7.9.7. After_class: Modify a hash-table with strings

7.9.7. After_class: Modify a hash-table with strings
	7.9. Activities

Resources

Knowledge about dynamic data structures.

Work Plan

The R-D laboratory of SAUCEM SL has received the following file corresponding to a hash table:

#ifndef HASH_TABLE_WITH_STRINGS_H__ 
#define HASH_TABLE_WITH_STRINGS_H__


#include <stdio.h>
#include <stdlib.h>
#include <string.h>


struct list_data
{
    char *data_string;
    char *key_string;
    struct list_data *next;
};

typedef struct list_data list_t;

struct hash_table_data 
{
    int size;           /* the size of the table */
    list_t **table;     /* the table elements    */
    int elements;       /*the number of elements   */
};

typedef struct hash_table_data  hash_table_t;

/**
 *  It creates a new (string) hash table.
 **/
 
hash_table_t *hash_table_create(int size);

/**
 *  Hash function
 **/
unsigned int hash(hash_table_t *hashtable, char *key_string);

/**
 *  It removes an element from the table
 **/
int hash_table_remove_data_string(hash_table_t *hashtable, char *key_string);

/**
 * It returns an element of the table. NULL if it is not found
 */

char* hash_table_lookup_data_string(hash_table_t *hashtable, char *key_string);

/**
 *It adds a new tuple in the hash table ((unique)key_string, data_string)
 */ 
int hash_table_add_data_string(hash_table_t *hashtable, char *key_string, char *data_string);

/**
 * Removing the hash table
 */
int hash_table_free(hash_table_t ** ptr_hashtable);

/**
 * Printing out all information (debuging)
 */ 
int hash_table_print(hash_table_t* hashtable);

/**
 *Resizing the table
 */
int hash_table_resize(hash_table_t* hashtable, int size);

#endif

It corresponds to the code of a table-hash optimized for text strings. The basic idea of a structure of this type of data structure is that it consists of a table (array) list, which allows quick access to key value pairs. This allows you to search much more efficiently than with other structures, introducing a hash function (which combines elements of an initial table). This function allows to the item you want to access more easily.

You also have the following implementation (.c file) for the previous .h file:

//
//gcc -Wall *.c -o data_string_hash_table
//valgrind ./data_string_hash_table
//

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "hash_table_with_strings.h"



/*create */
hash_table_t *hash_table_create(int size)
{
    int i;
    hash_table_t *new_table=NULL;    
    if (size<1)
    {
      return NULL;
    }
    /*space for the table */
    if ((new_table = calloc(1,sizeof(hash_table_t))) == NULL) 
    {
        return NULL;
    }        
    if ((new_table->table = calloc(1,sizeof(list_t*) * size)) == NULL) 
    {      
        free(new_table);
        return NULL;
    }
    /* Inicialization of the internal table */  
    for(i=0; i<size; i++) 
    {
      new_table->table[i] = NULL;
    }
    new_table->size = size;
    return new_table;
}

/*hash function to select the position of the array */ 
unsigned int hash(hash_table_t *hashtable, char *key_string)
 {
    if(hashtable==NULL)
    { 
      return 0;
    }
    if(key_string==NULL)
    { 
      return 0;
    }           
   unsigned int hashval=0;    
        
    for(; *key_string != '\0'; key_string++) 
    {
      hashval = *key_string + (hashval << 5) - hashval;    
    }
    return hashval % hashtable->size;
 }

 
/*Internal lookup_  */ 
list_t *hash_table_lookup_data_string_(hash_table_t *hashtable, char *key_string)
{
    if(hashtable==NULL)
    { 
      return NULL;
    }
    if(key_string==NULL)
    { 
      return NULL;
    }      
      
    list_t *list;
    unsigned int hashval = hash(hashtable, key_string);
    for(list = hashtable->table[hashval]; list != NULL; list = list->next) 
    {
        if (strcmp(key_string, list->key_string) == 0) 
	{
	  return list;
	}
    }
    return NULL;
}

