Bin packing implementation in C++ with STL

Question

This is my first time using this site so sorry for any bad formatting or weird formulations, I'll try my best to conform to the rules on this site but I might do some misstakes in the beginning.

I'm right now working on an implementation of some different bin packing algorithms in C++ using the STL containers. In the current code I still have some logical faults that needs to be fixed but this question is more about the structure of the program. I would wan't some second opinion on how you should structure the program to minimize the number of logical faults and make it as easy to read as possible. In it's current state I just feel that this isn't the best way to do it but I don't really see any other way to write my code right now.

The problem is a dynamic online bin packing problem. It is dynamic in the sense that items have an arbitrary time before they will leave the bin they've been assigned to.

In short my questions are:
How would the structure of a Bin packing algorithm look in C++?
Is STL containers a good tool to make the implementation be able to handle inputs of arbitrary lenght?
How should I handle the containers in a good, easy to read and implement way?

Some thoughts about my own code:
Using classes to make a good distinction between handling the list of the different bins and the list of items in those bins.
Getting the implementation as effective as possible.
Being easy to run with a lot of different data lengths and files for benchmarking.

#include <iostream>
#include <fstream>
#include <list>
#include <queue>
#include <string>
#include <vector>

using namespace std;

struct type_item {
    int size;
    int life;
    bool operator < (const type_item& input)
    {
        return size < input.size;
    }
};

class Class_bin {
    double load;
    list<type_item> contents;
    list<type_item>::iterator i;
public:
    Class_bin ();
    bool operator < (Class_bin);
    bool full (type_item);
    void push_bin (type_item);
    double check_load ();
    void check_dead ();
    void print_bin ();
};

Class_bin::Class_bin () {
    load=0.0;
}

bool Class_bin::operator < (Class_bin input){
    return load < input.load;
}

bool Class_bin::full (type_item input) {
    if (load+(1.0/(double) input.size)>1) {
        return false;
    }
    else {
        return true;
    }
}

void Class_bin::push_bin (type_item input) {
    int sum=0;

    contents.push_back(input);
    for (i=contents.begin(); i!=contents.end(); ++i) {
        sum+=i->size;
    }
    load+=1.0/(double) sum;
}

double Class_bin::check_load () {
    return load;
}

void Class_bin::check_dead () {
    for (i=contents.begin(); i!=contents.end(); ++i) {
        i->life--;
        if (i->life==0) {
            contents.erase(i);
        }
    }
}

void Class_bin::print_bin () {
    for (i=contents.begin (); i!=contents.end (); ++i) {
        cout << i->size << "  ";
    }
}


class Class_list_of_bins {
    list<Class_bin> list_of_bins;
    list<Class_bin>::iterator i;
public:

    void push_list (type_item);
    void sort_list ();
    void check_dead ();
    void print_list ();
private:
    Class_bin new_bin (type_item);
    bool comparator (type_item, type_item);
};

Class_bin Class_list_of_bins::new_bin (type_item input) {
    Class_bin temp;

    temp.push_bin (input);

    return temp;
}

void Class_list_of_bins::push_list (type_item input) {
    if (list_of_bins.empty ()) {
        list_of_bins.push_front (new_bin(input));
        return;
    }
    for (i=list_of_bins.begin (); i!=list_of_bins.end (); ++i) {
        if (!i->full (input)) {
            i->push_bin (input);
            return;
        }
    }
    list_of_bins.push_front (new_bin(input));
}

void Class_list_of_bins::sort_list () {
    list_of_bins.sort();
}

void Class_list_of_bins::check_dead () {
    for (i=list_of_bins.begin (); i !=list_of_bins.end (); ++i) {
        i->check_dead ();
    }
}

void Class_list_of_bins::print_list () {
    for (i=list_of_bins.begin (); i!=list_of_bins.end (); ++i) {
        i->print_bin ();
        cout << "\n";
    }
}


int main () {
    int i, number_of_items;

    type_item buffer;
    Class_list_of_bins bins;

    queue<type_item> input;

    string filename;
    fstream file;


    cout << "Input file name: ";
    cin >> filename;
    cout << endl;

    file.open (filename.c_str(), ios::in);

    file >> number_of_items;

    for (i=0; i<number_of_items; ++i) {
        file >> buffer.size;
        file >> buffer.life;
        input.push (buffer);
    }

    file.close ();

    while (!input.empty ()) {
        buffer=input.front ();
        input.pop ();
        bins.push_list (buffer);
    }

    bins.print_list ();

    return 0;
}

Note that this is just a snapshot of my code and is not yet running properly

Don't wan't to clutter this with unrelated chatter just want to thank the people who contributed, I will review my code and hopefully be able to structure my programming a bit better

Answer 1

Some thoughts:

Your names are kinda messed up in places.

