91 lines
2.1 KiB
C++
91 lines
2.1 KiB
C++
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/*
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* C++ Program to Implement Segmented Sieve
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*/
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#include <iostream>
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#include <cstring>
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#define MAX 46656
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#define LMT 216
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#define LEN 4830
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#define RNG 100032
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#define sq(x) ((x)*(x))
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#define mset(x,v) memset(x, v , sizeof(x))
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#define chkC(x,n) (x[n >> 6] & (1 << ((n >> 1) & 31)))
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#define setC(x,n) (x[n >> 6] |= (1 << ((n >> 1) & 31)))
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using namespace std;
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unsigned base[MAX/64], segment[RNG/64], primes[LEN];
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/*
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* Generates all the necessary prime numbers and marks them in base[]
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*/
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void sieve()
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{
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unsigned i, j, k;
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for (i = 3; i < LMT; i += 2)
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{
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if (!chkC(base, i))
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{
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for (j = i * i, k = i << 1; j < MAX; j += k)
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setC(base, j);
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}
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}
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for (i = 3, j = 0; i < MAX; i += 2)
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{
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if (!chkC(base, i))
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primes[j++] = i;
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}
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}
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/*
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* Returns the prime-count within range [a,b] and marks them in segment[]
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*/
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int segmented_sieve(int a, int b)
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{
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unsigned i, j, k, cnt = (a <= 2 && 2 <=b )? 1 : 0;
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if (b < 2)
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return 0;
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if (a < 3)
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a = 3;
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if (a % 2 == 0)
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a++;
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mset (segment, 0);
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for (i = 0; sq(primes[i]) <= b; i++)
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{
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j = primes[i] * ((a + primes[i] - 1) / primes[i]);
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if (j % 2 == 0) j += primes[i];
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for (k = primes[i] << 1; j <= b; j += k)
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{
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if (j != primes[i])
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setC(segment, (j - a));
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}
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}
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for (i = 0; i <= b - a; i += 2)
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{
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if (!chkC(segment, i))
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cnt++;
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}
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return cnt;
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}
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/*
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* Main
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*/
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int main()
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{
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sieve();
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int a, b;
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cout<<"Enter Lower Bound: ";
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cin>>a;
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cout<<"Enter Upper Bound: ";
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cin>>b;
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cout<<"Number of primes between "<<a<<" and "<<b<<": ";
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cout<<segmented_sieve(a, b)<<endl;
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return 0;
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}
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/*
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Enter Lower Bound: 7
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Enter Upper Bound: 600
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Number of primes between 7 and 600: 106
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------------------
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(program exited with code: 1)
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Press return to continue
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