FS2_Open Open source remastering of the Freespace 2 engine
bitarray.h
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1 /*
2  * Created by Ian "Goober5000" Warfield for the FreeSpace2 Source Code Project.
3  * You may not sell or otherwise commercially exploit the source or things you
4  * create based on the source.
5  */
6
7
8
9 #ifndef _BIT_ARRAY_H
10 #define _BIT_ARRAY_H
11
12 #include "globalincs/pstypes.h"
13
14
15 // the following four functions are adapted from http://www.codeproject.com/cpp/BitArray.asp; their explanation
16 // is as follows:
17 /*
18  * The Bit Array structure provides a compacted arrays of Booleans, with one bit for each Boolean value.
19  * A 0 (1) bit corresponds to the Boolean value false (true), respectively. We can look at a stream of bytes
20  * as a stream of bits; each byte contains 8 bits, so any n bytes hold n*8 bits. And the operation to
21  * manipulate this stream or bit array is so easy, just read or change the bit's state or make any Boolean
22  * operation on the whole bits array, like ‘AND’, ‘OR’, or ‘XOR’.
23  *
24  * As each byte contains 8 bits, we need to divide the bit number by 8 to reach the byte that holds the bit.
25  * Then, we can seek to the right bit in the reached byte by the remainder of dividing the bit number by 8.
26  * So to read or change the bit state, the operations will be like that.
27  *
28  * Note that to divide by 8, we need only to shift right by 3 (>>3), and to get the remainder of dividing
29  * by 8, we need only to AND with 7 (&7).
30  */
31
32 // returns bit state (0 or 1)
33 #define get_bit(array, bitnum) ((((ubyte *) array)[(bitnum) >> 3] >> ((bitnum) & 7)) & 1)
34
35 // sets bit to 1
36 #define set_bit(array, bitnum) (((ubyte *) array)[(bitnum) >> 3] |= (1 << ((bitnum) & 7)))
37
38 // clears bit to 0
39 #define clear_bit(array, bitnum) (((ubyte *) array)[(bitnum) >> 3] &= ~(1 << ((bitnum) & 7)))
40
41 // toggles bit (xor)
42 #define toggle_bit(array, bitnum) (((ubyte *) array)[(bitnum) >> 3] ^= (1 << ((bitnum) & 7)))
43
44
45 // calculate number of bytes from number of bits
46 #define calculate_num_bytes(num_bits) ((num_bits >> 3) + 1)
47
48
49 #endif // _BIT_ARRAY_H