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@tonious
Last active May 28, 2022
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A quick hashtable implementation in c.
/* Read this comment first: https://gist.github.com/tonious/1377667#gistcomment-2277101
* 2017-12-05
*
* -- T.
*/
#define _XOPEN_SOURCE 500 /* Enable certain library functions (strdup) on linux. See feature_test_macros(7) */
#include <stdlib.h>
#include <stdio.h>
#include <limits.h>
#include <string.h>
struct entry_s {
char *key;
char *value;
struct entry_s *next;
};
typedef struct entry_s entry_t;
struct hashtable_s {
int size;
struct entry_s **table;
};
typedef struct hashtable_s hashtable_t;
/* Create a new hashtable. */
hashtable_t *ht_create( int size ) {
hashtable_t *hashtable = NULL;
int i;
if( size < 1 ) return NULL;
/* Allocate the table itself. */
if( ( hashtable = malloc( sizeof( hashtable_t ) ) ) == NULL ) {
return NULL;
}
/* Allocate pointers to the head nodes. */
if( ( hashtable->table = malloc( sizeof( entry_t * ) * size ) ) == NULL ) {
return NULL;
}
for( i = 0; i < size; i++ ) {
hashtable->table[i] = NULL;
}
hashtable->size = size;
return hashtable;
}
/* Hash a string for a particular hash table. */
int ht_hash( hashtable_t *hashtable, char *key ) {
unsigned long int hashval;
int i = 0;
/* Convert our string to an integer */
while( hashval < ULONG_MAX && i < strlen( key ) ) {
hashval = hashval << 8;
hashval += key[ i ];
i++;
}
return hashval % hashtable->size;
}
/* Create a key-value pair. */
entry_t *ht_newpair( char *key, char *value ) {
entry_t *newpair;
if( ( newpair = malloc( sizeof( entry_t ) ) ) == NULL ) {
return NULL;
}
if( ( newpair->key = strdup( key ) ) == NULL ) {
return NULL;
}
if( ( newpair->value = strdup( value ) ) == NULL ) {
return NULL;
}
newpair->next = NULL;
return newpair;
}
/* Insert a key-value pair into a hash table. */
void ht_set( hashtable_t *hashtable, char *key, char *value ) {
int bin = 0;
entry_t *newpair = NULL;
entry_t *next = NULL;
entry_t *last = NULL;
bin = ht_hash( hashtable, key );
next = hashtable->table[ bin ];
while( next != NULL && next->key != NULL && strcmp( key, next->key ) > 0 ) {
last = next;
next = next->next;
}
/* There's already a pair. Let's replace that string. */
if( next != NULL && next->key != NULL && strcmp( key, next->key ) == 0 ) {
free( next->value );
next->value = strdup( value );
/* Nope, could't find it. Time to grow a pair. */
} else {
newpair = ht_newpair( key, value );
/* We're at the start of the linked list in this bin. */
if( next == hashtable->table[ bin ] ) {
newpair->next = next;
hashtable->table[ bin ] = newpair;
/* We're at the end of the linked list in this bin. */
} else if ( next == NULL ) {
last->next = newpair;
/* We're in the middle of the list. */
} else {
newpair->next = next;
last->next = newpair;
}
}
}
/* Retrieve a key-value pair from a hash table. */
char *ht_get( hashtable_t *hashtable, char *key ) {
int bin = 0;
entry_t *pair;
bin = ht_hash( hashtable, key );
/* Step through the bin, looking for our value. */
pair = hashtable->table[ bin ];
while( pair != NULL && pair->key != NULL && strcmp( key, pair->key ) > 0 ) {
pair = pair->next;
}
/* Did we actually find anything? */
if( pair == NULL || pair->key == NULL || strcmp( key, pair->key ) != 0 ) {
return NULL;
} else {
return pair->value;
}
}
int main( int argc, char **argv ) {
hashtable_t *hashtable = ht_create( 65536 );
ht_set( hashtable, "key1", "inky" );
ht_set( hashtable, "key2", "pinky" );
ht_set( hashtable, "key3", "blinky" );
ht_set( hashtable, "key4", "floyd" );
printf( "%s\n", ht_get( hashtable, "key1" ) );
printf( "%s\n", ht_get( hashtable, "key2" ) );
printf( "%s\n", ht_get( hashtable, "key3" ) );
printf( "%s\n", ht_get( hashtable, "key4" ) );
return 0;
}
@gourao
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gourao commented Jan 4, 2016

In ht_hash line 53, you need to initialize hashval to 0.

