bits.cpp

#include <cstdint>
#include <iostream>
using namespace std;

/*
 * Returns the bits in range msb to lsb from bits. The returned bits are
 * shifted to the least signifcant bit.  The bits are ordered from most to
 * least signifcant bit; where the most signifcant bit has the index <number of
 * bits> - 1 and the least has 0.
 *
 * The range is inclusive [msb,lsb]. E.g., bits(x, 1, 1) returns the second
 * bit.
 *
 * For example, let x = 10011010, then bits(x, 4, 1) = 00001101.
 */
template <typename T> inline T bits(T bits, size_t msb, size_t lsb) {
  T ones = ~static_cast<T>(0);
  return (~(ones << ((msb + 1) - lsb))) & (bits >> lsb);
}

template <typename T>
inline T assign_bits(T bits, size_t msb, size_t lsb, T value) {
  size_t size = msb - lsb + 1;
  T ones = ~static_cast<T>(0);

  T v = (~(ones << size) & value) << lsb;
  T clear_mask = ~(~(ones << size) << lsb);
  T r = bits & clear_mask;
  return r | v;
}

template <typename T> void print_bits(T b) {
  int size = sizeof(T) * 8;
  for (int i = size - 1; i >= 0; i--) {
    cout << (int)bits(b, i, i);
  }
  cout << endl;
}

class BitString {
public:
  int length;

  BitString() {}
};

int main() {
  uint32_t x = ~static_cast<uint32_t>(0);
  auto b = bits(x, 1, 0);
  cout << x << " = " << b << endl;

  uint8_t a = 4;
  print_bits(a);

  for (int i = 0; i < 16; i++) {
    print_bits(assign_bits((uint16_t)0, i + 1, i, (uint16_t)3));
  }

  cout << endl;
  uint64_t alter = 0;
  for (int i = sizeof(alter) * 8 - 1; i >= 0; i--) {
    if (i % 2 == 1) {
      alter = assign_bits(alter, i, i, (uint64_t)1);
    }
  }
  print_bits(alter);

  return 0;
}