• XOR

Many popular bit manipulation problems require to use XOR in code. In this article, I will introduce couple XOR problems and solution. XOR is represented by ^ in C language. XOR truth table is shown below. When two bits are the same, result is 0. When two bits are different, result is 1.

• Problem1: What is the minimum bit flip to become A from B. A and B are the input of this function. Output is the minimum number of bit flip. Assume A and B are 32-bit integer. Reference: leetcode.com/problems/minimum-bit-flips-to-..

uint32_t problem1(uint32_t A, uint32_t B)
{
    uint32_t count = 0;
    //Implement logic

    return count;
}

Based on the example above, iterate each bit position XOR result to increase the counter if XOR result is the 1. Another way is finding out number of 1s in A ^ B.

• Implementation and test

1. Create xor_problems ceedling project

   ceedling module:create[xor_problems]
   

2. xor_problems.h

   #ifndef XOR_PROBLEMS_H
   #define XOR_PROBLEMS_H
   #include <stdint.h>
   uint32_t problem1_1(uint32_t A, uint32_t B);
   uint32_t problem1_2(uint32_t A, uint32_t B);
   #endif // XOR_PROBLEMS_H
   

3. xor_problems.c

   #include "xor_problems.h"
   uint32_t problem1_1(uint32_t A, uint32_t B)
   {
    uint32_t count = 0;
   
    for(int i = 0; i < 32; i++)
    {
        if((A ^ B) & 1)
        {
            count++;
        }
        A >>= 1;
        B >>= 1;
    }
    return count;
   }
   uint32_t problem1_2(uint32_t A, uint32_t B)
   {
    uint32_t xor_result = A ^ B;
    uint32_t count = 0;
   
    for(int i = 0; i < 32; i++)
    {
        if(xor_result & 1)
        {
            count++;
        }
        xor_result >>= 1;
    }
   
    return count;
   }
   

4. test_xor_problems.c

   #include "unity.h"
   #include "xor_problems.h"
   void setUp(void)
   {
   }
   void tearDown(void)
   {
   }
   void test_xor_problems1_example(void)
   {
    uint32_t x = 10;
    uint32_t y = 7;
   
    TEST_ASSERT_EQUAL(3, problem1_1(x,y));
    TEST_ASSERT_EQUAL(3, problem1_2(x,y));
   }
   void test_xor_problems1_example2(void)
   {
    uint32_t x = 0;
    uint32_t y = 0;
   
    TEST_ASSERT_EQUAL(0, problem1_1(x,y));
    TEST_ASSERT_EQUAL(0, problem1_2(x,y));
   }
   void test_xor_problems1_example3(void)
   {
    uint32_t x = 10;
    uint32_t y = 5;
   
    TEST_ASSERT_EQUAL(4, problem1_1(x,y));
    TEST_ASSERT_EQUAL(4, problem1_2(x,y));
   }
   

5. Run test and result

   ceedling test:xor_problems
   

   

• Problem2: Every element appears twice except for one in given array. Find the single one. Assume array size is odd number. Reference: leetcode.com/problems/single-number

example#1 Input: [2, 5, 9, 9, 5] Output: 2

example#2 Input: [0] Output: 0

uint32_t problem2(uint32_t * arr, uint32_t arr_size)
{
    uint32_t single_one = arr[0];
    //Implement the logic

    return single_one;
}

• Implementation and test

Hint: A XOR A = 0, A XOR 0 = A.

A XOR A XOR B = ?

0 XOR B = B

Single one is B

1. Add function prototype in xor_problems.h

   uint32_t problem2(uint32_t * arr, uint32_t arr_size);
   

2. Add function in xor_problems.c

   uint32_t problem2(uint32_t * arr, uint32_t arr_size)
   {
    uint32_t single_one = arr[0];
   
    for(uint32_t i = 1; i < arr_size; i++)
    {
        single_one ^= arr[i];
    }
   
    return single_one;
   }
   

3. Add test cases in test_xor_problems.c

   void test_xor_problems2_example1(void)
   {
    uint32_t arr[] = {2, 5, 9, 9, 5};
    TEST_ASSERT_EQUAL(2, problem2(arr,5));
   }
   void test_xor_problems2_example2(void)
   {
    uint32_t arr[] = {0};
    TEST_ASSERT_EQUAL(0, problem2(arr,1));
   }
   

4. Run test and result

   ceedling test:xor_problems