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feat: add count distinct primes from binary string algorithm #2970

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@rudrakshtank rudrakshtank commented Jul 19, 2025 •
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@realstealthninja Could you please review this PR?

Description of Change

Added count_distinct_primes_from_binary_string.cpp to the bit_manipulation/ directory.

This program counts the number of distinct prime numbers that can be formed from a binary string using:

  • Any permutation of characters (i.e., swapping)
  • Changing '1's to '0's (but not the reverse)

It uses an efficient implementation of the Sieve of Eratosthenes for prime checking and bitmasking to explore all possible combinations.

Includes:

  • Doxygen-style documentation
  • A working main() with an example test case

Contributors guide

Checklist

  • Added description of change
  • File name matches File name guidelines
  • Added test case/example in main()
  • Added Doxygen-style documentation
  • Comments/documentation added as needed
  • PR title follows semantic commit guidelines
  • Confirmed this is not a duplicate contribution
  • I acknowledge that all my contributions will be made under the project's license

Notes:

Efficient bitmask + sieve-based solution to find all valid prime numbers from binary strings using allowed transformations.

#include <unordered_set>
#include <algorithm>

const int MAX = 1e6;
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@realstealthninja realstealthninja Aug 19, 2025

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max cannot be a negative number, hence replace int with uint32_t

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@realstealthninja realstealthninja Aug 24, 2025

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you still havent done this

Suggested change
const int MAX = 1e6;
const uint32_t MAX = 1e6;

#include <algorithm>

const int MAX = 1e6;
std::vector<bool> is_prime;
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@realstealthninja realstealthninja Aug 19, 2025

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Suggested change
std::vector<bool> is_prime;
staticstd::vector<bool> is_prime;

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@realstealthninja realstealthninja Aug 19, 2025

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Since you want to pre compute primes why not make this a constexpr? and assign in it directly? instead of a call from main. Also afaik vectors cant be used as constexpr in c++17 hence it might be preferred if you use std::array instead.

if any of this sounds confusing let me know I can explain or direct you to resources to learn about this feature of c++

std::unordered_set<int> seen;

for (int i = 2; i < limit; i++) {
if (__builtin_popcount(i) <= k && is_prime[i]) {
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__builtin_popcount is not part of the STL library it only exists in GCC.
thus this should be replaced with std::popcount()

Suggested change
if (__builtin_popcount(i) <= k && is_prime[i]) {
if (std::popcount(i) <= k && is_prime[i]) {

Comment on lines 19 to 22
#include <iostream>
#include <vector>
#include <unordered_set>
#include <algorithm>
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Suggested change
#include <iostream>
#include <vector>
#include <unordered_set>
#include <algorithm>
#include <bit> /// for std::popcount
#include <iostream>
#include <vector>
#include <unordered_set>
#include <algorithm>

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@realstealthninja realstealthninja Aug 24, 2025

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please add documentation for all the above headers as shown in the example

@realstealthninja realstealthninja changed the title (削除) Add count distinct primes from binary string algorithm (削除ここまで) (追記) feat: add count distinct primes from binary string algorithm (追記ここまで) Aug 19, 2025
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Thank you for the review, I'll get back to you with suggested changes
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I have done with the suggested changes, please review the changes

Comment on lines 71 to 77
void tests(){
precomputePrimes();
std::string s;
std::cin >> s;
std::cout << countPrimeBinaryStrings(s) << std::endl;
}

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tests should be outside the namespace and static, A test should be comparing a computed value to a known value using an assert statement

#include <unordered_set>
#include <algorithm>

const int MAX = 1e6;
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you still havent done this

Suggested change
const int MAX = 1e6;
const uint32_t MAX = 1e6;

Comment on lines 19 to 22
#include <iostream>
#include <vector>
#include <unordered_set>
#include <algorithm>
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@realstealthninja realstealthninja Aug 24, 2025

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please add documentation for all the above headers as shown in the example


// Example test cases
assert(countPrimeBinaryStrings("1") == 0); // Only "1" -> not prime
assert(countPrimeBinaryStrings("11") > 0); // Should form primes like 3
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@realstealthninja realstealthninja Aug 25, 2025

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you need to compare against the actual value that should be forming, comparing this way against 0 is too general. here from my estimation
01 not prime
10 prime
11 prime
so the answer should be three
you should be comparing against three

// Example test cases
assert(countPrimeBinaryStrings("1") == 0); // Only "1" -> not prime
assert(countPrimeBinaryStrings("11") > 0); // Should form primes like 3
assert(countPrimeBinaryStrings("101") > 0); // Can form primes like 5
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Same thing here, saying any binary string which is non zero and non 1 can form primes above 0 is not a great test.
here i estimate
001 not prime
100 not prime
101 prime

so the answer should be one, please compare against the actual number

/**
* @brief Precomputes all prime numbers up to MAX using Sieve of Eratosthenes.
*/
void precomputePrimes() {
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Again would recommend using constexpr to do the computation at compile time

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@Panquesito7 Panquesito7 Awaiting requested review from Panquesito7 Panquesito7 is a code owner

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