1. You have a lot of parameters named input, thats just meaningless
2. I'd expect full() to check whether it is full, not whether it can fit something else
3. I don't think push_bin pushes a bin
4. check_dead modifies the object (I'd expect something named check_*, to just tell me something about the object)
5. Don't put things like Class and type in the names of classes and types.
6. class_list_of_bins seems to describe what's inside rather then what the object is.
7. push_list doesn't push a list
8. Don't append stuff like _list to every method in a list class, if its a list object, we already know its a list method

I'm confused given the parameters of life and load as to what you are doing. The bin packing problem I'm familiar with just has sizes. I'm guessing that overtime some of the objects are taken out of bins and thus go away?

Some further thoughts on your classes

Class_list_of_bins is exposing too much of itself to the outside world. Why would the outside world want to check_dead or sort_list? That's nobodies business but the object itself. The public method you should have on that class really should be something like * Add an item to the collection of bins * Print solution * Step one timestep into the future

list<Class_bin>::iterator i;

Bad, bad, bad! Don't put member variables on your unless they are actually member states. You should define that iterator where it is used. If you want to save some typing add this: typedef list::iterator bin_iterator and then you use bin_iterator as the type instead.

EXPANDED ANSWER

Here is my psuedocode:

class Item
{
     Item(Istream & input)
     {
         read input description of item
     }

     double size_needed() { return actual size required (out of 1) for this item)
     bool alive() { return true if object is still alive}
     void do_timestep() { decrement life }
     void print() { print something }
}

class Bin
{
    vector of Items
    double remaining_space


    bool can_add(Item item) { return true if we have enough space}
    void add(Item item) {add item to vector of items, update remaining space}
    void do_timestep() {call do_timestep() and all Items, remove all items which indicate they are dead, updating remaining_space as you go}
    void print { print all the contents }
}

class BinCollection
{
   void do_timestep { call do_timestep on all of the bins }
   void add(item item) { find first bin for which can_add return true, then add it, create a new bin if neccessary }
   void print() { print all the bins }
}

Some quick notes:

• In your code, you converted the int size to a float repeatedly, that's not a good idea. In my design that is localized to one place
• You'll note that the logic relating to a single item is now contained inside the item itself. Other objects only can see whats important to them, size_required and whether the object is still alive
• I've not included anything about sorting stuff because I'm not clear what that is for in a first-fit algorithm.

Answer 2

How would the structure of a Bin packing algorithm look in C++?

Well, ideally you would have several bin-packing algorithms, separated into different functions, which differ only by the logic of the algorithm. That algorithm should be largely independent from the representation of your data, so you can change your algorithm with only a single function call.

You can look at what the STL Algorithms have in common. Mainly, they operate on iterators instead of containers, but as I detail below, I wouldn't suggest this for you initially. You should get a feel for what algorithms are available and leverage them in your implementation.

Is STL containers a good tool to make the implementation be able to handle inputs of arbitrary length?

It usually works like this: create a container, fill the container, apply an algorithm to the container.

Judging from the description of your requirements, that is how you'll use this, so I think it'll be fine. There's one important difference between your bin packing algorithm and most STL algorithms.

The STL algorithms are either non-modifying or are inserting elements to a destination. bin-packing, on the other hand, is "here's a list of bins, use them or add a new bin". It's not impossible to do this with iterators, but probably not worth the effort. I'd start by operating on the container, get a working program, back it up, then see if you can make it work for only iterators.

How should I handle the containers in a good, easy to read and implement way?

I'd take this approach, characterize your inputs and outputs:

• Input: Collection of items, arbitrary length, arbitrary order.
• Output: Collection of bins determined by algorithm. Each bin contains a collection of items.

Then I'd worry about "what does my algorithm need to do?"

• Constantly check bins for "does this item fit?"
• Your Class_bin is a good encapsulation of what is needed.
• Avoid cluttering your code with unrelated stuff like "print()" - use non-member help functions.

type_item

struct type_item {
    int size;
    int life;
    bool operator < (const type_item& input)
    {
        return size < input.size;
    }
};

It's unclear what life (or death) is used for. I can't imagine that concept being relevant to implementing a bin-packing algorithm. Maybe it should be left out?