@gourao
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gourao commented Jan 5, 2016

You also need to fix ht_remove:

        if (pair == hashtable->table[bin]) {
            hashtable->table[bin] = pair->next;
        }   

@maximgavrilov
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maximgavrilov commented Feb 26, 2016

This code makes terrible hash, because it is just first (as projected) or last (as implemented) 4-8 chars in string. Any strings set with the same prefix/suffix will trash your hash.

hashval = hashval << 8;
hashval += key[i];

Most popular way to calculate some simple hash for strings is something like hashval = key[i] + 31 * hashval, with possible modifications of some prime number instead of 31.

@CecilWard
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CecilWard commented May 26, 2016

Using signed arithmetic everywhere when should be using unsigned.

@b3ast-t
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b3ast-t commented Dec 19, 2016

can some one explain me this line of code please? hashval = hashval << 8; why bitshift??

@masein
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masein commented Dec 20, 2016

why do we use typedef ?

@nskforward
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nskforward commented Mar 25, 2017

How to implement "ht_free" function?

@lewis262626
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lewis262626 commented Mar 29, 2017

@ChangMyName it's just the hashing function used. You're free to use any others.

@exapsy
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exapsy commented Jun 5, 2017

This piece of code has a memory leak I think.
When you allocate the HashTable array in the ht_create() function, and then you return NULL in case malloc() couldn't allocate any memory, you don't free first the hashTable_t.
Which I think results in a memory leak.

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ghost commented Jul 19, 2017

You should use calloc in HT_create instade of malloc and then put null everywhere. It's cleaner and safer because malloc could return virtual memory even if memory is exhausted.

@msasad
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msasad commented Dec 1, 2017

Search operation in a hashtable must be O(1). In this implementation, it is O(n).

@dmcmax
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dmcmax commented Dec 4, 2017

there is a method/function to clear hast table or delete single key-value pair?

@rlegko
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rlegko commented Dec 5, 2017

I think the function ht_hash has some severe flaws.
First, as did owensss notice, the variable hashval is not initialized. On most architectures it will have the value that was left in the stack by the last function that used that location, maybe this one. This results in some random initialization on each call, which may return different values for identical keys.
Then the test hashval < ULONG_MAX is useless. As pointed above the purpose "the idea being that 'beef' and 'beefstake' should both end up in the same bin" by "checking to see if adding one more byte will roll it over" is completely missed. But why should 'beef' and 'beefstake' both end up in the same bin ? As much as possible, we want that different keys end up in different bins. It would be nice if we could make the computation of hashval with infinite precision, so that every character has its contribution to the result.
The good new is : you don't need infinite precision for that. This is because any way you take at the end the computed value modulo hashtable->size. The computations modulo some value M have an interesting property : as long as you do only additions and multiplications, the final value do not change if you replace any intermediate result by its value modulo M. Expressed in a more pedantic way, modulo M is an homomorphism. So you can take modulo hashtable->size at each step of the loop, which ensures that you will never roll over as long as hashtable->size is less than ULONG_MAX>>8, and the final result will be the same as if it was computed with infinite precision. Let's rewrite the function as follows :

int ht_hash( hashtable_t *hashtable, char *key ) {

unsigned long int hashval = 0;
int i = 0;

/* Convert our string to an integer */
while( i < strlen( key ) ) {
	hashval = ((hashval << 8) +  key[ i ]) %  hashtable->size;
	i++;
}
return hashval;

}

@tonious
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tonious commented Dec 6, 2017

Hey folks, original implementer here.

This was a quick code kata I wrote almost seven years ago. I've not updated it since then. And I'm not going to.

I'm happy to leave this here for the sake of retaining all the commentary, but I have some notes for coders that are new to this thread.

Do not use this in production.

It's public domain, it's just fundamentally buggy.

If you're writing code for a larger system, use a real data structures library. There's a perfectly servicable hash table implementation in glibc! These will be better implemented and better tested.

On the other hand if you're writing for an embedded system, you should probably not use a hash table. There are many better techniques for managing limited memory.

And on the gripping hand, if you're writing kernel code, well, you should be able to debug this with your eyes closed.

Read the thread before commenting.

This code has been hanging out for seven years. There are C programmers that are younger than this code sample.

You get the same points (zero) for not noticing that someone else has already found a given bug, as you would for copying their comment and submitting as your own.

I'd like to leave this thread here, but I don't want a stream of commenters rediscovering the same bugs.

This was not homework. This is not your homework.

This page is literally the first google result for 'Hash Table in C'. Do you think your markers don't know that? In fact, I've actually seen it handed to markers as example code that is evidence of plagiarism. It has some very distinctive bugs, and will not pass the dumbest of code similarity tests. Your only hope is that whoever marks this isn't paying attention and doesn't use automated plagiarism detection tools.

And whatever you're doing, do not submit it to my own grad advisor, at my own alma mater. That's the flirting-with-your-girlfriends-sister version of academic misconduct. Yes, this actually happened.

