#include <stddef.h>

size_t strlen(const char* str) 
{
    size_t len = 0;
    while (str[len])
        len++;
    return len;
}

char to_upper_char(char c) 
{
    if ('a' <= c && c <= 'z') {
        return c - 0x20;
    } else {
        return c;
    }
}

char to_lower_char(char c) 
{
    if ('A' <= c && c <= 'Z') {
        return c + 0x20;
    } else {
        return c;
    }
}

char* to_lower_str(char* str)
{
    for (size_t i = 0; i < strlen(str); i++) {
        str[i] = to_lower_char(str[i]);
    }
    return str;
}

char* to_upper_str(char* str)
{
    for (size_t i = 0; i < strlen(str); i++) {
        str[i] = to_upper_char(str[i]);
    }
    return str;
}