Doubt in understanding a piece of code

#1

why there is need to add mod;
for(int i=0;i+m<=n;i++){
int newhash=pref[i-m+1];
if(i-1>0){
newhash-=pref[i-1];
newhash+=mod;
newhash%=mod;

        if(newhash==(pat_hash*power(p,i))%mod){
            cout<<i<<endl;
        }
    }
}
#2

hi… so basically we add mod to avoid the result becoming negative… thats all

#3

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