hash.c

哈希表实现

/* +++Date last modified: 05-Jul-1997 */

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

#include "hash.h"

/*
** public domain code by Jerry Coffin, with improvements by HenkJan Wolthuis.
**
** Tested with Visual C 1.0 and Borland C 3.1.
** Compiles without warnings, and seems like it should be pretty
** portable.
*/

/*
** These are used in freeing a table.  Perhaps I should code up
** something a little less grungy, but it works, so what the heck.
*/

static void (*function)(void *) = (void (*)(void *))NULL;
static hash_table *the_table = NULL;


/* Initialize the hash_table to the size asked for.  Allocates space
** for the correct number of pointers and sets them to NULL.  If it
** can't allocate sufficient memory, signals error by setting the size
** of the table to 0.
*/

hash_table *construct_table(hash_table *table, size_t size)
{
      size_t i;
      bucket **temp;

      table -> size  = size;
      table -> table = (bucket * *)malloc(sizeof(bucket *) * size);
      temp = table -> table;

      if ( temp == NULL )
      {
            table -> size = 0;
            return table;
      }

      for (i=0;i<size;i++)
            temp[i] = NULL;
      return table;
}


/*
** Hashes a string to produce an unsigned short, which should be
** sufficient for most purposes.
*/

static unsigned hash(char *string)
{
      unsigned ret_val = 0;
      int i;

      while (*string)
      {
            i = *( int *)string;
            ret_val ^= i;
            ret_val <<= 1;
            string ++;
      }
      return ret_val;
}

/*
** Insert 'key' into hash table.
** Returns pointer to old data associated with the key, if any, or
** NULL if the key wasn't in the table previously.
*/

void *insert(char *key, void *data, hash_table *table)
{
      unsigned val = hash(key) % table->size;
      bucket *ptr;

      /*
      ** NULL means this bucket hasn't been used yet.  We'll simply
      ** allocate space for our new bucket and put our data there, with
      ** the table pointing at it.
      */

      if (NULL == (table->table)[val])
      {
            (table->table)[val] = (bucket *)malloc(sizeof(bucket));
            if (NULL==(table->table)[val])
                  return NULL;

            (table->table)[val] -> key = strdup(key);
            (table->table)[val] -> next = NULL;
            (table->table)[val] -> data = data;
            return (table->table)[val] -> data;
      }

      /*
      ** This spot in the table is already in use.  See if the current string
      ** has already been inserted, and if so, increment its count.
      */

      for (ptr = (table->table)[val];NULL != ptr; ptr = ptr -> next)
            if (0 == strcmp(key, ptr->key))
            {
                  void *old_data;

                  old_data = ptr->data;
                  ptr -> data = data;
                  return old_data;
            }

      /*
      ** This key must not be in the table yet.  We'll add it to the head of
      ** the list at this spot in the hash table.  Speed would be
      ** slightly improved if the list was kept sorted instead.  In this case,
      ** this code would be moved into the loop above, and the insertion would
      ** take place as soon as it was determined that the present key in the
      ** list was larger than this one.
      */

      ptr = (bucket *)malloc(sizeof(bucket));
      if (NULL==ptr)
            return 0;
      ptr -> key = strdup(key);
      ptr -> data = data;
      ptr -> next = (table->table)[val];
      (table->table)[val] = ptr;
      return data;
}


/*
** Look up a key and return the associated data.  Returns NULL if
** the key is not in the table.
*/

void *lookup(char *key, hash_table *table)
{
      unsigned val = hash(key) % table->size;
      bucket *ptr;

      if (NULL == (table->table)[val])
            return NULL;

      for ( ptr = (table->table)[val];NULL != ptr; ptr = ptr->next )
      {
            if (0 == strcmp(key, ptr -> key ) )
                  return ptr->data;
      }
      return NULL;
}

