bit_array - simple use

bit array,use less memory to describe a set of int number,

 

example:

/**
 * use bit string to store int numbers,if char[n] = 1,then n is include,else n is not exclude,
 */

#include <stdio.h>

// set a bit to 1,which indicate that a number is include,
void setbit(char data[], int pos);
// get a bit,1 = include,0 = exclulde,
char getbit(char data[], int pos);
// indicate the position of bit in byte[]
struct bit_of_ints {
	int a; // byte's subscript
	char b; // position in a single byte,0 - 7,
};
// get position struct by pos,
struct bit_of_ints figure_bit_of_ints(int pos);

int main() {
	int length = 100;
	// figure byte count
	int bytecount = (length >> 3) + (length % 8 == 0 ? 0 : 1);
	char data[bytecount];
	int i;
	// init byte[]
	for (i = 0; i < bytecount; i++) {
		data[i] = 0;
	}
	// set bits
	for (i = 0; i < length; i++) {
		if (i % 10 == 0) {
			setbit(data, i);
		}
	}
	// get bits
	for (i = 0; i < length; i++) {
		if (getbit(data, i) != 0) {
			printf("%d,", i);
		}
	}
	printf("\n");
	return 0;
}

struct bit_of_ints figure_bit_of_ints(int pos) {
	struct bit_of_ints boi;
	char b = pos % 8;
	boi.a = pos >> 3;
	switch (b) {
	case 0:
		boi.b = 0x80;
		break;
	case 1:
		boi.b = 0x40;
		break;
	case 2:
		boi.b = 0x20;
		break;
	case 3:
		boi.b = 0x10;
		break;
	case 4:
		boi.b = 0x08;
		break;
	case 5:
		boi.b = 0x04;
		break;
	case 6:
		boi.b = 0x02;
		break;
	case 7:
		boi.b = 0x01;
		break;
	default:
		break;
	}
	return boi;
}

void setbit(char data[], int pos) {
	struct bit_of_ints bis = figure_bit_of_ints(pos);
	data[bis.a] |= bis.b;
}

char getbit(char data[], int pos) {
	struct bit_of_ints bis = figure_bit_of_ints(pos);
	return data[bis.a] & bis.b;
}