模板 BSGS

大佬的手寫Hash。飛快

ll p,b,n;
class HASH
{
public:
    ll a[100005],inv[100005],mod;
    HASH()
    {
        memset(a,-1,sizeof(a));
        mod=100003;
    }
    ll Find(ll x)
    {
        ll idx=x%mod;
        while(a[idx]!=x&&a[idx]!=-1)idx=(idx+1)%mod;
        return a[idx]==x?inv[idx]:-1;
    }
    void Insert(ll x,ll val)
    {
        ll idx=x%mod;
        while(a[idx]!=-1)idx=(idx+1)%mod;
        a[idx]=x;
        inv[idx]=val;
    }
};
ll powmod(ll a,ll n,ll p)
{
    ll r=1;
    while(n)
    {
        if(n&1)r=(r*a)%p;
        a=(a*a)%p;
        n>>=1;
    }
    return r;
}
ll bsgs(ll b,ll n,ll p)
{
    HASH sh;
    ll m=ceil(sqrt(p*1.0)),temp=1;
    for(int i=0;i<=m;++i)
    {
        sh.Insert(temp,i);
        temp=temp*b%p;
        if(temp==1)break;
    }
    ll inv=powmod(powmod(b,m,p),p-2,p);
    temp=n;
    for(int i=0;i<=p/m;++i)
    {
        ll r=sh.Find(temp);
        if(r!=-1)return i*m+r;
        temp=temp*inv%p;
    }
    return -1;
}

map實現hash，常數大的飛起

typedef long long ll;
ll powermod(ll bit,ll n,ll mod)
{
    ll ans=1;
    bit = bit % mod;
    while(n)
    {
        if(n & 1) ans = ans * bit %mod;
        bit = bit * bit % mod;
        n>>=1;
    }
    return ans;
}
ll bsgs(ll a,ll b ,ll p)
{
    ll m ,inv,temp=1;
    m= ceil(sqrt(p+0.5));
    inv = powermod(a,p-m-1,p);
    map<ll ,ll> myp;
    myp[1]=m;
    for(int i=1;i<m;++i)
    {
       temp = (temp * a) %p;
       if(!myp[temp]) myp[temp] = i;
    }
    for(int i=0;i<m;++i)
    {
        if(myp[b])
        {
            ll num=myp[b];
            myp.clear();
            return i*m + (num == m? 0:num);
        }
        b = (b*inv)%p;
    }
    return -1;
}