174 lines
8.7 KiB
Markdown
174 lines
8.7 KiB
Markdown
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[Previous](14-04.html) [Table of Contents](index.html) [Next](14-06.html)
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------------------------ --------------------------------- --------------------
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**LISTING 14.3 L14-3.ASM**
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; Searches a buffer for a specified pattern. In case of a mismatch,
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; uses the value of the mismatched byte to skip across as many
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; potential match locations as possible (partial Boyer-Moore).
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; Returns start offset of first match searching forward, or NULL if
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; no match is found.
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; Tested with TASM.
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; C near-callable as:
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; unsigned char * FindString(unsigned char * BufferPtr,
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; unsigned int BufferLength, unsigned char * PatternPtr,
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; unsigned int PatternLength);
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parms struc
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dw 2 dup(?) ;pushed BP & return address
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BufferPtr dw ? ;pointer to buffer to be searched
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BufferLength dw ? ;# of bytes in buffer to be searched
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PatternPtr dw ? ;pointer to pattern for which to search
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PatternLength dw ? ;length of pattern for which to search
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parms ends
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.model small
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.code
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public _FindString
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_FindString proc near
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cld
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push bp ;preserve caller’s stack frame
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mov bp,sp ;point to our stack frame
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push si ;preserve caller’s register variables
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push di
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sub sp,256*2 ;allocate space for SkipTable
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; Create the table of distances by which to skip ahead on mismatches
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; for every possible byte value. First, initialize all skips to the
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; pattern length; this is the skip distance for bytes that don’t
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; appear in the pattern.
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mov ax,[bp+PatternLength]
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and ax,ax ;return an instant match if the pattern is
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jz InstantMatch ;0-length
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mov di,ds
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mov es,di ;ES=DS=SS
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mov di,sp ;point to SkipBuffer
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mov cx,256
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rep stosw
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dec ax ;from now on, we only need
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mov [bp+PatternLength],ax ; PatternLength - 1
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; Point to last (rightmost) byte of first potential pattern match
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; location in buffer.
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add [bp+BufferPtr],ax
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; Reject if buffer is too small, and set the count of the number of
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; potential pattern match locations in the buffer.
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sub [bp+BufferLength],ax
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jbe NoMatch
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; Set the skip values for the bytes that do appear in the pattern to
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; the distance from the byte location to the end of the pattern.
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; When there are multiple instances of the same byte, the rightmost
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; instance’s skip value is used. Note that the rightmost byte of the
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; pattern isn’t entered in the skip table; if we get that value for
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; a mismatch, we know for sure that the right end of the pattern has
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; already passed the mismatch location, so this is not a relevant byte
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; for skipping purposes.
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mov si,[bp+PatternPtr] ;point to start of pattern
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and ax,ax ;are there any skips to set?
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jz SetSkipDone ;no
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mov di,sp ;point to SkipBuffer
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SetSkipLoop:
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sub bx,bx ;prepare for word addressing off byte value
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mov bl,[si] ;get the next pattern byte
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inc si ;advance the pattern pointer
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shl bx,1 ;prepare for word lookup
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mov [di+bx],ax ;set the skip value when this byte value is
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; the mismatch value in the buffer
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dec ax
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jnz SetSkipLoop
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SetSkipDone:
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mov dl,[si] ;DL=rightmost pattern byte from now on
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dec si ;point to next-to-rightmost byte of pattern
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mov [bp+PatternPtr],si ; from now on
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; Search the buffer.
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std ;for backward REPZ CMPSB
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mov di,[bp+BufferPtr] ;point to first search location
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mov cx,[bp+BufferLength] ;# of match locations to check
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SearchLoop:
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mov si,sp ;point SI to SkipTable
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; Skip through until there’s a match for the rightmost pattern byte.
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QuickSearchLoop:
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mov bl,[di] ;rightmost buffer byte at this location
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cmp dl,bl ;does it match the rightmost pattern byte?
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jz FullCompare ;yes, so keep going
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sub bh,bh ;convert to a word
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add bx,bx ;prepare for look-up in SkipTable
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mov ax,[si+bx] ;get skip value from skip table for this
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; mismatch value
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add di,ax ;BufferPtr += Skip;
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sub cx,ax ;BufferLength -= Skip;
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ja QuickSearchLoop ;continue if any buffer left
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jmp short NoMatch
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; Return a pointer to the start of the buffer (for 0-length pattern).
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align 2
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InstantMatch:
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mov ax,[bp+BufferPtr]
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jmp short Done
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; Compare the pattern and the buffer location, searching from high
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; memory toward low (right to left).
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align 2
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FullCompare:
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mov [bp+BufferPtr],di ;save the current state of
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mov [bp+BufferLength],cx ; the search
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mov cx,[bp+PatternLength] ;# of bytes yet to compare
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jcxz Match ;done if only one character
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mov si,[bp+PatternPtr] ;point to next-to-rightmost bytes
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dec di ; of buffer location and pattern
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repz cmpsb ;compare the rest of the pattern
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jz Match ;that’s it; we’ve found a match
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; It’s a mismatch; let’s see what we can learn from it.
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inc di ;compensate for 1-byte overrun of REPZ CMPSB;
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; point to mismatch location in buffer
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; # of bytes that did match.
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mov si,[bp+BufferPtr]
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sub si,di
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; If, based on the mismatch character, we can’t even skip ahead as far
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; as where we started this particular comparison, then just advance by
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; 1 to the next potential match; otherwise, skip ahead from this
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; comparison location by the skip distance for the mismatch character,
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; less the distance covered by the partial match.
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sub bx,bx ;prepare for word addressing off byte value
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mov bl,[di] ;get the value of the mismatch byte in buffer
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add bx,bx ;prepare for word look-up
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add bx,sp ;SP points to SkipTable
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mov cx,[bx] ;get the skip value for this mismatch
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mov ax,1 ;assume we’ll just advance to the next
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; potential match location
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sub cx,si ;is the skip far enough to be worth taking?
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jna MoveAhead ;no, go with the default advance of 1
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mov ax,cx ;yes; this is the distance to skip ahead from
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; the last potential match location checked
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MoveAhead:
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; Skip ahead and perform the next comparison, if there’s any buffer
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; left to check.
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mov di,[bp+BufferPtr]
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add di,ax ;BufferPtr += Skip;
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mov cx,[bp+BufferLength]
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sub cx,ax ;BufferLength -= Skip;
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ja SearchLoop ;continue if any buffer left
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; Return a NULL pointer for no match.
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align 2
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NoMatch:
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sub ax,ax
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jmp short Done
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; Return start of match in buffer (BufferPtr - (PatternLength - 1)).
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align 2
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Match:
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mov ax,[bp+BufferPtr]
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sub ax,[bp+PatternLength]
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Done:
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cld ;restore default direction flag
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add sp,256*2 ;deallocate space for SkipTable
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pop di ;restore caller’s register variables
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pop si
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pop bp ;restore caller’s stack frame
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ret
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_FindString endp
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end
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------------------------ --------------------------------- --------------------
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[Previous](14-04.html) [Table of Contents](index.html) [Next](14-06.html)
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------------------------ --------------------------------- --------------------
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* * * * *
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Graphics Programming Black Book © 2001 Michael Abrash
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