k&r exercise

exe_2_6.c

#include <stdio.h>
#include <limits.h>

unsigned int getbits(unsigned int x, int p, int n);
unsigned int setbits(unsigned int x, int p, int n, unsigned int y);
void print_bits(unsigned int x);

int main() {
  print_bits(setbits((1 << 5) - 1, 3, 3, 10)); // x = 11111, y = 1010 => 10101

  return 0;
}

unsigned int getbits(unsigned int x, int p, int n) {
  return (x >> (p + 1 - n)) & ~(~0 << n);
}

unsigned int setbits(unsigned int x, int p, int n, unsigned int y) {
  return (x & (~0 << (p + 1))) | (getbits(y, n - 1, n) << (p + 1 - n)) | getbits(x, p - n, p - n + 1);
}

void print_bits(unsigned int x) {
  for (int i = CHAR_BIT * (int)sizeof(int) - 1; i >= 0; i--) {
    printf("%d", x >> i & 1);
  }
  printf("\n");
}

exe_2_7.c

#include <stdio.h>
#include <limits.h>

unsigned int getbits(unsigned int x, int p, int n);
unsigned int setbits(unsigned int x, int p, int n, unsigned int y);
unsigned int invert(unsigned int x, int p, int n);

void print_bits(unsigned int x);

int main() {
  print_bits(invert((1 << 5) - 1, 3, 3)); // x = 11111 -> 10001
  print_bits(invert(~0, 10, 5)); // x = 1..1 -> 1..100000111111

  return 0;
}

unsigned int getbits(unsigned int x, int p, int n) {
  return (x >> (p + 1 - n)) & ~(~0 << n);
}

unsigned int setbits(unsigned int x, int p, int n, unsigned int y) {
  return (x & (~0 << (p + 1))) | (getbits(y, n - 1, n) << (p + 1 - n)) | getbits(x, p - n, p - n + 1);
}

unsigned int invert(unsigned int x, int p, int n) {
  return setbits(x, p, n, ~getbits(x, p, n));
}

void print_bits(unsigned int x) {
  for (int i = CHAR_BIT * (int)sizeof(int) - 1; i >= 0; i--) {
    printf("%d", x >> i & 1);
  }
  printf("\n");
}

exe_2_8.c

#include <stdio.h>
#include <limits.h>

unsigned int rightrot(unsigned int x, int n);

void print_bits(unsigned int x);

int main() {
  print_bits(rightrot(10U, 4)); // 1010....0
  print_bits(rightrot(10U | (~0U << 4), 4)); // 10101....1
  return 0;
}

unsigned int rightrot(unsigned int x, int n) {
  for (int i = 0; i < n; i++) {
    x = (x >> 1) | ((x & 1) << (CHAR_BIT * (int)sizeof(int) - 1));
  }
  return x;
}


void print_bits(unsigned int x) {
  for (int i = CHAR_BIT * (int)sizeof(int) - 1; i >= 0; i--) {
    printf("%d", x >> i & 1);
  }
  printf("\n");
}

exe_2_9.c

#include <stdio.h>
#include <limits.h>

int bitcount(unsigned int x);
int bitcount_faster(unsigned int x);

int main() {
  int N = 329482135U;
  printf("%d, %d\n", bitcount(N), bitcount_faster(N));
  return 0;
}

int bitcount(unsigned int x) {
  int b;
  for (b = 0; x != 0; x >>= 1) {
    if (x & 1) b++;
  }
  return b;
}

int bitcount_faster(unsigned int x) {
  int b;
  for (b = 0; x != 0; x &= x - 1) {
    b++;
  }
  return b;
}