Skip to content

Instantly share code, notes, and snippets.

Embed
What would you like to do?
Ported of Firebase pushid generator in C, origin: https://gist.github.com/mikelehen/3596a30bd69384624c11#file-generate-pushid-js
/*
* Fancy ID generator that creates 20-character string identifiers with the
* following properties:
*
* 1. They're based on timestamp so that they sort *after* any existing ids.
* 2. They contain 72-bits of random data after the timestamp so that IDs won't
* collide with other clients' IDs.
* 3. They sort *lexicographically* (so the timestamp is converted to characters
* that will sort properly).
* 4. They're monotonically increasing. Even if you generate more than one in
* the same timestamp, the
* latter ones will sort after the former ones. We do this by using the previous
* random bits
* but "incrementing" them by 1 (only in the case of a timestamp collision).
* author: hungnv@opensource.com.vn
* origin: https://gist.github.com/mikelehen/3596a30bd69384624c11#file-generate-pushid-js
*/
#include <stdio.h>
#include <stdlib.h>
#include <sys/time.h>
#include <math.h>
static char DICT[] = "-0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz";
#include <sys/time.h>
#include <malloc.h>
#include <string.h>
#include <zlib.h>
long long last_gen_time = 0L;
long long current_timestamp() {
struct timeval te;
gettimeofday(&te, NULL);
long long milliseconds = te.tv_sec * 1000LL + te.tv_usec / 1000;
return milliseconds;
}
double
randr() {
return (double) rand() / (double) (RAND_MAX + 1.0);
}
int main() {
srand(current_timestamp());
long long mls = current_timestamp();
int i;
char last_random_char[72] = "";
int collison = (mls == last_gen_time) ? 1 : 0;
char time_stamp_char[8] = "";
for (i = 7; i >= 0; i--) {
time_stamp_char[i] = DICT[(int) (mls % 64)];
printf("time stamp char at %d: %c\n", i, time_stamp_char[i]);
mls = mls / 64;
}
if (mls != 0) {
printf("mls should be 0");
return 1;
}
if (!collison) {
for (i = 0; i < 12; i++) {
double fl = randr() * 64;
printf("floor: %lf\n", fl);
// last_random_char[i] = (char) (fl);
last_random_char[i] = (char) floor(fl);
}
} else {
int last_value_int = 0;
for (i = 11; i >= 0 && last_random_char[i] == 63; i--) {
last_value_int = i;
last_random_char[i] = 0;
}
last_random_char[last_value_int]++;
}
time_stamp_char[8] = '\0';
char id[20] = "";
strcpy(id, time_stamp_char);
for (i = 0; i < 12; i++) {
int g = (int) last_random_char[i];
//printf("id_: %d\n", g);
strncat(id, &DICT[g], 1);
}
printf("final: %s \n", id);
printf("id length: %zu\n", strlen(id));
//printf("current time: %lld\n", mls);
return 0;
}
@stefanfisk

This comment has been minimized.

Copy link

commented Mar 6, 2016

Line 79 is a buffer overflow.

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment
You can’t perform that action at this time.