C/C++ efficient bit array

Question

Can you recommend efficient/clean way to manipulate arbitrary length bit array?

Right now I am using regular int/char bitmask, but those are not very clean when array length is greater than datatype length.

std vector<bool> is not available for me.

Answer 1

Since you mention C as well as C++, I'll assume that a C++-oriented solution like boost::dynamic_bitset might not be applicable, and talk about a low-level C implementation instead. Note that if something like boost::dynamic_bitset works for you, or there's a pre-existing C library you can find, then using them can be better than rolling your own.

Warning: None of the following code has been tested or even compiled, but it should be very close to what you'd need.

To start, assume you have a fixed bitset size N. Then something like the following works:

typedef uint32_t word_t;
enum { WORD_SIZE = sizeof(word_t) * 8 };

word_t data[N / 32 + 1];

inline int bindex(int b) { return b / WORD_SIZE; }
inline int boffset(int b) { return b % WORD_SIZE; }

void set_bit(int b) { 
    data[bindex(b)] |= 1 << (boffset(b)); 
}
void clear_bit(int b) { 
    data[bindex(b)] &= ~(1 << (boffset(b)));
}
int get_bit(int b) { 
    return data[bindex(b)] & (1 << (boffset(b));
}
void clear_all() { /* set all elements of data to zero */ }
void set_all() { /* set all elements of data to one */ }

As written, this is a bit crude since it implements only a single global bitset with a fixed size. To address these problems, you want to start with a data struture something like the following:

struct bitset { word_t *words; int nwords; };

and then write functions to create and destroy these bitsets.

struct bitset *bitset_alloc(int nbits) {
    struct bitset *bitset = malloc(sizeof(*bitset));
    bitset->nwords = (n / WORD_SIZE + 1);
    bitset->words = malloc(sizeof(*bitset->words) * bitset->nwords);
    bitset_clear(bitset);
    return bitset;
}

void bitset_free(struct bitset *bitset) {
    free(bitset->words);
    free(bitset);
}

Now, it's relatively straightforward to modify the previous functions to take a struct bitset * parameter. There's still no way to re-size a bitset during its lifetime, nor is there any bounds checking, but neither would be hard to add at this point.

Answer 2

boost::dynamic_bitset if the length is only known in run time.

std::bitset if the length is known in compile time (although arbitrary).

Answer 3

I've written a working implementation based off Dale Hagglund's response to provide a bit array in C (BSD license).

https://github.com/noporpoise/BitArray/

Please let me know what you think / give suggestions. I hope people looking for a response to this question find it useful.

Answer 4

This posting is rather old, but there is an efficient bit array suite in C in my ALFLB library.

For many microcontrollers without a hardware-division opcode, this library is EFFICIENT because it doesn't use division: instead, masking and bit-shifting are used. (Yes, I know some compilers will convert division by 8 to a shift, but this varies from compiler to compiler.)

It has been tested on arrays up to 2^32-2 bits (about 4 billion bits stored in 536 MBytes), although last 2 bits should be accessible if not used in a for-loop in your application.

See below for an extract from the doco. Doco is http://alfredo4570.net/src/alflb_doco/alflb.pdf, library is http://alfredo4570.net/src/alflb.zip

Enjoy,
Alf

//------------------------------------------------------------------
BM_DECLARE( arrayName, bitmax);
        Macro to instantiate an array to hold bitmax bits.
//------------------------------------------------------------------
UCHAR *BM_ALLOC( BM_SIZE_T bitmax); 
        mallocs an array (of unsigned char) to hold bitmax bits.
        Returns: NULL if memory could not be allocated.
//------------------------------------------------------------------
void BM_SET( UCHAR *bit_array, BM_SIZE_T bit_index);
        Sets a bit to 1.
//------------------------------------------------------------------
void BM_CLR( UCHAR *bit_array, BM_SIZE_T bit_index);
        Clears a bit to 0.
//------------------------------------------------------------------
int BM_TEST( UCHAR *bit_array, BM_SIZE_T bit_index); 
        Returns: TRUE (1) or FALSE (0) depending on a bit.
//------------------------------------------------------------------
int BM_ANY( UCHAR *bit_array, int value, BM_SIZE_T bitmax); 
        Returns: TRUE (1) if array contains the requested value (i.e. 0 or 1).
//------------------------------------------------------------------
UCHAR *BM_ALL( UCHAR *bit_array, int value, BM_SIZE_T bitmax);
        Sets or clears all elements of a bit array to your value. Typically used after a BM_ALLOC.  
        Returns: Copy of address of bit array
//------------------------------------------------------------------
void BM_ASSIGN( UCHAR *bit_array, int value, BM_SIZE_T bit_index);
        Sets or clears one element of your bit array to your value.
//------------------------------------------------------------------
BM_MAX_BYTES( int bit_max); 
        Utility macro to calculate the number of bytes to store bitmax bits.
        Returns: A number specifying the number of bytes required to hold bitmax bits.
//------------------------------------------------------------------

