# Examples

```           *************************************
* Hashing, Linked Lists, Trees, Forking *
*************************************

**** Hashing ****
(From R.K. Irvine)
I have used many hash functions.  Each one has advantages
and disadvantages.  Here is a powerful but simple one.
It simply does a crc16 on the character string.  The crc16 is
a great hash function since it is designed to produce a
unique code for different character strings.  So for "reasonable"
text it gives a good distribution of hash numbers (a big problem
to solve in general).  The crc16 function I list below uses nibbles
to build the crc from each byte, this is faster than a bit by bit and
less room than a 256 entry table needed for a byte by byte (fits
in cache).  So, it is fast (no multiply, just shifts and masks),
well behaved and simple.  If you have a VAX you could even use the
"crc" instruction instead of the crc16 function - what a CISC.
To use, do a crc16 on the string and the truncate the 16 bit
result to the size you need (this should be a power of 2).

/*	crc16 - crc16 routine
*
*	R.K. Irvine
*
*	This routine returns the crc16 for the string pointed
*	to by "in".
*	crc16 is given by:  x^16 + x^15 + x^2 + 1
*/
unsigned short
crc16(register char *in) {
register unsigned int	n, crc;

static unsigned short crc16l[] = {
0x0000,0xc0c1,0xc181,0x0140,
0xc301,0x03c0,0x0280,0xc241,
0xc601,0x06c0,0x0780,0xc741,
0x0500,0xc5c1,0xc481,0x0440,
};

static unsigned short crc16h[] = {
0x0000,0xcc01,0xd801,0x1400,
0xf001,0x3c00,0x2800,0xe401,
0xa001,0x6c00,0x7800,0xb401,
0x5000,0x9c01,0x8801,0x4400,
};

crc = 0;
while (n = (unsigned char)(*in++)) {
n ^= crc;
crc = crc16l[n&0x0f] ^ crc16h[(n>>4)&0x0f] ^ (crc>>8);
}
return(crc);
}

/**********************************
* Linked Linear List Trial Program
**********************************
*/
#include <stdio.h>

typedef struct _list_item {
int               val;
struct _list_item *next;
} list_item;

list_item *tail=NULL;

main()
{

print_list_items();
}

list_item *entry;
int value;
{
list_item *new_list_item;

new_list_item=(list_item*)malloc(sizeof(list_item));
if (entry==NULL){
printf("First list_item in list\n");
}
else {
entry->next = new_list_item;
printf("Adding %d to list. Last value was %d \n",value,entry->val);
}
new_list_item->val  = value;
new_list_item->next = NULL;
return new_list_item;
}

print_list_items()
{
list_item *ptr_to_list_item;

for (ptr_to_list_item= head;ptr_to_list_item!= NULL; ptr_to_list_item=ptr_to_list_item->next) {
printf("Value is %d \n", ptr_to_list_item->val);
}
}

*** Trees ***
/*compile using gcc */
#include <stdio.h>
#include <stdlib.h>

#define TRUE	1
#define FALSE	0

struct node {
int key;
int count;
struct node *left;
struct node *right;
int bal;
};

int search(int x, struct node **p)
{
struct node *p1, *p2;
int h;

if (*p == NULL) {
*p          = (struct node *) malloc(sizeof (struct node));
(*p)->key   = x;
(*p)->count = 1;
(*p)->left  = NULL;
(*p)->right = NULL;
(*p)->bal   = 0;
return(TRUE);
};
if (x == (*p)->key) {
(*p)->count++;
return(FALSE);
};
if (x < (*p)->key) {
if ((h=search(x, &((*p)->left))) == TRUE)
switch ((*p)->bal) {
case 1:  (*p)->bal = 0;
return(FALSE);
case 0:  (*p)->bal = -1;
return(h);
case -1: p1 = (*p)->left;
if (p1->bal == -1) {
(*p)->left = p1->right;
p1->right  = *p;
(*p)->bal  = 0;
*p         = p1;
} else {
p2         = p1->right;
p1->right  = p2->left;
p2->left   = p1;
(*p)->left = p2->right;
p2->right  = *p;
(*p)->bal  = (p2->bal == -1?  1 : 0);
p1->bal    = (p2->bal ==  1? -1 : 0);
*p         = p2;
}
(*p)->bal = 0;
return(FALSE);
};
} else {
if ( (h=search(x, &((*p)->right))) == TRUE)
switch ((*p)->bal) {
case -1: (*p)->bal = 0;
return(FALSE);
case 0:  (*p)->bal = 1;
return(h);
case 1:  p1 = (*p)->right;
if (p1->bal == 1) {
(*p)->right = p1->left;
p1->left    = *p;
(*p)->bal   = 0;
*p          = p1;
} else {
p2          = p1->left;
p1->left    = p2->right;
p2->right   = p1;
(*p)->right = p2->left;
p2->left    = *p;
(*p)->bal   = (p2->bal ==  1? -1 : 0);
p1->bal     = (p2->bal == -1?  1 : 0);
*p          = p2;
}
(*p)->bal = 0;
return(FALSE);
}
}
}

void visit(struct node *p)
{
if (p != NULL) {
printf("%d(%d)", p->key, p->bal);
if (p->left != NULL)
printf("\tl:%d", p->left->key);
if (p->right != NULL)
printf("\tr:%d", p->right->key);
printf("\n");
visit(p->left);
visit(p->right);
}
}

main()
{
int i, j;
struct node *root;
root = (struct node *) NULL;
for (i = 0; i < 20; i++) {
j = rand() % 100;
printf("-----%d-----\n", j);
search(j, &root);
visit(root);
}
}

*** Forking ***

main(){
int pid, status, died;
switch(pid=fork()){
case -1: printf("can't fork\n");
exit(-1);
case 0 : printf("I'm the child.\n");
exit(3);
default: printf("I'm the parent.\n");
died= wait(&status);
printf("The child, pid= %d, has returned %d.\n",pid,status>>8);
printf("It sent a %d signal to parent\n",(status & 0xff));

}
}
```