Question

Recently, I was given a small task of using graph data structure as core to make a web application. I started out with an idea of simple path optimization problem, which can be completed in few days. The problem is that I am not able decide the correct framework for this task. Using just PHP was the only thing i could think of given the time constraint.

So, how can I represent a graph data structure using PHP’s custom data structure( array).

Furthermore,can you suggest some other frameworks on which i can work for this task.

Answer 1

You can use the array to keep an adjacency list.

PHP’s array has two uses: it can be a list of objects, or an associative array, which associates one object with another. You can also use an associative array as a poor man’s set, by keeping the using the data from the set as keys in the associative array. Since it is usual to associate data to vertices, and edges, we can actually make use of this in a natural way. Following is an example of an undirected graph class.

<?php

/**
 * Undirected graph implementation.
 */
class Graph
{
  /**
   * Adds an undirected edge between $u and $v in the graph.
   * 
   * $u,$v can be anything.
   *
   * Edge (u,v) and (v,u) are the same.
   * 
   * $data is the data to be associated with this edge.
   * If the edge (u,v) already exists, nothing will happen (the
   * new data will not be assigned).
   */
  public function add_edge($u,$v,$data=null)
  {
    assert($this->sanity_check());
    assert($u != $v);

    if ($this->has_edge($u,$v))
      return;

    //If u or v don't exist, create them.
    if (!$this->has_vertex($u))
      $this->add_vertex($u);
    if (!$this->has_vertex($v))
      $this->add_vertex($v);
    
    //Some sanity.
    assert(array_key_exists($u,$this->adjacency_list));
    assert(array_key_exists($v,$this->adjacency_list));

    //Associate (u,v) with data.
    $this->adjacency_list[$u][$v] = $data;
    //Associate (v,u) with data.
    $this->adjacency_list[$v][$u] = $data;

    //We just added two edges
    $this->edge_count += 2;

    assert($this->has_edge($u,$v));

    assert($this->sanity_check());
  }

  public function has_edge($u,$v)
  {
    assert($this->sanity_check());

    //If u or v do not exist, they surely do not make up an edge.
    if (!$this->has_vertex($u))
      return false;
    if (!$this->has_vertex($v))
      return false;


    //some extra sanity.
    assert(array_key_exists($u,$this->adjacency_list));
    assert(array_key_exists($v,$this->adjacency_list));
    
    //This is the return value; if v is a neighbor of u, then its true.
    $result = array_key_exists($v,$this->adjacency_list[$u]);

    //Make sure that iff v is a neighbor of u, then u is a neighbor of v
    assert($result == array_key_exists($u,$this->adjacency_list[$v]));

    return $result;
  }

  /**
   * Remove (u,v) and return data.
   */
  public function remove_edge($u,$v)
  {
    assert($this->sanity_check());

    if (!$this->has_edge($u,$v))
      return null;
    
    assert(array_key_exists($u,$this->adjacency_list));
    assert(array_key_exists($v,$this->adjacency_list));
    assert(array_key_exists($v,$this->adjacency_list[$u]));
    assert(array_key_exists($u,$this->adjacency_list[$v]));

    //remember data.
    $data = $this->adjacency_list[$u][$v];

    unset($this->adjacency_list[$u][$v]);
    unset($this->adjacency_list[$v][$u]);
    
    //We just removed two edges.
    $this->edge_count -= 2;

    assert($this->sanity_check());

    return $data;
  }

  //Return data associated with (u,v)
  public function get_edge_data($u,$v)
  {
    assert($this->sanity_check());
    
    //If no such edge, no data.
    if (!$this->has_edge($u,$v))
      return null;

    //some sanity.
    assert(array_key_exists($u,$this->adjacency_list));
    assert(array_key_exists($v,$this->adjacency_list[$u]));
    
    
    return $this->adjacency_list[$u][$v]; 
  }

  /**
   * Add a vertex. Vertex must not exist, assertion failure otherwise.
   */
  public function add_vertex($u,$data=null)
  {
    assert(!$this->has_vertex($u));

    //Associate data.
    $this->vertex_data[$u] = $data;
    //Create empty neighbor array.
    $this->adjacency_list[$u] = array();

    assert($this->has_vertex($u));
    assert($this->sanity_check());
  }

  public function has_vertex($u)
  {
    assert($this->sanity_check());
    assert(array_key_exists($u,$this->vertex_data) == array_key_exists($u,$this->adjacency_list));
    return array_key_exists($u,$this->vertex_data);
  }

  //Returns data associated with vertex, null if vertex does not exist.
  public function get_vertex_data($u)
  {
    assert($this->sanity_check());

    if (!array_key_exists($u,$this->vertex_data))
      return null;

    return $this->vertex_data[$u];
  }
  
  //Count the neighbors of a vertex.
  public function count_vertex_edges($u)
  {
    assert($this->sanity_check());

    if (!$this->has_vertex($u))
      return 0;

