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2008-11-03T05:19:45Z

12 revision(s)

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==Understanding the codes==

===buzhash.c===

====Satic Values====
hash_t(unsigned long) BUZINIT
hash_t(unsigned long) buztbl[256]

====void random_perm( int *C)====
input : 256 int array
work : Random changing between values in the array
output : No
used in ressed_buztable

====void ressed_buztable====
input : No
work : Change the values(BUZINIT, buztbl[256]) to random
output : No

*test
<pre>

=====How it works=====
e.g. changing of the value BUZINIT

C[0] = 1
Before
BUZINIT = D4A96F89
11010100 10101001 01101111 10001001

BUZINIT = NUZINIT << 1 | C[0]

After
BUZINIT = A952DF13
10101001 01010010 11011111 00010011

-------------------------------------------------------------------------

C[0] = 0

Before
BUZINIT = A952DF13
10101001 01010010 11011111 00010011

BUZINIT = NUZINIT << 1 | C[0]

After
BUZINIT = 52A5BE26
01010010 10100101 10111110 00100110

--------------------------------------------------------------------------
... 256 times with random C[0] = (0 or 1)
</pre>

===== to get binary information=====
<pre>
//Use this function
void bits(unsigned long a) {
unsigned long displayMask = 1 << 31;
int c;
for (c = 1; c <= 32; c++) {
putchar( a & displayMask ? '1' : '0' );
a = a << 1;
if ( c%8 == 0) {
putchar(' ');
}
}
putchar('\n');
}
</pre>

=====How to use &, |, <<, >>=====
*e.g. How it works? a = a<<2 | 3;
<pre>
1) the initial value of 'a'
6A54B7C4 (01101010 01010100 10110111 11000100)

2) a << 2 is run
6A54B7C4 (01101010 01010100 10110111 11000100) to
A952DF10 (10101001 01010010 11011111 00010000)

3) a | 3 is run
a = A952DF10 (10101001 01010010 11011111 00010000)
3 = 3 (00000000 00000000 00000000 00000011)
a | 3 = A952DF13 (10101001 01010010 11011111 00010011)
</pre>

====void dump_buztbl====
input : No
work : print BUZINIT and buztbl[]
output : No

====hash_t buzhash( char * key )====
input : char *key => char array (e.g. "bbbbbbb0")
work : No
output : hash h (explain below)

=====How to return value(hash h)=====
<pre>
e.g. char *key => "abcdef0"
BUZINIT = 1783936964L
buztbl[97] = 11100011 10011101 11100101 10000010(base2)

hast_t h = BUZINIT // h = 1783936964
while ( *key ) // a -> b -> c -> d -> e

h = ((h << 1) | (h >> (sizeof(hash_t)*8-1))) ^ buztbl[ (unsigned)*key++ ]; //*key = a
// h = 01101010 01010100 10110111 11000100
// ((h << 1) | (h >> (sizeof(hash_t)*8-1)))
// '0'1101010 01010100 10110111 11000100 move first value '0'
// 11010100 10101001 01101111 1000100'0' to the end
//buztbl[ (unsigned)*key++ ] //buztbl[97] ( 97='a' )
//11100011 10011101 11100101 10000010
// 11010100 10101001 01101111 10001000 ^ (xor)
// 11100011 10011101 11100101 10000010
// h = 00110111 00110100 10001010 00001010(base2)
h = ((h << 1) | (h >> (sizeof(hash_t)*8-1))) ^ buztbl[ (unsigned)*key++ ]; //*key = b
h = ((h << 1) | (h >> (sizeof(hash_t)*8-1))) ^ buztbl[ (unsigned)*key++ ]; //*key = c
h = ((h << 1) | (h >> (sizeof(hash_t)*8-1))) ^ buztbl[ (unsigned)*key++ ]; //*key = d
h = ((h << 1) | (h >> (sizeof(hash_t)*8-1))) ^ buztbl[ (unsigned)*key++ ]; //*key = e

return h;
</pre>

====hash_t buzhashn( char * key, int n )====
input : char *key => char array (e.g. "bbbbbbb0")
int n (e.g. 3)
work : No
output : hash h (same as buzhash but use only use n values from *key)

=====Keep editing=====

SE250:lab-5:dcho040 - Revision history

Mark: 12 revision(s)