strstrip Code

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pal

Joined: 26 Aug 2008
Posts: 227

pal 10 Jul 2009, 11:49

Hey I made an strstrip code i.e. a code that takes in a string, only allows certain characters to be in the string and outputs the string. I was just wondering if anyone could comment on how I could make it better etc.

I was thinking comparing multiple bytes at once would make it faster, maybe using a rep for the first @@ loop (but I couldn't think of the right one to use) etc.

Code:

proc        strstrip,szAllowed,szIn,szOut
       push    edi
 push    esi
 mov             edx,[szAllowed]
     mov             esi,[szIn]
  mov             edi,[szOut]
 mov             ecx,-1
removeLoop:
   add             ecx,1
       mov             al,byte [esi+ecx]
   test    al,al
       jz              removeEnd
   push    ecx
 mov             ecx,-1
      
    @@: add             ecx,1
               cmp             byte [edx+ecx],0
            jz              @F
          cmp             byte [edx+ecx],al
           jne             @B
          stosb   
@@:     pop             ecx
 jmp             removeLoop
  
removeEnd:
  xor             eax,eax
     xor             ecx,ecx
     pop             esi
 pop             edi
 ret
endp

Cheers for any comments, pal.

10 Jul 2009, 11:49

eskizo

Joined: 22 Nov 2005
Posts: 59

eskizo 10 Jul 2009, 13:40

Code:

; At least, remove the last loop and put this:
add esp, ecx

10 Jul 2009, 13:40

baldr

Joined: 19 Mar 2008
Posts: 1651

baldr 16 Jul 2009, 08:27

pal,

Having strlen(szAllowed) computed beforehand can reduce inner loop to repne scasb:

Code:

        xor     eax, eax
        mov     ecx, -1
        mov     edi, szAllowed
  repne scasb
        not     ecx             ;note that ecx == strlen(szAllowed)+1
        mov     edx, ecx

        mov     esi, szIn
        mov     ebx, szOut

next:   lodsb
        mov     ecx, edx
        mov     edi, szAllowed
  repne scasb                   ;this scans szAllowed with trailing \0
        jnz     next
        mov     [ebx], al       ;here we come if character in szAllowed (\0 too)
        inc     ebx
        test    ecx, ecx        ;ecx will be 0 iff character was \0
        jnz     next

done:

Probably bit map of allowed characters (32 bytes) and bt instruction will eliminate the need to scan szAllowed for each character in szIn.

16 Jul 2009, 08:27

pal

Joined: 26 Aug 2008
Posts: 227

pal 16 Jul 2009, 09:14

Cheers baldr. I dunno why I didn't use a repne scasb in the first place, it must have just skipped my mind.

16 Jul 2009, 09:14

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