Sqids (pronounced "squids") is a small library that lets you generate YouTube-looking IDs from numbers. It's good for link shortening, fast & URL-safe ID generation and decoding back into numbers for quicker database lookups.

The library is built using GNU Autotools.

./bootstrap
./configure
make

The library includes several blocklists that are enabled and built by default.

You have the option to build it without them by passing --disable-default-blocklist to configure.

Unlike Hashids, Sqids relies heavily on dynamic memory allocation.

You can still override the memory management functions if needed, by reassigning sqids_mem_alloc/sqids_mem_free.

Sqids defines a thread-safe sqids_errno with the following possible values:

Keep in mind that you should first test the function result and then inspect sqids_errno - if a function succeeds, sqids_errno is left untouched.

sqids_t *
sqids_new(char *alphabet, int min_len, sqids_bl_t *blocklist)

Sqids structure constructor.

If you pass NULL null as the alphabet, SQIDS_DEFAULT_ALPHABET will be used.

A NULL blocklist will result in no blocklist at all. See the blocklist API below for further information.

The returned structure should be freed using sqids_free.

In case of failure, NULL is returned and sqids_errno is set accordingly.

/* free a sqids structure */
void
sqids_free(sqids_t *sqids)

Sqids structure destructor.

Deallocates the Sqids structure including the alphabet and the blocklist, if present.

void
sqids_shuffle(char *alphabet)

Consistent shuffle.

Chances are you won't be using this anywhere outside Sqids.

char *
sqids_encode(sqids_t *sqids, unsigned int num_cnt, unsigned long long *nums)

Encode function.

Encodes an array of numbers to a string hash.

In case of failure, NULL is returned and sqids_errno is set accordingly.

char *
sqids_vencode(sqids_t *sqids, unsigned int num_cnt, ...)

Variadic version of sqids_encode.

int
sqids_num_cnt(sqids_t *sqids, char *s)

Number counting function.

Returns the count of numbers in an hash. Keep in mind that this function can return 0 as empty strings are not considered errors.

In case of failure, -1 is returned and sqids_errno is set accordingly.

int
sqids_decode(sqids_t *sqids, char *s, unsigned long long *nums, unsigned int num_max)

Decode function.

Decodes a hash back to numbers. Keep in mind that this function can return 0 as empty strings are not considered errors.

Result is the count of numbers decoded in the hash.

In case of failure, -1 is returned and sqids_errno is set accordingly.

sqids_bl_t *
sqids_bl_new(int (*match_func)(char *, char *))

Blocklist constructor.

Returns a new empty blocklist, which itself is a doubly linked list.

If you pass NULL as match_func, it will default to sqids_bl_match.

In case of failure, NULL is returned and sqids_errno is set accordingly.

void
sqids_bl_free(sqids_bl_t *bl)

Blocklist destructor.

Frees the blocklist and all its data.

sqids_bl_node_t *
sqids_bl_add_tail(sqids_bl_t *bl, char *s)

Adds a word to the end of the blocklist.

Result is a pointer to the new blocklist node.

In case of failure, NULL is returned and sqids_errno is set accordingly.

sqids_bl_node_t *
sqids_bl_add_head(sqids_bl_t *bl, char *s)

Adds a word to the beginning of the blocklist.

Result is a pointer to the new blocklist node.

In case of failure, NULL is returned and sqids_errno is set accordingly.

sqids_bl_node_t *
sqids_bl_find(sqids_bl_t *bl, char *s)

Tests if a string matches any of the bad words in the blocklist.

Result is pointer to the matching blocklist node, or NULL if no match is found.

int
sqids_bl_match(char *s, char *bad_word)

Default blocklist match function.

Tests if a string matches a bad word.

Result is 1 in case of a match, 0 otherwise.

sqids_bl_t *
sqids_bl_list_LANG(int (*match_func)(char *, char *))

Will either return the combined blocklist of a language-specific one. If you pass NULL as match_func, sqids_bl_match will be used as default.

In case of failure, NULL is returned and sqids_errno is set accordingly.

A command-line utility is provided so one can easily encode/decode hashes and experiment with the library.

Simple encode & decode:

#include <sqids.h>

sqids_t *sqids = sqids_new(SQIDS_DEFAULT_ALPHABET, 0, sqids_bl_list_all(NULL));

unsigned long long nums[] = {1, 2, 3};
char *hash = sqids_encode(sqids, 3, nums);  // => "86Rf07"
sqids_decode(sqids, hash, nums, 3);         // => 3

sqids_mem_free(hash);
sqids_free(sqids);

Enforce a minimum length:

#include <sqids.h>

sqids_t *sqids = sqids_new(SQIDS_DEFAULT_ALPHABET, 10, sqids_bl_list_all(NULL));

unsigned long long nums[] = {1, 2, 3};
char *hash = sqids_encode(sqids, 3, nums);  // => "86Rf07xd4z"
sqids_decode(sqids, hash, nums, 3);         // => 3

sqids_mem_free(hash);
sqids_free(sqids);

Using a custom alphabet will produce different results:

#include <sqids.h>

sqids_t *sqids = sqids_new("FxnXM1kBN6cuhsAvjW3Co7l2RePyY8DwaU04Tzt9fHQrqSVKdpimLGIJOgb5ZE", 0, sqids_bl_list_all(NULL));

unsigned long long nums[] = {1, 2, 3};
char *hash = sqids_encode(sqids, 3, nums);  // => "B4aajs"
sqids_decode(sqids, hash, nums, 3);         // => 3

sqids_mem_free(hash);
sqids_free(sqids);

Custom blocklist is just as simple:

#include <sqids.h>

sqids_bl_t *bl = sqids_bl_new(NULL);
sqids_bl_add_tail(bl, "86Rf07");
sqids_t *sqids = sqids_new(SQIDS_DEFAULT_ALPHABET, 0, bl);

unsigned long long nums[] = {1, 2, 3};
char *hash = sqids_encode(sqids, 3, nums);  // => "se8ojk"
sqids_decode(sqids, hash, nums, 3);         // => 3

sqids_mem_free(hash);
sqids_free(sqids);

For a more thorough example of the API, take a look at src/main.c.

MIT