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Use an improved hash function for symbol tables.

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The hash algorithm has been modified to include a rotate at each step. This should improve the quality of hashes and reduce the number of collisions. However, probably the more important change for performance is to do the modulo computation by repeated subtraction rather than by calling a slow library function.
This commit is contained in:
Stephen Heumann 2023-03-06 21:54:14 -06:00
parent 3406dbd3ae
commit 3ac55a64bf
2 changed files with 26 additions and 20 deletions
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44
CCommon.asm
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@ -44,37 +44,43 @@ lb1 lda [fromPtr],Y
Hash start cc
hashSize equ 876 # hash buckets - 1
sum equ 0 hash
disp equ 0 disp into hash table
length equ 2 length of string
subroutine (4:sPtr),4
stz sum default to bucket 0
lda [sPtr] set the length of the string
tax
and #$00FF
sta length
ldy #1 start with char 1
lda [sPtr] if 1st char is '~', start with char 6
txa if 1st char is '~', start with char 6
and #$FF00
cmp #'~'*256
bne lb1
bne lb0
ldy #6
lb1 lda [sPtr],Y get the value to add in
and #$3F3F
cpy length if there is only 1 char left then
bne lb2
and #$00FF and out the high byte
lb2 clc add it to the sum
adc sum
sta sum
iny next char
lb0 lda #0 initial value is 0
bra lb2 while there are at least 2 chars left
lb1 asl a rotate sum left one bit
adc [sPtr],Y add in next two bytes
iny advance two chars
iny
cpy length
ble lb1
mod2 sum,#hashSize+1 return disp
asl sum
asl sum
lb2 cpy length
blt lb1
bne lb3 if there is 1 char left then
asl a rotate sum left one bit
sta disp
lda [sPtr],Y
and #$00FF and out the high byte
adc disp add last byte to the sum
sec
lb3 sbc #hashSize+1 disp := (sum mod (hashSize+1)) << 2
bcs lb3
adc #hashSize+1
asl a
asl a
sta disp
return 2:sum
return 2:disp return disp
end

2
Header.pas
View File

@ -18,7 +18,7 @@ uses CCommon, MM, Scanner, Symbol, CGI;
{$segment 'HEADER'}
const
symFileVersion = 38; {version number of .sym file format}
symFileVersion = 39; {version number of .sym file format}
var
inhibitHeader: boolean; {should .sym includes be blocked?}