strstri implementation in C++

strstri.cpp

template<typename T, typename U>
static const T* stristr(const T* string, const T* substring, U(*tolwr)(U))
{
    const T *a, *b;
    b = substring;
    if(*b == 0)
        return string;
    for(; *string != 0; string += 1)
    {
        if(*string != *b)
            continue;
        a = string;
        while(1)
        {
            if(tolwr(*b) == 0)
                return string;
            if(tolwr(*a++) != tolwr(*b++))
                break;
        }
        b = substring;
    }
    return nullptr;
}