How to rewrite this C++ code to Nim properly

[maxcroud]

Hello,

I have some C++ code which context I want to re-create in Nim. Briefly, there 
is buffer of bytes. As it's just a pointer, I can easily designate an offset 
(current). Do not care about unpack_int, it's just a function combining some 
bytes from the buffer into an integer.

**The questions is how to make this buffer concept in Nim to be safe and 
efficient?**

I wanted to use slices, but they holds an immutuable sequence. 
    
    
    const unsigned char* current, start, end;
    
    void reset(const void* data, int size) {
            start = (const unsigned char*) data;
            end = start + size;
            current = start;
    }
    
    int get_int() {
            int i;
            current = unpack_int(current, &i);
            return i;
    }
    
    int unpack_int(const unsigned char *src, int *out) {
            int sign = (*src >> 6) & 1;
            *out = *src & 0x3F;
            do {
                    if(!(*src & 0x80)) break;
                    src++;
                    *out |= (*src & (0x7F)) << (6);
                    
                    if(!(*src & 0x80)) break;
                    src++;
                    *out |= (*src & (0x7F)) << (6+7);
                    
                    if(!(*src & 0x80)) break;
                    src++;
                    *out |= (*src & (0x7F)) << (6+7+7);
                    
                    if(!(*src & 0x80)) break;
                    src++;
                    *out |= (*src & (0x7F)) << (6+7+7+7);
            } while(0);
            
            src++;
            *out ^= -sign; // if (sign) *i = ~(*i)
            return src;
    }