CODEWITHSUNDEEP
c++matrixc++11bit-manipulationtranspose
Dejwi
Dejwi

Reputation: 4487

C++ map matrix column into rank - bit manipulations

I've got (binary) matrices represented by uint64_t (from C++11). And I'd like to be able to efficiently map from any column into first rank. For example

0 1 0 0 0 0 0 0
0 1 0 0 0 0 0 0
0 1 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 1 0 0 0 0 0 0
0 1 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 1 0 0 0 0 0 0

uint64_t matrice = 0x4040400040400040uLL;
uint64_t matrice_2 = map(matrice, ColumnEnum::Column2);

1 1 1 0 1 1 0 1
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0

matrice_2 contains 0xED00000000000000uLL;

Upvotes: 3

Views: 442

Answers (3)

ipc
ipc

Reputation: 8143

Great question. I really enjoyed the hacking. Here is my solution:

uint64_t map(uint64_t x, int column)
{
  x = (x >> (7 - column)) & 0x0101010101010101uLL;
  x = (x | (x >> 7)) & 0x00FF00FF00FF00FFuLL;
  x = (x | (x >> 14))& 0x000000FF000000FFuLL;
  x = (x | (x >> 28))& 0x00000000000000FFuLL;
  return x << 56;
}

A working example can be found at ideone, where the call is really map(matrice, ColumnEnum::Column2).

Upvotes: 3

Dietmar K&#252;hl
Dietmar K&#252;hl

Reputation: 153792

A nice little riddle. Here is a reasonably readable version:

matrice = (matrice >> (8ull - column)) & 0x0101010101010101ull;
uint64_t result((  ((matrice >>  0ul)  & 0x01ull)
                 | ((matrice >>  7ul)  & 0x02ull)
                 | ((matrice >> 14ull) & 0x04ull)
                 | ((matrice >> 21ull) & 0x08ull)
                 | ((matrice >> 28ull) & 0x10ull)
                 | ((matrice >> 35ull) & 0x20ull)
                 | ((matrice >> 42ull) & 0x40ull)
                 | ((matrice >> 49ull) & 0x80ull)) << 56ull);

Upvotes: 3

PiotrNycz
PiotrNycz

Reputation: 24347

First define bitmask for every column:

uint64_t columns[8] = {
  0x8080808080808080uLL,
  0x4040404040404040uLL,
  //...
  0x0101010101010101uLL
};

by applying column bitmask to your "matrice" you get only this column:

uint64_t col1 = matrice & columns[1]; // second column - rest is empty

by shifting you can get only first column case:

uint64_t col0 = (col1 << 1); // second column - rest is empty
//                       ^ this number is just zero based index of column,,,

Now first bit is on right place - just set the next 7 bits:

 col0 |= (col0 & (1 << 55)) << 7; // second bit...
 // ....

Or just use std::bitset<64>, I would do....

Upvotes: 1

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