Can someone explain this implementation of a hashtable?

This is the problem setter's implementation of a hash table. I am fairly new to hash tables, and the only ones I have seen make use of structures for forming the lists. Can someone explain this implementation, which is given in the editorial, though not explained.

I can't understand how collisions have been taken care of. If possible, please explain the role of vis[], head[] and nextcell[].

http://ift.tt/1BxSJJN

#include<bits/stdc++.h>
using namespace std;

#define MEM(a, b) memset(a, (b), sizeof(a))
#define CLR(a) memset(a, 0, sizeof(a))
#define MAX(a, b) ((a) > (b) ? (a) : (b))
#define MIN(a, b) ((a) < (b) ? (a) : (b))
#define ABS(X) ( (X) > 0 ? (X) : ( -(X) ) )
#define S(X) ( (X) * (X) )
#define SZ(V) (int )V.size()
#define FORN(i, n) for(i = 0; i < n; i++)
#define FORAB(i, a, b) for(i = a; i <= b; i++)
#define ALL(V) V.begin(), V.end()
#define IN(A, B, C)  ((B) <= (A) && (A) <= (C))

typedef pair<int,int> PII;
typedef pair<double, double> PDD;
typedef vector<int> VI;
typedef vector<PII > VP;

#define AIN(A, B, C) assert(IN(A, B, C))

typedef long long int LL;

char S1[250005], S2[250005];
int len1, len2;

int vis[35000000];
int head[35000000];
int nextCell[35000000];
int info[35000000];
int node;

LL prime2 = 34369934, prime1 = 9584612342;
int mark;
int cnt;

void insert(int hash, int start)
{
    if(vis[hash] != mark)
    {
        vis[hash] = mark;
        head[hash] = -1;
    }

    info[node] = start;
    nextCell[node] = head[hash];
    head[hash] = node++;
}

int check(int hash, int at, int len)
{
    if(vis[hash] != mark) return 0;

    int flag, j, i;
    for(i = head[hash]; i != -1; i = nextCell[i])
    {
        int val = info[i];
        flag = 1;
        for(j = 0; j < len; j++)
        {
            cnt++;
            if(S1[val + j] != S2[at + j])
            {
                flag = 0;
                break;
            }
        }

        if(flag) 
            return 1;
    }

    return 0;
}

int possible(int len)
{
    mark++;
    node = 0;

    if(len == 0) return -1;

    LL hash = 0, mul = 1;
    int i;

    for(i = 0; i < len; i++)
    {
        hash = (hash * prime1 + S1[i]) % prime2;
        mul = (mul * prime1) % prime2;
    }

    mul = (prime2 - mul) % prime2;

    insert(hash, 0);
    for(; i < len1; i++)
    {
        hash = (hash * prime1 + S1[i] + mul * S1[i - len]) % prime2;
        insert(hash, i - len + 1);
    }

    hash = 0;
    for(i = 0; i < len; i++)
    {
        hash = (hash * prime1 + S2[i]) % prime2;
    }

    if(check(hash, 0, len)) return 0;
    for(; i < len2; i++)
    {
        hash = (hash * prime1 + S2[i] + mul * S2[i - len]) % prime2;
        if(check(hash, i - len + 1, len)) return i - len + 1;
    }

    return -1;
}

int main()
{
    //Some processing
}