/*remove  */ 
int hash_table_remove_data_string(hash_table_t *hashtable, char *key_string)
{
    if(hashtable==NULL)
    { 
      return -1;
    }
    if(key_string==NULL)
    { 
      return -2;
    }      
    
    list_t *list;
    list_t *prev;
    unsigned int hashval = hash(hashtable, key_string);
    
    for(list = hashtable->table[hashval], prev=NULL; list != NULL; prev=list, list = list->next) 
    {
        if (strcmp(key_string, list->data_string) == 0) 
	{
	  if(prev==NULL){
	    hashtable->table[hashval]=list->next;	    
	  }else{
	    prev->next=list->next;	  
	  } 
	 free(list->data_string);
	 free(list->key_string);
	 free(list);
	 hashtable->elements--;
	 return 0;
	}
    }
    return -1;
}


/* lookup */
char* hash_table_lookup_data_string(hash_table_t *hashtable, char *key_string)
{      
  list_t* ptr=hash_table_lookup_data_string_(hashtable,key_string);
  
  if(ptr==NULL)
  {
    return NULL;
    
  }
  else
  {  
    return ptr->data_string;  
  }
}

/* Adding element */ 
int hash_table_add_data_string(hash_table_t *hashtable, char *key_string, char *data_string)
{
    if(hashtable==NULL)
    { 
      return -1;
    }
    if(key_string==NULL)
    { 
      return -2;
    }      
    if(data_string==NULL)
    { 
      return -3;
    }   
    list_t *new_list=NULL;
    list_t *current_list=NULL;
    unsigned int hashval = hash(hashtable, key_string);
    if ((new_list = malloc(sizeof(list_t))) == NULL) 
    {     
      return -1;
    }
    current_list = hash_table_lookup_data_string_(hashtable, key_string);
    if (current_list != NULL)
    {
      return -2;
    }
    /* Insert into list by the head */
    new_list->key_string = strdup(key_string);
    new_list->data_string = strdup(data_string);
    new_list->next = hashtable->table[hashval];
    hashtable->table[hashval] = new_list;
    hashtable->elements++;
    return 0;
}

/* Removing */ 
int hash_table_free(hash_table_t ** ptr_hashtable)
{   /*Safety issues*/
    if(ptr_hashtable == NULL)
    {
      return -1;
      
    }
    hash_table_t* hashtable=NULL;
    hashtable= (*ptr_hashtable);
    if (hashtable == NULL) 
    { 
      return -2;
    }
    int i=0;
    list_t *list;
    list_t *temp;    
    for(i=0; i<hashtable->size; i++) 
    {
        list = hashtable->table[i];
        while(list!=NULL) {
            temp = list;
            list = list->next;
            free(temp->data_string);
	    free(temp->key_string);
            free(temp);
	    hashtable->elements--;
        }
    }
    /* Free the table itself */
    free(hashtable->table);
    free(hashtable);
    (*ptr_hashtable)=NULL;
    return 0;
}

/* Printing out the table */ 
int hash_table_print(hash_table_t* hashtable)
{
   if(hashtable==NULL)
    { 
      return -1;
    }
    
    int aux_printing=0;
    int i=0;
    list_t *list=NULL;

    if (hashtable == NULL) 
      { 
	return 0;
      }
    
    for(i=0; i<hashtable->size; i++)  
      {
        list = hashtable->table[i];
        while(list!=NULL) 
	{
            printf("\n[%d][%d](key,value):\t \%20s : \%20s",i, (aux_printing++),
		   list->key_string,list->data_string);	   
            list = list->next;
        }
      }    
  return 0 ;    
}

