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 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 | #include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <assert.h>
typedef unsigned char uchar;
uchar get_bit(uchar ch, int bit_idx) {
return ch >> (7 - bit_idx) & 1;
}
// Ex: get_bit(10011100₂, 3) should return 1.
// ▲
uchar set_bit(uchar ch, int bit_idx, uchar new_value) {
if(new_value) { // bit_idx==2
// Set bit (to 1) // ↓
return ch | (1 << (7 - bit_idx)); // bitwise-or with a byte like this: 00100000
}
else {
// Unset bit (to 0) // bit_idx==2
assert(new_value == 0); // ↓
return ch & (~ (1 << (7 - bit_idx))); // bitwise-and with a byte like this: 11011111
}
// ~▒ means bitwise-NOT of ▒. It flips every 0 to 1, and 1 to 0.
// ~00100000₂ == 11011111₂
// Note: The notation for binary bits in the comments is not valid C code and is for illustration.
//
// 1 is the same as 00000001₂
//
// 0x01 == 00000001₂ == 1
// 0x80 == 10000000₂ == 128
}
/*
uchar get_bits(uchar ch, int start_bit, int end_bit) { // end_bit is inclusive
uchar result = 0x00;
int start_bit_idx = end_bit - start_bit + 1;
for(int bit_idx = start_bit, result_bit_idx = start_bit_idx; bit_idx <= end_bit; bit_idx++, result_bit_idx++) {
uchar bit_value = get_bit(ch, bit_idx);
result = set_bit(result, result_bit_idx, bit_value);
printf("bit_idx==%d result_bit_idx==%d bit_value==%d result==0x%02x\n",
bit_idx, result_bit_idx, bit_value, result);
}
return result;
}
uchar get_bits(uchar ch, int start_bit, int end_bit) { // end_bit is inclusive
return (ch << start_bit) >> (8 - (end_bit - start_bit + 1));
}
*/
uchar get_bits(uchar ch, int start_bit, int end_bit) { // end_bit is inclusive
ch <<= start_bit;
int num_bits = end_bit - start_bit + 1;
ch >>= 8 - num_bits;
return ch;
}
int main(int argc, char* argv[]) {
assert( get_bit(0xcb, 1) == 1 ); // 0xcb == 11001011₂
assert( get_bit(0xcb, 3) == 0 ); // 0xcb == 11001011₂
assert( set_bit(0x00, 0, 1) == 0x80 ); // 10000000₂
assert( set_bit(0xcb, 2, 1) == 0xeb ); // 0xcb == 11001011₂, 0xeb == 11101011₂
assert( set_bit(0xeb, 2, 0) == 0xcb ); // 0xcb == 11001011₂, 0xeb == 11101011₂
assert( get_bits(0x9c, 3, 6) == 0x0e);
// get_bits(10011100₂, 3, 6) ==> 00001110₂
// ‾‾‾‾ ‾‾‾‾
return EXIT_SUCCESS;
}
/* vim: set tabstop=4 shiftwidth=4 fileencoding=utf-8 noexpandtab: */
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