Can anyone tell me what this function is doing? (The code has been written in C,

Question

Can anyone tell me what this function is doing? (The code has been written in C, and we assume that we have 32 bit integers).

int myFunction(int x) {

int mask1, mask2, mask4, mask8, mask16;
  
mask2 = 0x33 + (0x33 << 8);
mask2 += mask2 << 16;
mask1 = mask2 ^ (mask2 << 1);
mask4 = 0x0F + (0x0F << 8);
mask4 += mask4 << 16;
mask8 = 0xFF + (0xFF << 16);
mask16 = 0xFF + (0xFF << 8);
  
x = (x & mask1) + ((x >> 1) & mask1);
x = (x & mask2) + ((x >> 2) & mask2);
x = (x & mask4) + ((x >> 4) & mask4);
x = (x & mask8) + ((x >> 8) & mask8);
x = (x & mask16) + ((x >> 16) & mask16);
  
return x;
}

Explanation / Answer

Working of function is given in comments below ------->

int myFunction(int x) {

        int mask1, mask2, mask4, mask8, mask16;
       
        //0x33(16) = 51(10) = 110011(2)
        mask2 = 0x33 + (0x33 << 8);//110011 left shift by 8 so it become 11001100000000(2) = 13056(10); 51+13056 = 13107

        mask2 += mask2 << 16;//mask2 = mask2 + (mask2 left shifted by 16)

        mask1 = mask2 ^ (mask2 << 1);//mask 1 = mask2 ^ (mask 2 left shift by 1)

        mask4 = 0x0F + (0x0F << 8);//0x0F(16) = 1111(2); mask 4 = 1111 + (1111 left shift by 8)

        mask4 += mask4 << 16;//mask4 = mask4 + (mask4 left shift by 16)

        mask8 = 0xFF + (0xFF << 16);//0xFF(16)=11111111(2); mask8 = 11111111 + (11111111 left shift by 16)

        mask16 = 0xFF + (0xFF << 8);//mask16 = 11111111 + (11111111 left shift by 8)

      
        //& is bitwise and; >> is right shift
        x = (x & mask1) + ((x >> 1) & mask1);//(bitwaise and between x and mask1) + (bitwise and between (x right shift by 1) and mask1)

        x = (x & mask2) + ((x >> 2) & mask2);//(bitwaise and between x and mask2) + (bitwise and between (x right shift by 2) and mask2)

        x = (x & mask4) + ((x >> 4) & mask4);//(bitwaise and between x and mask4) + (bitwise and between (x right shift by 4) and mask4)

        x = (x & mask8) + ((x >> 8) & mask8);//(bitwaise and between x and mask8) + (bitwise and between (x right shift by 8) and mask8)

        x = (x & mask16) + ((x >> 16) & mask16);//(bitwaise and between x and mask16) + (bitwise and between (x right shift by 16) and mask16)

  
        return x;
    }

To know better about this function try this program main.cpp -------->

#include <iostream>
using namespace std;
int myFunction(int x) {

        int mask1, mask2, mask4, mask8, mask16;
        cout<<"mask1 = "<<mask1<<"; mask2 = "<<mask2<<"; mask4 = "<<mask4<<"; mask8 = "<<mask8<<"; mask16 = "<<mask16;
        cout<<"; 0x33 = "<<0x33<<endl;//0x33(16) = 51(10) = 110011(2)
        mask2 = 0x33 + (0x33 << 8);//110011 left shift by 8 so it become 11001100000000(2) = 13056(10); 51+13056 = 13107
        cout<<"mask2 = "<<mask2<<endl;
        mask2 += mask2 << 16;//mask2 = mask2 + (mask2 left shifted by 16)
        cout<<"mask2 = "<<mask2<<endl;
        mask1 = mask2 ^ (mask2 << 1);//mask 1 = mask2 ^ (mask 2 left shift by 1)
        cout<<"mask1 = "<<mask1<<endl;
        mask4 = 0x0F + (0x0F << 8);//0x0F(16) = 1111(2); mask 4 = 1111 + (1111 left shift by 8)
        cout<<"mask4 = "<<mask4<<endl;
        mask4 += mask4 << 16;//mask4 = mask4 + (mask4 left shift by 16)
        cout<<"mask4 = "<<mask4<<endl;
        mask8 = 0xFF + (0xFF << 16);//0xFF(16)=11111111(2); mask8 = 11111111 + (11111111 left shift by 16)
        cout<<"mask8 = "<<mask8<<endl;
        mask16 = 0xFF + (0xFF << 8);//mask16 = 11111111 + (11111111 left shift by 8)
        cout<<"mask16 = "<<mask16<<endl;
      
        //& is bitwise and; >> is right shift
        x = (x & mask1) + ((x >> 1) & mask1);//(bitwaise and between x and mask1) + (bitwise and between (x right shift by 1) and mask1)
        cout<<"x = "<<x<<endl;
        x = (x & mask2) + ((x >> 2) & mask2);//(bitwaise and between x and mask2) + (bitwise and between (x right shift by 2) and mask2)
        cout<<"x = "<<x<<endl;
        x = (x & mask4) + ((x >> 4) & mask4);//(bitwaise and between x and mask4) + (bitwise and between (x right shift by 4) and mask4)
        cout<<"x = "<<x<<endl;
        x = (x & mask8) + ((x >> 8) & mask8);//(bitwaise and between x and mask8) + (bitwise and between (x right shift by 8) and mask8)
        cout<<"x = "<<x<<endl;
        x = (x & mask16) + ((x >> 16) & mask16);//(bitwaise and between x and mask16) + (bitwise and between (x right shift by 16) and mask16)
        cout<<"x = "<<x<<endl;
  
        return x;
    }
int main()
{
    myFunction(10);
    return 0;
}

Output ------->

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