February 2008

selection sort

Selection sort in an algorithm which sorts a sequence of elements. The smallest element is removed from the sequence and added to the end of a new sorted sequence. This continues until all elements have been moved to the new sequence.

Selection sort is an O(n²) algorithm. The redcode implemention is 10 instructions long and requires 1.5n²+5.5n-8 cycles in the worst case, making it approximately twice as fast as Bubble Sort.

Implementation in Redcode:
          org    outer+1

temp equ (outer-1)

outer mov.b x, y
mov <p, temp
inner slt <p, *p p, p
y djn inner, #LENGTH-1
add.b x, p
mov *p, @p
mov temp, *p
x djn outer, #LENGTH-1

Monday, 25 February 2008

Bubble sort

Bubble sort is an O(n²) algorithm to sort a sequence of elements into order. Ilmari Karonen's Bubbly Sort 1b is a 10 line implementation of Bubble sort which requires 3.5n²-0.5n-4 cycles in the worst case.

My alternative code is shown below:

Implementation in Redcode:
          org    inner

temp equ (outer-1)

outer mov.b x, y
mov.f q, p

inner slt {p, <p
y djn inner, #LENGTH-1
jmz next, y

mov @p, temp
mov temp, *p
djn inner, y
next djn outer, #LENGTH-1

This alternative implementation runs slightly faster than Ilmari's, at the cost of 1 extra line of code. The number of cycles in the worst case is 3n²-5, which also beats Gnome sort.

Tuesday, 19 February 2008

Comb sort

Comb sort (also known as Dobosiewicz sort) makes a number of passes through a sequence of elements, exchanging any elements which are out of order. With each pass the distance between compared elements decreases. The algorithm terminates when all elements are in order.

In the redcode implementation, the value of the temp element is used as a flag to determine whether or not any exchanges occurred in the last pass.

Implementation in Redcode:
          ra     equ 4
rb equ 5

org inner

temp equ (outer-1)

outer mov #LENGTH, y
sub.b gap, y
mov.f q, p
sub.b gap, p
sub temp, temp

inner slt {p, <p
y djn inner, #LENGTH-(LENGTH*ra/rb)
jmz done, y

mov @p, temp
mov temp, *p
djn inner, y

done mul #ra, gap
gap div #rb, #(LENGTH*ra/rb)

jmn outer, gap
nop >gap, {outer
jmn outer, temp


Tuesday, 12 February 2008


Quicksort is an algorithm developed by C. A. R. Hoare in 1960 to sort a sequence of elements into order. Quicksort works by dividing a sequence into two sub-sequences.

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 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