Misc/abrash-black-book
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James Gregory 2013-12-30 18:34:31 +11:00
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14-06.md
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@ -9,12 +9,12 @@ good as or better than—sometimes *much* better than—brute-force
searching. (For short patterns, you might want to use **REPNZ SCASB,**
thereby getting the best of both worlds.)
Know your data and use your smarts. Don’t stop thinking just because
you’re implementing a big-name algorithm; you know more than it does.
Know your data and use your smarts. Don't stop thinking just because
you're implementing a big-name algorithm; you know more than it does.
### Further Optimization of Boyer-Moore {#Heading6}
We can do substantially better yet than Listing 14.3 if we’re willing to
We can do substantially better yet than Listing 14.3 if we're willing to
accept tighter limits on the data. Limiting the length of the
searched-for pattern to a maximum of 255 bytes allows us to use the
**XLAT** instruction and generally tighten the critical loop. (Be aware,
@ -35,7 +35,7 @@ about 60 percent faster than Listing 14.3.
; Requires that the pattern be no longer than 255 bytes, and that
; there be a match for the pattern somewhere in the buffer (ie., a
; copy of the pattern should be placed as a sentinel at the end of
; the buffer if the pattern isn’t already known to be in the buffer).
; the buffer if the pattern isn't already known to be in the buffer).
; Tested with TASM.
; C near-callable as:
; unsigned char * FindString(unsigned char * BufferPtr,
@ -58,14 +58,14 @@ about 60 percent faster than Listing 14.3.
public _FindString
_FindString proc near
cld
push bp ;preserve caller’s stack frame
push bp ;preserve caller's stack frame
mov bp,sp ;point to our stack frame
push si ;preserve caller’s register variables
push si ;preserve caller's register variables
push di
sub sp,256 ;allocate space for SkipTable
; Create the table of distances by which to skip ahead on mismatches
; for every possible byte value. First, initialize all skips to the
; pattern length; this is the skip distance for bytes that don’t
; pattern length; this is the skip distance for bytes that don't
; appear in the pattern.
mov di,ds
mov es,di ;ES=DS=SS
@ -106,7 +106,7 @@ about 60 percent faster than Listing 14.3.
mov bx,sp ;point to SkipTable for XLAT
SearchLoop:
sub ah,ah ;used to convert AL to a word
; Skip through until there’s a match for the first pattern byte.
; Skip through until there's a match for the first pattern byte.
QuickSearchLoop:
; See if we have a match at the first buffer location.
REPT 8 ;unroll loop 8 times to reduce branching
@ -132,21 +132,21 @@ about 60 percent faster than Listing 14.3.
dec di ;point to next destination byte to compare (SI
; points to next-to-rightmost source byte)
repz cmpsb ;compare the rest of the pattern
jz Match ;that’s it; we’ve found a match
; It’s a mismatch; let’s see what we can learn from it.
jz Match ;that's it; we've found a match
; It's a mismatch; let's see what we can learn from it.
inc di ;compensate for 1-byte overrun of REPZ CMPSB;
; point to mismatch location in buffer
; # of bytes that did match.
mov si,[bp+BufferPtr]
sub si,di
; If, based on the mismatch character, we can’t even skip ahead as far
; If, based on the mismatch character, we can't even skip ahead as far
; as where we started this particular comparison, then just advance by
; 1 to the next potential match; otherwise, skip ahead from this
; comparison location by the skip distance for the mismatch character,
; less the distance covered by the partial match.
mov al,[di] ;get the value of the mismatch byte in buffer
xlat ;get the skip value for this mismatch
mov cx,1 ;assume we’ll just advance to the next
mov cx,1 ;assume we'll just advance to the next
; potential match location
sub ax,si ;is the skip far enough to be worth taking?
jna MoveAhead ;no, go with the default advance of 1
@ -167,9 +167,9 @@ about 60 percent faster than Listing 14.3.
Done:
cld ;restore default direction flag
add sp,256 ;deallocate space for SkipTable
pop di ;restore caller’s register variables
pop di ;restore caller's register variables
pop si
pop bp ;restore caller’s stack frame
pop bp ;restore caller's stack frame
ret
_FindString endp
end
@ -182,15 +182,15 @@ it as a parameter.
### Know What You Know {#Heading7}
Here we’ve turned up our nose at a repeated string instruction, we’ve
gone against the grain by comparing backward, and yet we’ve speeded up
Here we've turned up our nose at a repeated string instruction, we've
gone against the grain by comparing backward, and yet we've speeded up
our code quite a bit. All this without any restrictions or special
requirements (excluding Listing 14.4)—and without any new information.
Everything we needed was sitting there all along; we just needed to
think to look at it.
As Yogi Berra might put it, “You don’t know what you know until you know
it.”
As Yogi Berra might put it, "You don't know what you know until you know
it."
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