bitrot16 : demonstration of logical shift-and-rotate on signed integer

bitrot16.c

#include <stdint.h>
#include <stdio.h>

// perform a logical bit shift with rotation on a signed 16-bit integer
// second argument is number of bits:
// positive to shift right, negative shift left
int16_t bitrot16(int16_t x, int n) {
    // use a union with unsigned type,
    // so all our bitwise ops ignore sign
    union { int16_t si; uint16_t ui; } u;
    u.si = x;
    if (n > 0) { // shift right
	// low bits wrap to high
	u.ui = u.ui >> n | (u.ui << (16-n));
    } else { // shift left
	int m = n * -1;
	// high bits go left and wrap
	u.ui = u.ui << m | (u.ui >> (16-m));
    }
    return u.si;
}


// testing
void print_bits(int16_t x) {
    union { int16_t si; uint16_t ui; } u;
    u.si = x;
    for (int i=0; i<16; ++i) {
	if (u.ui & (1 << (15-i))) { 
	    printf("1");
	} else {
	    printf("0");
	}
    }
    printf("\n");
}

// test
int main(void) {
    int x;

    x = 0xdead;
    for  (int i=0; i<16; ++i) {
	print_bits(bitrot16(x, i));
    }

    for  (int i=0; i<16; ++i) {
	print_bits(bitrot16(x, i * -1));
    }
    
    x = 0xbeef;
    for  (int i=0; i<16; ++i) {
	print_bits(bitrot16(x, i));
    }

    for  (int i=0; i<16; ++i) {
	print_bits(bitrot16(x, i));
    }
    
    for  (int i=0; i<16; ++i) {
	print_bits(bitrot16(x, i * -1));
    }

    return 0;
}