DESCRIPTION

This class calculates the shortest path between input and output coordinate systems in a graph that defines the relationships between them. This class is primarely designed to analyze gene-related coordinate systems. See Bio::Coordinate::GeneMapper.

Note that this module can not be used to manage graphs.

Technically the graph implemented here is known as Directed Acyclic Graph (DAG). DAG is composed of vertices (nodes) and edges (with optional weights) linking them. Nodes of the graph are the coordinate systems in gene mapper.

The shortest path is found using the Dijkstra's algorithm. This algorithm is fast and greedy and requires all weights to be positive. All weights in the gene coordinate system graph are currently equal (1) making the graph unweighted. That makes the use of Dijkstra's algorithm an overkill. A simpler and faster breadth-first would be enough. Luckily the difference for small graphs is not significant and the implementation is capable of taking weights into account if needed at some later time.

Input format

The graph needs to be primed using a hash of hashes where there is a key for each node. The second keys are the names of the downstream neighboring nodes and values are the weights for reaching them. Here is part of the gene coordiante system graph:

    $hash = {
             '6' => undef,
             '3' => {
                     '6' => 1
                    },
             '2' => {
                     '6' => 1,
                     '4' => 1,
                     '3' => 1
                    },
             '1' => {
                     '2' => 1
                    },
             '4' => {
                     '5' => 1
                    },
             '5' => undef
            };

Note that the names need to be positive integers. Root should be '1' and directness of the graph is taken advantage of to speed calculations by assuming that downsream nodes always have larger number as name.

An alternative (shorter) way of describing input is to use hash of arrays. See Bio::Coordinate::Graph::hash_of_arrays.

new

graph

 Title   : graph
 Usage   : $obj->graph($my_graph)
 Function: Read/write method for the graph structure
 Example :
 Returns : hash of hashes grah structure
 Args    : reference to a hash of hashes

hash_of_arrays

 Title   : hash_of_arrays
 Usage   : $obj->hash_of_array(%hasharray)
 Function: An alternative method to read in the graph structure.
           Hash arrays are easier to type. This method converts
           arrays into hashes and assigns equal values "1" to
           weights.
 Example : Here is an example of simple structure containing a graph.
           my $DAG = {
                      6  => [],
                      5  => [],
                      4  => [5],
                      3  => [6],
                      2  => [3, 4, 6],
                      1  => [2]
                     };
 Returns : hash of hashes graph structure
 Args    : reference to a hash of arrays

shortest_path

 Title   : shortest_path
 Usage   : $obj->shortest_path($a, $b);
 Function: Method for retrieving the shortest path between nodes.
           If the start node remains the same, the method is sometimes
           able to use cached results, otherwise it will recalculate
           the paths.
 Example :
 Returns : array of node names, only the start node name if no path
 Args    : name of the start node
         : name of the end node

dijkstra

 Title   : dijkstra
 Usage   : $graph->dijkstra(1);
 Function: Implements Dijkstra's algorithm.
           Returns or sets a list of mappers. The returned path
           description is always directed down from the root.
           Called from shortest_path().
 Example :
 Returns : Reference to a hash of hashes representing a linked list
           which contains shortest path down to all nodes from the start
           node. E.g.:
            $res = {
                      '2' => {
                               'prev' => '1',
                               'dist' => 1
                             },
                      '1' => {
                               'prev' => undef,
                               'dist' => 0
                             },
                    };
 Args    : name of the start node