namespace StringUtils
{
    namespace impl
    {
        inline bool startsWith(const char* s1, const char* s2);
    }

    inline const char* strstr(const char* s1, const char* s2)
    {
        char ch = s2[0];

        if(0 == ch)
            return s1;

        while(*s1)
        {
            if(*s1 == ch && impl::startsWith(s1, s2))
                return s1;
            ++s1;
        }
        return 0;
    }

    inline void toupper(const char* s1, char* s2)
    {
        int ch = *s1;
        while(ch != 0)
        {
            if(ch >= 'a' && ch <= 'z')
                *s2 = ch - 32;
            else
                *s2 = ch;
            ++s1;
            ++s2;

            ch = *s1;
        }
        *s2 = 0;
    }

    inline void tolower(const char* s1, char* s2)
    {
        int ch = *s1;

        while(ch != 0)
        {
            if(ch >= 'A' && ch <= 'Z')
                *s2 = ch + 32;
            else
                *s2 = ch;
            ++s1;
            ++s2;

            ch = *s1;
        }
        *s2 = 0;
    }

    namespace impl
    {
        inline bool startsWith(const char* s1, const char* s2)
        {
            while(*s1 && *s2)
            {
                if(*s1++ != *s2++)
                    return false;
            }

            if(*s2)
                return false;
            return true;
        }
    }
}