math - Triple Modular Multiplicaiton -

i calculating following sum:

(a[x]+ma[x-1]+2ma[x-2]+3m*a[x-3]+....)%mod (mod=1e9+7)

for this, using loop.

long long mulmod(long long a,long long  b,long long c) {     if (a == 0 || b == 0)         return 0;     if (a == 1)         return b;     if (b == 1)         return a;     long long a2 = mulmod(a, b / 2, c);     if ((b & 1) == 0)      {         return (a2 + a2) % c;     }      else      {         return ((a % c) + (a2 + a2)) % c;     } }  res=a[x]%mod; for(i=x-1;i>=1;i--)     res=(res%mod+mulmod(mulmod(x-i,m,mod),a[i],mod))%mod; 

however, still giving me overflow errors. basic error, guess in (abc)%mod.

thank you.

you need incorporate following modular arithmetic identities program avoid overflow:

(a + b + ...) mod c = (a mod c + b mod c + ... mod c) mod c

and

(a * b * ...) mod c = (a mod c * b mod c * ... mod c) mod c