/*resize*/
int hash_table_resize(hash_table_t* hashtable, int size)
{
  if(hashtable==NULL)
  {
    return -1;
  }
  if(size<0)
  {
    return -2;
  }  
  //Creating and isolated 
  hash_table_t* ht2=hash_table_create(size);
  int i=0;
  list_t *list;
  list_t *temp;    
  for(i=0; i<hashtable->size; i++) 
    {
        list = hashtable->table[i];
        while(list!=NULL) 
	{
            temp = list;
            list = list->next;
	    hash_table_add_data_string(ht2,temp->key_string,temp->data_string);
            free(temp->data_string);
	    free(temp->key_string);
	    free(temp);		    
	}
    }
  free(hashtable->table);  
  (*hashtable)=(*ht2);
  free(ht2);
  return 0;
}


int main()
{
  printf("\n ***Checking the code \n");
  printf("\n ***Creating hash table with 1 queue \n");
  hash_table_t* h1 =hash_table_create(1);
  printf("\n ***Adding 4 elements \n");
  hash_table_add_data_string(h1,"uno","1");
  hash_table_add_data_string(h1,"one","1");
  hash_table_add_data_string(h1,"two","2");
  hash_table_add_data_string(h1,"two","22"); //It should fail
  hash_table_add_data_string(h1,"three","3");
  hash_table_add_data_string(h1,"four", "4");
  
  hash_table_remove_data_string(h1,"uno");
  char* element_two=NULL;
  element_two=hash_table_lookup_data_string(h1,"two");
  if (element_two!=NULL)
  {
    printf ("\n *** Element \"two\" is: %s", element_two);
  }
  
  
  printf("\n***Dumping the queue \n");
  hash_table_print(h1);
  printf("\n***Resizing the table to 4 queues \n");    
  hash_table_resize(h1,4);
  printf("\n***Dumping the queue \n");
  hash_table_print(h1);  
  printf("\n***Releasing the hashtable \n");
  hash_table_free(&h1);
  return 0; 
}

Using both files, you are asked to:

Download the code, analyze it, and try to understand its internals. Try to store many items. Why is there a function to resize? What advantage can bring in?
Try to modify the code so that the inner table of the hash-table is automatically resized. SAUCEM team seems well that when the table has a higher occupancy of 0.75 and doubling its size is less than 0.25 when it is reduced by half.
SAUCEM also asked to use the initial code to implement a logistics application, containing elements corresponding to a shopping cart. Each element must save the amount of items available in the store, the price of each item, and a textual description of the product:
```
 struct item{
	   int amount_available;
	   int produc_id;
	   char* produc_description;
	   float price;
	   };	   
	 
```
They ask also perform a test application that allows a user to perform the following operations: (1) Add a new product, (2) remove a product, and (3) change the number of elements to an item.
Try to exploit the structure again so that instead of recording a text string, it stores the following data structure information concerning a WiFi, described as follows:
```
enum network_mode
}
    Auto,
    AdHoc,
    Managed,
    Master,
    Repeater,
    Secondary,
    Monitor,
    Unknown
};

struct ap_scan_info
{
    unsigned char mac[6];        // Cell MAC
    char essid[15];              // ESSID
    enum network_mode mode;      // Operation mode (enumeration)
    int channel;                 // Channel for transmission
    unsigned short encrypted;    // Boolean stating if net is encrypted 
    unsigned int quality[2];     // Two integers stating quality current/max
 };	    
```
Hints: You will have to change the key you use to calculate the hash function. We recommend using the MAC is a unique identifier of the network and a key of the table. We also call a program, test, you enter 10000-elements in the hash table. How long does a search takes? How long before a single list (Note: You can try making a resize with 1 on the initial array of the table)?
Finally, the asked to generalize the given code to reuse in future prototypes within the company labs. Try to generalize the code implementing the dependent part of the data (i.e. creation, destruction, dump data) in specific functions. For that the lab SAUCEM have thought of using the technique of "function pointers" and ask you to add this solution to your code. The team has estimated it needs a pointer to int hash function(void * key) (depending on application), this function will be passed as a parameter when the hash table is created: int hash_table_create (int size, int (* hash_routine) ( void *))). They also believe that you can add optional pointers to functions to print and delete a node.

UC3M

Telematic/Audiovisual Syst./Communication Syst. Engineering

Systems Architecture

September 2017 - January 2018