    //some sanity.    
    assert (array_key_exists($u,$this->adjacency_list));
    
    return count($this->adjacency_list[$u]);
  }

  /**
   * Return an array of neighbor vertices of u.
   * If $with_data == true, then it will return an associative array, like so:
   * {neighbor => data}.
   */
  public function get_edge_vertices($u,$with_data=false)
  {
    assert($this->sanity_check());
    
    if (!array_key_exists($u,$this->adjacency_list))
      return array();

    $result = array();

    if ($with_data) {
      foreach( $this->adjacency_list[$u] as $v=>$data)
      {
        $result[$v] = $data;
      }
    } else {

      foreach( $this->adjacency_list[$u] as $v=>$data)
      {
        array_push($result, $v);
      }
    }

    return $result;
  }

  //Removes a vertex if it exists, and returns its data, null otherwise.
  public function remove_vertex($u)
  {
    assert($this->sanity_check());

    //If the vertex does not exist,
    if (!$this->has_vertex($u)){
      //Sanity.
      assert(!array_key_exists($u,$this->vertex_data));
      assert(!array_key_exists($u,$this->adjacency_list));
      return null;
    }

    //We need to remove all edges that this vertex belongs to.
    foreach ($this->get_edge_vertices($u) as $v)
    {
      $this->remove_edge($u,$v);
    }


    //After removing all such edges, u should have no neighbors.
    assert($this->count_vertex_edges($u) == 0);

    //sanity.
    assert(array_key_exists($u,$this->vertex_data));
    assert(array_key_exists($u,$this->adjacency_list));
    
    //remember the data.
    $data = $this->vertex_data[$u];

    //remove the vertex from the data array.
    unset($this->vertex_data[$u]);
    //remove the vertex from the adjacency list.
    unset($this->adjacency_list[$u]);
    
    assert($this->sanity_check());

    return $data;
  }

  public function get_vertex_count()
  {
    assert($this->sanity_check());
    return count($this->vertex_data);
  }
  public function get_edge_count()
  {
    assert($this->sanity_check());
    
    //edge_count counts both (u,v) and (v,u)
    return $this->edge_count/2;
  }

  public function get_vertex_list($with_data=false)
  {
    $result = array();
    
    if ($with_data)
      foreach ($this->vertex_data as $u=>$data)
        $result[$u]=$data;
    else
      foreach ($this->vertex_data as $u=>$data)
        array_push($result,$u);
    
    return $result;
  }


  public function edge_list_str_array($ordered=true)
  {
    $result_strings = array();
    foreach($this->vertex_data as $u=>$udata)
    {
      foreach($this->adjacency_list[$u] as $v=>$uv_data)
      {
        if (!$ordered || ($u < $v))
          array_push($result_strings, '('.$u.','.$v.')');
      }
    }
    return $result_strings;
  }

  public function sanity_check()
  {
    if (count($this->vertex_data) != count($this->adjacency_list))
      return false;

    $edge_count = 0;

    foreach ($this->vertex_data as $v=>$data)
    {

      if (!array_key_exists($v,$this->adjacency_list))
        return false;

      $edge_count += count($this->adjacency_list[$v]);
    }

    if ($edge_count != $this->edge_count)
      return false;

    if (($this->edge_count % 2) != 0)
      return false;

    return true;
  }

  /**
   * This keeps an array that associates vertices with their neighbors like so:
   *
   * {<vertex> => {<neighbor> => <edge data>}}
   *
   * Thus, each $adjacency_list[$u] = array( $v1 => $u_v1_edge_data, $v2 => $u_v2_edge_data ...)
   *
   * The edge data can be null.
   */
  private $adjacency_list = array();

  /**
   * This associates each vertex with its data.
   *
   * {<vertex> => <data>}
   *
   * Thus each $vertex_data[$u] = $u_data
   */
  private $vertex_data = array();

  /**
   * This keeps tracks of the edge count so we can retrieve the count in constant time,
   * instead of recounting. In truth this counts both (u,v) and (v,u), so the actual count
   * is $edge_count/2.
   */
  private $edge_count = 0;
}


$G = new Graph();

for ($i=0; $i<5; ++$i)
{
  $G->add_vertex($i);
}

for ($i=5; $i<10; ++$i)
{
  $G->add_edge($i,$i-5);
}

print 'V: {'.join(', ',$G->get_vertex_list())."}\n";
print 'E: {'.join(', ',$G->edge_list_str_array())."}\n";

$G->remove_vertex(1);

print 'V: {'.join(', ',$G->get_vertex_list())."}\n";
print 'E: {'.join(', ',$G->edge_list_str_array())."}\n";

$G->remove_vertex(1);

print 'V: {'.join(', ',$G->get_vertex_list())."}\n";
print 'E: {'.join(', ',$G->edge_list_str_array())."}\n";
?>