/*
** Delete a key from the hash table and return associated
** data, or NULL if not present.
*/

void *del(char *key, hash_table *table)
{
      unsigned val = hash(key) % table->size;
      void *data;
      bucket *ptr, *last = NULL;

      if (NULL == (table->table)[val])
            return NULL;

      /*
      ** Traverse the list, keeping track of the previous node in the list.
      ** When we find the node to delete, we set the previous node's next
      ** pointer to point to the node after ourself instead.  We then delete
      ** the key from the present node, and return a pointer to the data it
      ** contains.
      */

      for (last = NULL, ptr = (table->table)[val];
            NULL != ptr;
            last = ptr, ptr = ptr->next)
      {
            if (0 == strcmp(key, ptr -> key))
            {
                  if (last != NULL )
                  {
                        data = ptr -> data;
                        last -> next = ptr -> next;
                        free(ptr->key);
                        free(ptr);
                        return data;
                  }

                  /*
                  ** If 'last' still equals NULL, it means that we need to
                  ** delete the first node in the list. This simply consists
                  ** of putting our own 'next' pointer in the array holding
                  ** the head of the list.  We then dispose of the current
                  ** node as above.
                  */

                  else
                  {
                        data = ptr->data;
                        (table->table)[val] = ptr->next;
                        free(ptr->key);
                        free(ptr);
                        return data;
                  }
            }
      }

      /*
      ** If we get here, it means we didn't find the item in the table.
      ** Signal this by returning NULL.
      */

      return NULL;
}

/*
** free_table iterates the table, calling this repeatedly to free
** each individual node.  This, in turn, calls one or two other
** functions - one to free the storage used for the key, the other
** passes a pointer to the data back to a function defined by the user,
** process the data as needed.
*/

static void free_node(char *key, void *data)
{
      (void) data;

      if (function)
            function(del(key,the_table));
      else  del(key,the_table);
}

/*
** Frees a complete table by iterating over it and freeing each node.
** the second parameter is the address of a function it will call with a
** pointer to the data associated with each node.  This function is
** responsible for freeing the data, or doing whatever is needed with
** it.
*/

void free_table(hash_table *table, void (*func)(void *))
{
      function = func;
      the_table = table;

      enumerate( table, free_node);
      free(table->table);
      table->table = NULL;
      table->size = 0;

      the_table = NULL;
      function = (void (*)(void *))NULL;
}

/*
** Simply invokes the function given as the second parameter for each
** node in the table, passing it the key and the associated data.
*/

void enumerate( hash_table *table, void (*func)(char *, void *))
{
      unsigned i;
      bucket *temp;

      for (i=0;i<table->size; i++)
      {
            if ((table->table)[i] != NULL)
            {
                  for (temp = (table->table)[i];
                        NULL != temp;
                        temp = temp -> next)
                  {
                        func(temp -> key, temp->data);
                  }
            }
      }
}

#ifdef TEST

#include <stdio.h>

void printer(char *string, void *data)
{
      printf("%s: %s\n", string, (char *)data);
}

int main(void)
{
      hash_table table;

      char *strings[] = {
            "The first string",
            "The second string",
            "The third string",
            "The fourth string",
            "A much longer string than the rest in this example.",
            "The last string",
            NULL
            };

      char *junk[] = {
            "The first data",
            "The second data",
            "The third data",
            "The fourth data",
            "The fifth datum",
            "The sixth piece of data"
            };

      int i;
      void *j;

      construct_table(&table,200);

      for (i = 0; NULL != strings[i]; i++ )
            insert(strings[i], junk[i], &table);

      for (i=0;NULL != strings[i];i++)
      {
            printf("\n");
            enumerate(&table, printer);
            del(strings[i],&table);
      }

      for (i=0;NULL != strings[i];i++)
      {
            j = lookup(strings[i], &table);
            if (NULL == j)
                  printf("\n'%s' is not in table",strings[i]);
            else  printf("\nERROR: %s was deleted but is still in table.",
                  strings[i]);
      }
      free_table(&table, NULL);
      return 0;
}

#endif /* TEST */