1 /*
2  * 2-7. Write a function invert(x,p,n) that returns x with the n bits
3  * that begin at position p inverted (i.e., 1 changed into 0 and vice versa),
4  * leaving the others unchanged.
5  */
6 
7 #include <stdio.h>
8 
9 unsigned setbits(unsigned x, int p, int n, unsigned y);
10 unsigned getbits(unsigned x, int p, int n);
11 unsigned invert(unsigned x, int p, int n);
12 
13 main()
14 {
15 	printf("%u: %u\n", 0xabcdf0f1, invert(0xabcdef01, 12, 9));
16 	printf("%u: %u\n", 0x1dd45678, invert(0x12345678, 27, 7));
17 	printf("%u: %u\n", 0x92fab03a, invert(0x92fab3da, 9, 5));
18 	printf("%u: %u\n", 0x1babc26a, invert(0x1ba83dea, 17, 11));
19 }
20 
21 /*
22 a    b    c    d    e    f    0    1
23 1010 1011 1100 1101 1110 1111 0000 0001
24 1010 1011 1100 1101 1111 0000 1111 0001
25 a    b    c    d    f    0    f    1
26 
27 1    2    3    4    5    6    7    8
28 0001 0010 0011 0100 0101 0110 0111 1000
29 0001 1101 1101 0100 0101 0110 0111 1000
30 1    d    d    4    5    6    7    8
31 
32 9    2    f    a    b    3    d    a
33 1001 0010 1111 1010 1011 0011 1101 1010
34 1001 0010 1111 1010 1011 0000 0011 1010
35 9    2    f    a    b    0    3    a
36 
37 1    b    a    8    3    d    e    a
38 0001 1011 1010 1000 0011 1101 1110 1010
39 0001 1011 1010 1011 1100 0010 0110 1010
40 1    b    a    b    c    2    6    a
41  */
42 
43 unsigned invert(unsigned x, int p, int n) {
44 	return setbits(x, p, n, ~getbits(x, p, n));
45 }
46 
47 unsigned setbits(unsigned x, int p, int n, unsigned y)
48 {
49 	return ((x >> (p+1)) << (p+1)) | (getbits(y, n-1, n) << (p-n+1)) | (x & ~(~0 << (p-n+1)));
50 }
51 
52 unsigned getbits(unsigned x, int p, int n)
53 {
54 	return (x >> (p+1-n)) & ~(~0 << n);
55 }