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/* hash.c by Adam Rogoyski <apoc@laker.net> Temperanc on EFNet irc
* Copyright (C) 1998, 1999 Adam Rogoyski
* --- GNU General Public License Disclaimer ---
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*/
/* This file contains a simple hash for looking up changes we may have
* made to specific offsets in the file. When we are editing a large
* file that cannot fit in memory, when we reread a portion of the file
* from disk, it will be the original, and not the changed, so we need
* to lookup changes and add them to the current buffer in memory.
* The Hash function is just a mod 1009 into the hash table of 0-1008
* entries. Collisions are handled with just a list of entries for
* that index in the hash. The key/pair is offset/byte-value.
*/
#include "hexedit.h"
/* All the books say this should usually be a prime number */
#define HASH_TABLE_SIZE 1009
/* The really complex and thought out hash function. */
#define HASH(x) (x % HASH_TABLE_SIZE)
/* list of chained collisions */
struct hash_list
{
unsigned long offset;
unsigned char value;
struct hash_list *next;
};
struct hash_table
{
struct hash_list *chain;
};
/* hash table */
static struct hash_table *table = NULL;
void
init_hash ()
/* set each location empty by setting *next chain to -1 */
{
int i = 0;
if (table)
{
struct hash_list *p = NULL;
for (i = 0; i < HASH_TABLE_SIZE; i++)
{
while (table[i].chain)
{
p = table[i].chain->next;
free (table[i].chain);
table[i].chain = p;
}
}
free (table);
}
table = malloc (HASH_TABLE_SIZE * sizeof (struct hash_table));
if (!table)
die_horribly ("Not enough memory to create hash table\n", NULL);
for (i = 0; i < HASH_TABLE_SIZE; i++)
{
table[i].chain = NULL;
}
}
void
clear_hash ()
{
int i = 0;
struct hash_list *p = NULL;
for (i = 0; i < HASH_TABLE_SIZE; i++)
{
while (table[i].chain)
{
p = table[i].chain;
table[i].chain = table[i].chain->next;
free (p);
}
}
}
void
insert_hash_entry (unsigned long offset, unsigned char value)
{
int i = HASH(offset);
if (!table[i].chain)
/* empty location in table */
{
table[i].chain = malloc (sizeof (struct hash_list));
if (!table[i].chain)
die_horribly (NOT_ENOUGH_MEMORY, NULL);
table[i].chain->offset = offset;
table[i].chain->value = value;
table[i].chain->next = NULL;
}
else
/* Has at least one element in table. Insert the new collision
* into the front of the chain.
*/
{
struct hash_list *p = table[i].chain;
/* First, make sure entry isn't already here, if it is, we
* change it to the new value.
*/
while (p)
{
if (p->offset == offset)
{
p->value = value;
break;
}
p = p->next;
}
if (p)
return; /* already in hash */
p = malloc (sizeof (struct hash_list));
if (!p)
die_horribly (NOT_ENOUGH_MEMORY, NULL);
/* build the new entry */
p->offset = offset;
p->value = value;
p->next = table[i].chain;
/* now put entry in the front of the list */
table[i].chain = p;
}
}
void
delete_hash_entry (unsigned long offset)
{
int i = HASH(offset);
struct hash_list *p = table[i].chain;
/* hash entry not here */
if (!p)
return;
/* one or more at this index */
else
{
struct hash_list *last = NULL;
while (p)
{
if (p->offset == offset)
break;
last = p;
p = p->next;
}
/* if we found it delete it */
if (p)
{
/* if item isn't first element */
if (last)
{
last->next = p->next; /* skip over p */
free (p); /* then delete it */
}
else
{
table[i].chain = p->next; /* skip over p */
free (p); /* then delete it */
}
}
}
}
int
hash_lookup (unsigned long offset, unsigned char *c)
/* return -1 on lookup failure. Store value in *c if successful */
{
int i = HASH(offset);
struct hash_list *p = table[i].chain;
while (p)
{
if (p->offset == offset)
break;
p = p->next;
}
if (p)
{
if (c)
*c = p->value;
return 0;
}
return -1;
}
#ifdef DEBUG_HASH
void
print_hash_table ()
{
int i = 0;
for (i = 0; i < HASH_TABLE_SIZE; i++)
{
fprintf (stderr, "%d: ", i);
/* empty entry */
if (!table[i].chain)
fprintf (stderr, "empty\n");
else
{
struct hash_list *p = table[i].chain;
while (p)
{
fprintf (stderr, "(%ld,%d) ", p->offset, p->value);
p = p->next;
}
fprintf (stderr, "\n");
}
}
}
#endif
void
commit_changes_in_hash (FILE *fp)
/* cycle through the hash and write back all the entries, and clear
* the hash while we're at it.
*/
{
int i = 0;
int result = 0;
struct hash_list *p = NULL;
for (i = 0; i < HASH_TABLE_SIZE; i++)
{
p = table[i].chain;
while (table[i].chain)
{
result = fseek (fp, table[i].chain->offset, SEEK_SET);
if (result)
die_horribly ("Cannot write new file correctly", "fseek");
result = fwrite (&table[i].chain->value, 1, 1, fp);
if (!result && ferror (fp))
die_horribly ("File writing error", "ferror");
else if (!result && feof (fp))
die_horribly ("End of File Reached?", "feof");
p = table[i].chain->next;
free (table[i].chain);
table[i].chain = p;
}
}
}
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