Significant C++ code execution slowdown - c++

Question

So I have to solve one USACO problem involving computing all the primes <= 100M and printing these of them which are palindromes while the restrictions are 16MB memory and 1 sec executions time. So I had to make a lot of optimisations.
Please take a look at the following block of code:
for(int i = 0; i < all.size(); ++i)
{
if(all[i] < a) continue;
else if(all[i] > b) break;
if(isPrime(all[i]))
{
char buffer[50];
//toString(all[i], buffer);
int c = all[i];
log10(2);
buffer[3] = 2;
//buffer[(int)log10(all[i])+1] = '\n';
//buffer[(int)log10(all[i])+2] = '\0';
//fputs(buffer, pFile);
}
}
Now, it executes in the satisfying 0.5 sec range, but when I change log10(2) to log10(all[i]) it skyrockets nearly to 2 seconds! For no apparent reason. I'm assigning all[i] to the variable c and it doesn't slow down the execution at all, but when I pass all[i] as parameter, it makes the code 4 times slower! Any ideas why this is happening and how I can fix it?
Whole code:
/*
ID: xxxxxxxx
PROG: pprime
LANG: C++11
*/
#include <fstream>
#include <iostream>
#include <vector>
#include <queue>
#include <stack>
#include <set>
#include <string>
#include <cstring>
#include <algorithm>
#include <list>
#include <ctime>
#include <cstdio>
using namespace std;
typedef struct number Number;
ifstream fin("pprime.in");
ofstream fout("pprime.out");
int MAXN = 100000000;
unsigned short bits[2000000] = {};
vector<int> primes;
vector<int> all;
int a, b;
short getBit(int atPos)
{
int whichNumber = (atPos-1) / 16;
int atWhichPosInTheNumber = (atPos-1) % 16;
return ((bits[whichNumber] & (1 << atWhichPosInTheNumber)) >> atWhichPosInTheNumber);
}
void setBit(int atPos)
{
int whichNumber = (atPos-1) / 16;
int atWhichPosInTheNumber = (atPos-1) % 16;
int old = bits[whichNumber];
bits[whichNumber] = bits[whichNumber] | (1 << atWhichPosInTheNumber);
}
void calcSieve()
{
for(int i = 2; i < MAXN; ++i)
{
if(getBit(i) == 0)
{
for(int j = 2*i; j <= (MAXN); j += i)
{
setBit(j);
}
primes.push_back(i);
}
}
}
int toInt(list<short> integer)
{
int number = 0;
while(!integer.empty())
{
int current = integer.front();
integer.pop_front();
number = number * 10 + current;
}
return number;
}
void toString(int number, char buffer[])
{
int i = 0;
while(number != 0)
{
buffer[i] = number % 10 + '0';
number /= 10;
}
}
void DFS(list<short> integer, int N, int atLeast)
{
if(integer.size() > N)
{
return;
}
if(!(integer.size() > 0 && (integer.front() == 0 || integer.back() % 2 == 0)) && atLeast <= integer.size())
{
int toI = toInt(integer);
if(toI <= b) all.push_back(toInt(integer));
}
for(short i = 0; i <= 9; ++i)
{
integer.push_back(i);
integer.push_front(i);
DFS(integer, N, atLeast);
integer.pop_back();
integer.pop_front();
}
}
bool isPrime(int number)
{
for(int i = 0; i < primes.size() && number > primes[i]; ++i)
{
if(number % primes[i] == 0) return false;
}
return true;
}
int main()
{
int t = clock();
ios::sync_with_stdio(false);
fin >> a >> b;
MAXN = min(MAXN, b);
int N = (int)log10(b) + 1;
int atLeast = (int)log10(a) + 1;
for(short i = 0; i <= 9; ++i)
{
list<short> current;
current.push_back(i);
DFS(current, N, atLeast);
}
list<short> empty;
DFS(empty, N, atLeast);
sort(all.begin(), all.end());
//calcSieve
calcSieve();
//
string output = "";
int ends = clock() - t;
cout<<"Exexution time: "<<((float)ends)/CLOCKS_PER_SEC<<" seconds";
cout<<"\nsize: "<<all.size()<<endl;
FILE* pFile;
pFile = fopen("pprime.out", "w");
for(int i = 0; i < all.size(); ++i)
{
if(all[i] < a) continue;
else if(all[i] > b) break;
if(isPrime(all[i]))
{
char buffer[50];
//toString(all[i], buffer);
int c = all[i];
log10(c);
buffer[3] = 2;
//buffer[(int)log10(all[i])+1] = '\n';
//buffer[(int)log10(all[i])+2] = '\0';
//fputs(buffer, pFile);
}
}
ends = clock() - t;
cout<<"\nExexution time: "<<((float)ends)/CLOCKS_PER_SEC<<" seconds";
ends = clock() - t;
cout<<"\nExexution time: "<<((float)ends)/CLOCKS_PER_SEC<<" seconds";
fclose(pFile);
//fout<<output;
return 0;
}

Answer 1

I think you've done this backwards. It seems odd to generate all the possible palindromes (if that's what DFS actually does... that function confuses me) and then check which of them are prime. Especially since you have to generate the primes anyway.
The other thing is that you are doing a linear search in isPrime, which is not taking advantage of the fact that the array is sorted. Use a binary search instead.
And also, using list instead of vector for your DFS function will hurt your runtime. Try using a deque.
Now, all that said, I think that you should do this the other way around. There are a huge number of palindromes that won't be prime. What's the point in generating them? A simple stack is all you need to check if a number is a palindrome. Like this:
bool IsPalindrome( unsigned int val )
{
int digits[10];
int multiplier = 1;
int *d = digits;
// Add half of number's digits to a stack
while( multiplier < val ) {
*d++ = val % 10;
val /= 10;
multiplier *= 10;
}
// Adjust for odd-length palindrome
if( val * 10 < multiplier ) --d;
// Check remaining digits
while( val != 0 ) {
if(*(--d) != val % 10) return false;
val /= 10;
}
return true;
}
This avoids the need to call log10 at all, as well as eliminates all that palindrome generation. The sieve is pretty fast, and after that you'll only have a few thousand primes to test, most of which will not be palindromes.
Now your whole program becomes something like this:
calcSieve();
for( vector<int>::iterator it = primes.begin(); it != primes.end(); it++ ) {
if( IsPalindrome(*it) ) cout << *it << "\n";
}
One other thing to point out. Two things actually:
int MAXN = 100000000;
unsigned short bits[2000000] = {};
bits is too short to represent 100 million flags.
bits is uninitialised.
To address both these issues, try:
unsigned short bits[1 + MAXN / 16] = { 0 };