tuple_t extEuclid(unsigned int a, unsigned int b) {
if(a < b) { // a must be the greater one
unsigned int t = a;
a = b;
b = t;
}
if(b == 0) return (tuple_t){1, 0, a};
tuple_t t = extEuclid(b, a % b);
return (tuple_t){t.b, t.a - (a/b) * t.b, t.c};
}
/* Print n strings that are hashed to 0 by universal_hash seeded with seed
* although character can be anything <255, only 'pretty' characters are chosen
* for the 'prettiness' of the output */
void collide_clever(unsigned int size, unsigned int seed, int n) {
static bool first = true;
int randSize, sum, gen, i, x, toChange, *randArr = NULL;
unsigned char y, *line = malloc(sizeof(*line) * MAX_LEN);
// index randsize will be '\0'. Therefore, randsize < MAXLEN.
#if I_DUNNO_STATS
unsigned char **used = malloc(sizeof(*used) * n);
int useCount = 0;
#endif
if(size > 255) {
fprintf(stderr, "Size is too big. Outside assignment scope!\n");
fprintf(stderr, "I had spent ages addressing that but after nerf,\n");
fprintf(stderr, "and reached the state where generated string hash to one index for any size.\n");
fprintf(stderr, "But no point working further on and submitting risky long code.\n");
fprintf(stderr, "Anyway, terminating in sass :P\n");
exit(EXIT_FAILURE);
}
// initialize universal hash's random array
if(first == true) { // some say there are still first == true for most CompSc students
first = false;
srand(seed);
universal_hash(line, size);
}
// find out which index to work on
randArr = getRandArray(seed, size);
i = 0;
while(randArr[i] == 0)
i++;
toChange = i;
#if GET_TIME
clock_t start = clock();
fprintf(stderr, "N: %d\nSIZE: %d\n", n, size);
#endif
// initialize my random array
while(n > 0) {
sum = 0;
randSize = rand() % (MAX_LEN - INIT_SIZE - 1) + INIT_SIZE;
line[randSize] = '\0';
for(i = 0; i < randSize; i++) {
if(i != toChange) {
line[i] = rand() % (MAX_ASCII - START_ASCII - 1) + START_ASCII;
sum += (line[i] * randArr[i]);
}
}
// explanation of how and why this works and the algorithm is in the report, question 7.
gen = size - (sum % size);
tuple_t t = extEuclid(size, randArr[toChange]);
x = t.b * gen / t.c;
if(x < 0)
y = size - ((-x) % size);
else
y = x % size;
if(y < START_ASCII && size + y < MAX_ASCII) {
y += size * (1 + (START_ASCII - y) / size);
// minimum multiple of size to add to make y fall within given range
} else if(y == '\n' || y == 127 || y <= 32) { // 127 = DEL ASCII
// if the string genrated has newline character, solution is not valid since
// strings are loaded with fgets().
// this happens is about 0.3% of the time.
// Forum post requirement: avoid characters from 0-32.
continue;
// had attempted to add minimum multiple of size,
// faster to generate new rather than resolve
}
line[toChange] = y;
#if I_DUNNO_STATS
for(i = 0; i < useCount; i++) {
// make sure it is not duplicate
if(strcmp((char*) used[i], (char*) line) == 0)
break;
}
used[useCount] = malloc(sizeof(unsigned char) * (strlen((char*)line) + 1));
strcpy((char*)used[useCount], (char*)line);
// fprintf(stderr, "USED %d: %s\n", useCount, used[useCount]);
useCount++;
#endif
fprintf(stdout, "%s\n", line);
n--;
}
#if GET_TIME
clock_t end = clock();
fprintf(stderr, "TIME TAKEN: %.5f s\n", ((double) end - start) / CLOCKS_PER_SEC);
#endif
#if I_DUNNO_STATS
for(i = 0; i < useCount; i++) {
free(used[i]);
}
free(used);
#endif
free(randArr);
free(line);
}
int modInv(int n, int m) {
int x, y, gcd;
extEuclid(n, m, x, y, gcd);
if (gcd == 1) return x % m;
return 0;
}