Saturday, March 3, 2007

25 Byte Sort Routine

On the topic of sorting, I came across something cool the other day in Michael Abrash's Graphics Programming Black Book that I thought I'd share: a 25-byte long integer sort routine (that is to say, the routine itself is only 25 bytes long).

Here it is, your Moment of Zen:

; Sorts an array of ints. C-callable (small model). 25 bytes.
; void sort (int n, int a[]);
; Courtesy of David Stafford.

      .model small
        public _sort

top:    mov     dx,[bx]     ; swap two adjacent ints
        xchg    dx,[bx+2]
        xchg    dx,[bx]

        cmp     dx,[bx]     ; did we put them in the right order?
        jl      top         ; no, swap them back

        inc     bx          ; go to the next integer
        inc     bx
        loop    top

_sort:  pop     dx          ; get return address (entry point)
        pop     cx          ; get count
        pop     bx          ; get pointer
        push    bx          ; restore pointer
        dec     cx          ; decrement count
        push    cx          ; save count
        push    dx          ; save return address
        jg      top         ; if cx > 0


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