@tombustayoung
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tombustayoung commented Dec 6, 2017

big deal

@igor375
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igor375 commented Dec 27, 2017

working variant below:
a) we have no need for hash table size more than possible elements count
b) in case of duplicate hash, just store value to the next free slot
c) just for less details - fixed key/value length

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

struct entry_s {
	char not_empty;
	char key[7];
	char val[16];
};

typedef struct entry_s entry_t;

struct ht_s {
	int size;
	struct entry_s *tab;	
};

typedef struct ht_s ht_t;


/* Create a new ht. */
ht_t *ht_create(int size) {
	ht_t *ht = NULL;
	int i;

	if (size < 1) return NULL;

	/* Allocate the tab itself. */
	if ((ht = malloc(sizeof(ht_t))) == NULL )
		return NULL;
	/* Allocate pointers to the head nodes. */
	if ((ht->tab = malloc(sizeof( entry_t) * size)) == NULL)
		return NULL;
	for (i = 0; i < size; i++)
		memset(&ht->tab[i], 0, sizeof(entry_t));

	ht->size = size;
	return ht;	
}

/* Hash a string for a particular hash tab. */
int ht_hash( ht_t *ht, char *key ) {
	unsigned int hv;
	int i = 0;

	/* Convert our string to an integer */
	while (i < strlen(key)) {
		hv += key[i];
		i++;
	}
	return hv % ht->size;
}

/* Insert a key-val pair into a hash tab. */
void ht_set(ht_t *ht, char *key, char *val) {
	int ind = ht_hash(ht, key), i = ind;

	for (; i < ht -> size; i++)
		if (!ht->tab[i].not_empty) {
			ht->tab[i].not_empty = 1;
			strcpy(ht->tab[i].key, key);
			strcpy(ht->tab[i].val, val);
			return;
		}
	for (i = 0; i < ind; i++)
		if (!ht->tab[i].not_empty) {
			ht->tab[i].not_empty = 1;
			strcpy(ht->tab[i].key, key);
			strcpy(ht->tab[i].val, val);
			return;
		}
}

/* Retrieve a key-val pair from a hash tab. */
char *ht_get(ht_t *ht, char *key) {
	int ind = ht_hash(ht, key), i = ind;

	for (; i < ht->size; i++)
		if ((ht->tab[i].not_empty) && !strcmp(ht->tab[i].key, key))
			return ht->tab[i].val;
	for (i = 0; i < ind; i++)
		if ((ht->tab[i].not_empty) && !strcmp(ht->tab[i].key, key))
			return ht->tab[i].val;
	return "not found";	
}


int main(void) {

	ht_t *ht = ht_create(4);

	ht_set(ht, "key1", "inky");
	ht_set(ht, "key2", "pinky");
	ht_set(ht, "key3", "blinky");
	ht_set(ht, "kez2", "floyd");

	printf( "%s\n", ht_get( ht, "key1" ) );
	printf( "%s\n", ht_get( ht, "key2" ) );
	printf( "%s\n", ht_get( ht, "key3" ) );
	printf( "%s\n", ht_get( ht, "kez2" ) );
	printf( "%s\n", ht_get( ht, "key4" ) );

	return 0;
}```

 

@etale-cohomology
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etale-cohomology commented Feb 7, 2018

@igor375 I like your hash table implementation, but it doesn't ever free memory =(

@perlboy-emeritus
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perlboy-emeritus commented Mar 10, 2018

This is a hashing algorithm I implemented for a client in 1992. The client needed
to store hundreds of thousands of graphics files in an MRP system on AIX servers but
long before database BLOBs were released into the wild. (Full disclosure: I would
not have used BLOBs even if they had been available.) The idea was to create a
set of directories numbered 0..SOME NUMBER and find the image files by hashing a
normalized string that represented a filename. This one's signature has been
modified for use in hash.c. The actual implementation's return expression was:

return (hash % PRIME) % QUEUES;

where PRIME = 23017 and QUEUES = 503. Both are prime numbers, PRIME to encourage
a uniform pseudo-random distribution and QUEUES was the number of file system
directories deemed needed to hold the collection and its expected growth at the
time, as in 0..QUEUES-1. I tested the distribution of keys exhaustively for
uniformity before implementing. The client was pleased and when last I consulted
to them in 2004, the algorithm was still in use. Weinberger's algorithm was
published in Programming in C++, Dewhurst & Stark, PH, 1989, ISBM 0-13-723156-3;
Copyright © Peter Weinberger, AT&T Bell Laboratories, Inc., 1989. Obviously, you
may need permission to use it in a commercial distribution but not to study or as
I did, for a client who uses it only internally. It is so well-known that
anti-plagiarism software will detect it, but the point is, don't reinvent the wheel. Stand
on the shoulders of giants. If you're a student, your project might be to use this or some
other hashing algorithm in a novel and a unique to you, application, of course, citing
your sources, unless you're studying numerical programming, and then all bets are off.