This is personal preference, but I don't like giving operator< to my objects. Objects are usually non-trivial and have many meanings of less-than. For example, one algorithm might want all the alive items sorted before the dead items. I typically wrap that in another struct for clarity:

struct type_item {
    int size;
    int life;
    struct SizeIsLess {
      // Note this becomes a function object, which makes it easy to use with
      // STL algorithms.
      bool operator() (const type_item& lhs, const type_item& rhs)
      {
          return lhs.size < rhs.size;
      }
    }
};

vector<type_item> items;
std::sort(items.begin, items.end(), type_item::SizeIsLess);

Class_bin

class Class_bin {
    double load;
    list<type_item> contents;
    list<type_item>::iterator i;
public:
    Class_bin ();
    bool operator < (Class_bin);
    bool full (type_item);
    void push_bin (type_item);
    double check_load ();
    void check_dead ();
    void print_bin ();
};

• I would skip the Class_ prefix on all your types - it's just a bit excessive, and it should be clear from the code. (This is a variant of hungarian notation. Programmers tend to be hostile towards it.)
• You should not have a class member i (the iterator). It's not part of class state. If you need it in all the members, that's ok, just redeclare it there. If it's too long to type, use a typedef.
• It's difficult to quantify "bin1 is less than bin2", so I'd suggest removing the operator<.
• bool full(type_item) is a little misleading. I'd probably use bool can_hold(type_item). To me, bool full() would return true if there is zero space remaining.
• check_load() would seem more clearly named load().
• Again, it's unclear what check_dead() is supposed to accomplish.
• I think you can remove print_bin and write that as a non-member function, to keep your objects cleaner.
• Some people on StackOverflow would shoot me, but I'd consider just making this a struct, and leaving load and item list public. It doesn't seem like you care much about encapsulation here (you're only need to create this object so you don't need do recalculate load each time).

Class_list_of_bins

class Class_list_of_bins {
    list<Class_bin> list_of_bins;
    list<Class_bin>::iterator i;
public:

    void push_list (type_item);
    void sort_list ();
    void check_dead ();
    void print_list ();
private:
    Class_bin new_bin (type_item);
    bool comparator (type_item, type_item);
};

• I think you can do without this class entirely.
• Conceptually, it represents a container, so just use an STL container. You can implement the methods as non-member functions. Note that sort_list can be replaced with std::sort.
• comparator is too generic a name, it gives no indication of what it compares or why, so consider being more clear.

Overall Comments

Overall, I think the classes you've picked adequately model the space you're trying to represent, so you'll be fine.

I might structure my project like this:

struct bin {
  double load;  // sum of item sizes.
  std::list<type_item> items;

  bin() : load(0) { }
};

// Returns true if the bin can fit the item passed to the constructor.
struct bin_can_fit {
  bin_can_fit(type_item &item) : item_(item) { }
  bool operator()(const bin &b) {
    return item_.size < b.free_space;
  }
 private:
  type_item item_;
};

// ItemIter is an iterator over the items.
// BinOutputIter is an output iterator we can use to put bins.
template <ItemIter, BinOutputIter>
void bin_pack_first_fit(ItemIter curr, ItemIter end, BinOutputIter output_bins) {
  std::vector<bin> bins;  // Create a local bin container, to simplify life.
  for (; curr != end; ++curr) {
    // Use a helper predicate to check whether the bin can fit this item.
    // This is untested, but just for an idea.
    std::vector<bin>::iterator bin_it =
        std::find_if(bins.begin(), bins.end(), bin_can_fit(*curr));
    if (bin_it == bins.end()) {
      // Did not find a bin with enough space, add a new bin.
      bins.push_back(bin);
      // push_back invalidates iterators, so reassign bin_it to the last item.
      bin_it = std::advance(bins.begin(), bins.size() - 1);
    }

    // bin_it now points to the bin to put the item in.
    bin_it->items.push_back(*curr);
    bin_it->load += curr.size();
  }
  std::copy(bins.begin(), bins.end(), output_bins);  // Apply our bins to the destination.
}

void main(int argc, char** argv) {
  std::vector<type_item> items;
  // ... fill items
  std::vector<bin> bins;
  bin_pack_first_fit(items.begin(), items.end(), std::back_inserter(bins));
}

Answer 3

This interview gives some great insight into the rationale behind the STL. This may give you some inspiration on how to implement your algorithms the STL-way.