Answer 5

You can use std::bitset

int main() {
  const bitset<12> mask(2730ul); 
  cout << "mask =      " << mask << endl;

  bitset<12> x;

  cout << "Enter a 12-bit bitset in binary: " << flush;
  if (cin >> x) {
    cout << "x =        " << x << endl;
    cout << "As ulong:  " << x.to_ulong() << endl;
    cout << "And with mask: " << (x & mask) << endl;
    cout << "Or with mask:  " << (x | mask) << endl;
  }
}

Answer 6

I know it's an old post but I came here to find a simple C bitset implementation and none of the answers quite matched what I was looking for, so I implemented my own based on Dale Hagglund's answer. Here it is :)

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>

typedef uint32_t word_t;
enum { BITS_PER_WORD = 32 };
struct bitv { word_t *words; int nwords; int nbits; };

struct bitv* bitv_alloc(int bits) {
    struct bitv *b = malloc(sizeof(struct bitv));

    if (b == NULL) {
        fprintf(stderr, "Failed to alloc bitv\n");
        exit(1);
    }

    b->nwords = (bits >> 5) + 1;
    b->nbits  = bits;
    b->words  = malloc(sizeof(*b->words) * b->nwords);

    if (b->words == NULL) {
        fprintf(stderr, "Failed to alloc bitv->words\n");
        exit(1);
    }

    memset(b->words, 0, sizeof(*b->words) * b->nwords);

    return b;
}

static inline void check_bounds(struct bitv *b, int bit) {
    if (b->nbits < bit) {
        fprintf(stderr, "Attempted to access a bit out of range\n");
        exit(1);
    }
}

void bitv_set(struct bitv *b, int bit) {
    check_bounds(b, bit);
    b->words[bit >> 5] |= 1 << (bit % BITS_PER_WORD);
}

void bitv_clear(struct bitv *b, int bit) {
    check_bounds(b, bit);
    b->words[bit >> 5] &= ~(1 << (bit % BITS_PER_WORD));
}

int bitv_test(struct bitv *b, int bit) {
    check_bounds(b, bit);
    return b->words[bit >> 5] & (1 << (bit % BITS_PER_WORD));
}

void bitv_free(struct bitv *b) {
    if (b != NULL) {
        if (b->words != NULL) free(b->words);
        free(b);
    }
}

void bitv_dump(struct bitv *b) {
    if (b == NULL) return;

    for(int i = 0; i < b->nwords; i++) {
        word_t w = b->words[i];

        for (int j = 0; j < BITS_PER_WORD; j++) {
            printf("%d", w & 1);
            w >>= 1;
        }

        printf(" ");
    }

    printf("\n");
}

void test(struct bitv *b, int bit) {
    if (bitv_test(b, bit)) printf("Bit %d is set!\n", bit);
    else                   printf("Bit %d is not set!\n", bit);
}

int main(int argc, char *argv[]) {
    struct bitv *b = bitv_alloc(32);

    bitv_set(b, 1);
    bitv_set(b, 3);
    bitv_set(b, 5);
    bitv_set(b, 7);
    bitv_set(b, 9);
    bitv_set(b, 32);
    bitv_dump(b);
    bitv_free(b);

    return 0;
}

Answer 7

I use this one:

//#include <bitset>
#include <iostream>
//source http://stackoverflow.com/questions/47981/how-do-you-set-clear-and-toggle-a-single-bit-in-c
#define BIT_SET(a,b) ((a) |= (1<<(b)))
#define BIT_CLEAR(a,b) ((a) &= ~(1<<(b)))
#define BIT_FLIP(a,b) ((a) ^= (1<<(b)))
#define BIT_CHECK(a,b) ((a) & (1<<(b)))

/* x=target variable, y=mask */
#define BITMASK_SET(x,y) ((x) |= (y))
#define BITMASK_CLEAR(x,y) ((x) &= (~(y)))
#define BITMASK_FLIP(x,y) ((x) ^= (y))
#define BITMASK_CHECK(x,y) ((x) & (y))

Answer 8

I have recently released BITSCAN, a C++ bit string library which is specifically oriented towards fast bit scanning operations. BITSCAN is available here. It is in alpha but still pretty well tested since I have used it in recent years for research in combinatorial optimization (e.g. in BBMC, a state of the art exact maximum clique algorithm). A comparison with other well known C++ implementations (STL or BOOST) may be found here.

I hope you find it useful. Any feedback is welcome.

Answer 9

In micro controller development, some times we need to use 2-dimentional array (matrix) with element value of [0, 1] only. That means if we use 1 byte for element type, it wastes the memory greatly (memory of micro controller is very limited). The proposed solution is that we should use 1 bit matrix (element type is 1 bit).

http://htvdanh.blogspot.com/2016/09/one-bit-matrix-for-cc-programming.html

Answer 10

I recently implemented a small header-only library called BitContainer just for this purpose. It focuses on expressiveness and compiletime abilities and can be found here: https://github.com/EddyXorb/BitContainer

It is for sure not the classical way to look at bitarrays but can come in handy for strong-typing purposes and memory efficient representation of named properties.

Example:

constexpr Props props(Prop::isHigh(),Prop::isLow()); // intialize BitContainer of type Props with strong-type Prop

constexpr bool result1 = props.contains(Prop::isTiny()) // false
constexpr bool result2 = props.contains(Prop::isLow())  // true