/* Hash a string for a particular hash table. */
int ht_hash( size_t ht_size, char *key ) {
    unsigned int hash = 0, g;
    register char *p;   /* pointer used to scan and access string */

    /* Convert our string to an integer using Peter Weinberger's algorithm */
    for (p = key; *p; p++) {
        hash = (hash << 4) + *p;
        /* assumes a 32 bit int size */
        if ((g = hash & 0xf0000000)) {
            hash ^= g >> 24;
            hash ^= g;
        }
    }
    return hash % ht_size;
}

@tonious
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tonious commented Apr 27, 2018

Nice. I wish I knew about that hash when I wrote the my version 😆

I was more focused on memory management at the time. Being a little further on in my career, I think I'd start with a test suite and build backwards.

@OSTnm
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OSTnm commented May 11, 2018

hashval is not initialized to 0;

int ht_hash(hashtable_t *hashtable, char *key) {

**unsigned long int hashval = 0;**
int i = 0;

/* Convert our string to an integer */
while(hashval < ULONG_MAX && i < strlen(key)) {
	hashval = hashval << 8;
	hashval += key[i];
	i++;
}

return hashval % hashtable->size;

}

@Liron24
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Liron24 commented May 26, 2018

Hi,
This is very informative, thank you for sharing :)
I have a project in C which I need to use some kind of DB to store information which is basically a large table with a lot of fields for each entry. The code should work online and I'm always getting more entries and I don't know the hash table size in advance.
I also need to keep that DB sorted since I need to compare between the fields and make decision accordingly.
I want to use the hash table for having the field that I need to decide for as the key and a struct with all the other fields as values.
I was wondering about:
1.Can I dynamically enlarge the table and not set a predefined size to it? Any Ideas how? Would it make is much more compliacted in calculating the keys-to-hashes?
2.Is there a fast/efficient way to keeo the table sorted by the keys?
3.Can I make the key to be int? How the ht_hash would impact by that?

In case yo have other ideas for me regarding DBs I can use in C, I would love to hear them.

Thank you!

@fermosh
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fermosh commented Jul 17, 2018

@elinux9000
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elinux9000 commented Jul 30, 2018

Memory leak if table allocation fails.

/* Allocate pointers to the head nodes. */
if ((ht->tab = malloc(sizeof( entry_t) * size)) == NULL)
	return NULL;

should be

/* Allocate pointers to the head nodes. */
if ((ht->tab = malloc(sizeof( entry_t) * size)) == NULL)
{ 
    free(ht);
    return NULL;
}

@yanichik
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yanichik commented Dec 3, 2018

@tonious what is the purpose of typedefing the entry_s and hashtable_s structs to entry_t & hashtable_t ?
thanks.

@datvuthanh
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datvuthanh commented Sep 17, 2019

Delete entries

void ht_del( hashtable_t *hashtable, char *key){
    
    int bin = 0;
    int flag =0;
    
    entry_t *temp,*pair;
    
    bin = ht_hash (hashtable,key);
    
    pair = hashtable->table[bin]; // Go to index node
   
    temp = pair;
    while(pair != NULL) {
        if(strcmp(key,pair->key) == 0 ){
            flag = 1;
            if(pair == hashtable->table[bin]){
                hashtable->table[bin] = pair->next;
            }
            else{
                temp->next = pair->next;
            }
            free(pair);
            break;
        }
        temp = pair;
        pair = pair->next;
    }
    if (flag){
        printf("Data deleted successfully from Hash Table\n");
    }
    else{
        printf("Given data is not present in hash table!!!!\n");
    }
}

@fgl82
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fgl82 commented Nov 3, 2019

@tonious Hey, I'm using this on my program, I don't care if it has bugs as for my purposes it works just fine.
Do I need to do anything else other than mention where I took this code from?
Thanks.

@tonious
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tonious commented Nov 4, 2019

@fgl82, I just took a look at simplemenu, and I agree. This actually isn't a horrible solution for your purposes. It's still a horrible solution for all other purposes 😁.

I'd recommend using a better hashing algorithm. perlboy-emeritus has a great comment on this. But that's a fine stroke, and won't matter in your use case.

FWIW, I have a gameboy zero build that I really like, but emulation station really is too heavy for the way I use it. I can't wait to see how simplemenu turns out.

@fgl82
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fgl82 commented Dec 10, 2019

@tonious, I'm just seeing this reply! Man, thanks for this, it's working great after tweaking it just a bit, have had no complaints so far!

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