位数组

看了《C和指针》后，参考网上的代码自己实现的版本（真的不是纯抄的）。这是5.9 编程练习的第4题。

头文件bits.h：

/* bits.h */
#ifndef BITS_H_INCLUDED
#define BITS_H_INCLUDED

void set_bit(char bit_array[], unsigned int bit_number);

void clear_bit(char bit_array[], unsigned int bit_number);

void reverse_bit(char bit_array[], unsigned int bit_number);

void assign_bit(char bit_array[], unsigned int bit_number, int value);

int test_bit(char bit_array[], unsigned int bit_number);

#endif // BITS_H_INCLUDED

源代码bits.c

/* bits.c */
#include "bits.h"

#define BASE 8
#define SHIFT 3
#define MASK 7

void set_bit(char bit_array[], unsigned int bit_number)
{
    bit_array[bit_number >> SHIFT] |= 1 << (bit_number & MASK);
}

void clear_bit(char bit_array[], unsigned int bit_number)
{
    bit_array[bit_number >> SHIFT] &= ~(1 << (bit_number & MASK));
}

void reverse_bit(char bit_array[], unsigned int bit_number)
{
    bit_array[bit_number >> SHIFT] ^= 1 << (bit_number & MASK);
}

void assign_bit(char bit_array[], unsigned int bit_number, int value)
{
    if (value == 1)
    {
        set_bit(bit_array, bit_number);
    }
    else if (value == 0)
    {
        clear_bit(bit_array, bit_number);
    }
}

int test_bit(char bit_array[], unsigned int bit_number)
{
    return (bit_array[bit_number >> SHIFT] & (1 << (bit_number & MASK)))
        >> (bit_number & MASK);
}

 测试用例：

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

#include "bits.h"

int main()
{
    char bits[128];
    memset(bits, 0, sizeof(bits));
    set_bit(bits, 234);
    printf("%d\n", test_bit(bits, 234));
    clear_bit(bits, 234);
    printf("%d\n", test_bit(bits, 234));
    reverse_bit(bits, 235);
    printf("%d\n", test_bit(bits, 235));
    reverse_bit(bits, 235);
    printf("%d\n", test_bit(bits, 235));
    assign_bit(bits, 1024, 1);
    printf("%d\n", test_bit(bits, 1024));

    return 0;
}

答案版本：

头文件bitarray.h

#ifndef BITARRAY_H_INCLUDED
#define BITARRAY_H_INCLUDED

/*
 * Prototypes for a suite of functions that implement an array of bits in a
 * character array.
 */

// Set a specific bit
void set_bit(char bit_array[], unsigned int bit_number);

// Clear a specific bit
void clear_bit(char bit_array[], unsigned int bit_number);

// Assign a value to a bit
void assign_bit(char bit_array[], unsigned int bit_number, int value);

// Test a specific bit
int test_bit(char bit_array[], unsigned int bit_number);

#endif // BITARRAY_H_INCLUDED

源代码bitarray.c

/* bitarray.cpp -- Implements an array of bits in a character array. */

#include <limits.h>

#include "bitarray.h"

static unsigned int character_offset(unsigned int bit_number);
static unsigned int bit_offset(unsigned int bit_number);

void set_bit(char bit_array[], unsigned int bit_number)
{
    bit_array[character_offset(bit_number)] |=
        1 << bit_offset(bit_number);
}

void clear_bit(char bit_array[], unsigned int bit_number)
{
    bit_array[ character_offset( bit_number ) ] &=
        ~(1 << bit_offset(bit_number));
}

void assign_bit(char bit_array[], unsigned int bit_number, int value)
{
    if (value != 0)
        set_bit(bit_array, bit_number);
    else
        clear_bit(bit_array, bit_number);
}

int test_bit(char bit_array[], unsigned int bit_number)
{
    return (bit_array[ character_offset( bit_number ) ] &
        1 << bit_offset( bit_number )) != 0;
}

static unsigned int character_offset(unsigned int bit_number)
{
    return bit_number / CHAR_BIT;
}

static unsigned int bit_offset(unsigned int bit_number)
{
    return bit_number % CHAR_BIT;
}