reverse string and replace '\' symbols with '/'

reverse.c

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

unsigned int reverse_and_replace(unsigned char* str)
{
    unsigned int replace_count = 0;
    unsigned int str_len = 0;
    const unsigned char from = '\\';
    const unsigned char to = '/';
    int i;
    
    if (str == NULL)
        return 0;

    str_len = strlen(str);
    for (i = 0; i < str_len/2; i++)     // we can use str_len >> 1 or another variable 
    {                                   // but compiler will optimise it for us
        unsigned char ch;

        if (str[i] == from)
        {
            str[i] = to;
            replace_count++;
        }
        
        ch = str[i];
        str[i] = str[str_len - i - 1];
        str[str_len - i - 1] = ch;

        if (str[i] == from)
        {
            str[i] = to;
            replace_count++;
        }
    }

    // check middle char
    if ((str_len % 2) && (str[str_len/2] == from))
    {
        str[str_len/2] = to;
        replace_count++;
    }

    return replace_count;
}

int main()
{
    int ret = 0;

    char* test1 = NULL;
    struct _test {    
        char str[20]; 
        char str_rev[20];
        int replaces;
    } test[] = { 
                 { "abc\\123", "321/cba", 1 },         // Test string with odd number of chars
                 { "ab\\c\\01234", "43210/c/ba", 2 },  // Test string with even number of chars
                 { "", "", 0 },                        // Test empty string
               };
    int i;

    // Test NULL 
    int num = reverse_and_replace(test1);
    if (num != 0 || test1 != NULL)
    {
        printf ("TEST FAILED: string = %s, number of symbols converted\n", test1, num);
        ret = 1;
    }

    // All other tests
    for (i = 0; i < sizeof(test)/sizeof(test[0]); i++)
    {
        num = reverse_and_replace(test[i].str);
        if (test[i].replaces != num || strncmp(test[i].str, test[i].str_rev, strlen(test[i].str)) != 0)
        {
            printf ("TEST FAILED: string = %s, number of symbols converted\n", test[i].str, num);
            ret = 1;
        }
    }

    return ret;
}