1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 | #include <stdio.h>
#include <stdlib.h>
#include <assert.h>
/* [from office hours]
* This is a solution to the worksheet from Fri 11/17.
*/
unsigned char get_bits(unsigned char* buf, int start_bit, int end_bit) {
int start_byte_idx = start_bit / 8;
int end_byte_idx = end_bit / 8;
unsigned char ch0 = buf[start_byte_idx];
int start0 = start_bit % 8;
int end0 = start_byte_idx == end_byte_idx ? end_bit % 8 : 7;
int num_bits0 = (end0 - start0 + 1);
unsigned char bits0 = (ch0 << start0);
bits0 = bits0 >> (8 - num_bits0);
unsigned char ch1 = buf[end_byte_idx];
int start1 = 0;
int end1 = start_byte_idx == end_byte_idx ? 0 : end_bit % 8;
int num_bits1 = end1 + 1;
unsigned char bits1 = (ch1 << start1);
bits1 = bits1 >> (8 - num_bits1);
return bits0 << num_bits1 | bits1;
}
int main(int argc, char* argv[]) {
unsigned char buf[256] = {255, 127, 63, 31}; // initializes to zero
/* printf("%d\n", get_bits(buf, 2, 6)); */
/* assert( get_bits(buf, 2, 6) == 32 - 1 ); */
printf("%d\n", get_bits(buf, 5, 10));
assert( get_bits(buf, 5, 10) == 59 );
return EXIT_SUCCESS;
}
/* vim: set tabstop=4 shiftwidth=4 fileencoding=utf-8 noexpandtab: */
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