What is the cause for the degradation of environment?
Capitalism, corruption, consuming society? - OVERPOPULATION!
Please, save the Planet - kill yourself...

Sunday, November 30, 2014

A Flawed Research on a TSP Algorithm

A Travelling Salesman Problem (TSP) is a well known computational challenge. A lot of algorithms were developed to solve it or its special cases.

I came around an article authored by Fang Liu 'A dual population parallel ant colony optimization algorithm for solving the travelling salesman problem'. In this article he proposed a modification of an Ant Colony System algorithm for solving TSP and presented results obtained by his algorithm. In the table with results all looked fine - his algorithm was able to provide very good solutions for the TSP instances from TSPLIB (which is the common testing ground for TSP algorithms).

So the researcher presented good results... it seems. But then he decided to show the best routes his algorithm was able to find and annotated them with the corresponding route costs. Lets take a look at one of them. Here you are his best route for the 'att48' instance from the TSPLIB:

Route that claims to be optimal (but the cost is very wrong)
The optimal route for 'att48' and its cost is well-known (it applies to all TSPLIB instanses). Its cost is approximately 33523 (there are different approaches to rounding distances between points). So what we see at the picture above should be the optimal route (or extremely close to it). But dear reader, do you think that you see optimal route? Humans are able to provide very good solutions to TSP instances that consists of not too many points. I bet you can draw far better route yourself. The route from this picture is 1, 8, 46, 33, 20, 17, 43, 27, 19, 37, 6, 30, 36, 28, 7, 18, 44, 31, 38, 9, 40, 15, 12, 11, 47, 21, 13, 25, 14, 23, 3, 22, 16, 41, 34, 2, 29, 5, 48, 39, 32, 24, 42, 10, 45, 35, 4, 26, 1 and its cost is 41052 which is whooping 22% far from optimal! The same story for another illustration in the article.

Here take a look at the optimal route which cost is really 33523:
Actually optimal route for 'att48' with Cost = 33523
So what we can conclude? I do believe that the routes demonstrated in the article are the best routes found by given algorithm, but costs are put-up for the routes and for the table of results in the article as well. I think that author developed an algorithm that wasn't able to find good solutions and provided fraud table with the put-up testing results. And clearly this article wasn't reviewed by scientist that have knowledge in TSP area because these plots are so obviously flawed that one can't overlook it!

No one usually shows plots of the routes they find with their algorithms. I wonder if there are more modern algorithms with the put-up results?

Sunday, November 2, 2014

How to Create Delauney Triangulation Graph from a .shp-file Using NetworkX

For my own project I needed to create a graph based on a Delauney triangulation using NetworkX python library. And a special condition was that all the edges must be unique. Points for triangulation stored in a .shp-file.

I was lucky enough to find this tread on Delauney triangulation using NetworkX graphs. I made a nice function out of it for point NetworkX graphs processing. This function preserves nodes attributes (which are lost after triangulation) and calculates lengths of the edges. It can be further improved (and most likely will be) but even at the current state it is very handy.

Example of use:
import networkx as nx
import scipy.spatial
import matplotlib.pyplot as plt

path = '/directory/'
f_path = path + 'filename.shp'
G = nx.read_shp(f_path)

GD = createTINgraph(G, show = True)

Code for the function:
import networkx as nx
import scipy.spatial
import matplotlib.pyplot as plt

def createTINgraph(point_graph, show = False, calculate_distance = True):
  Creates a graph based on Delaney triangulation

  @param point_graph: either a graph made by read_shp() from another NetworkX's point graph
  @param show: whether or not resulting graph should be shown, boolean
  @param calculate_distance: whether length of edges should be calculated
  @return - a graph made from a Delauney triangulation

  @Copyright notice: this code is an improved (by Yury V. Ryabov, 2014, riabovvv@gmail.com) version of
                    Tom's code taken from this discussion

  TIN = scipy.spatial.Delaunay(point_graph)
  edges = set()
  # for each Delaunay triangle
  for n in xrange(TIN.nsimplex):
      # for each edge of the triangle
      # sort the vertices
      # (sorting avoids duplicated edges being added to the set)
      # and add to the edges set
      edge = sorted([TIN.vertices[n,0], TIN.vertices[n,1]])
      edges.add((edge[0], edge[1]))
      edge = sorted([TIN.vertices[n,0], TIN.vertices[n,2]])
      edges.add((edge[0], edge[1]))
      edge = sorted([TIN.vertices[n,1], TIN.vertices[n,2]])
      edges.add((edge[0], edge[1]))

  # make a graph based on the Delaunay triangulation edges
  graph = nx.Graph(list(edges))

  #add nodes attributes to the TIN graph from the original points
  original_nodes = point_graph.nodes(data = True)
  for n in xrange(len(original_nodes)):
    XY = original_nodes[n][0] # X and Y tuple - coordinates of the original points
    graph.node[n]['XY'] = XY
    # add other attributes
    original_attributes = original_nodes[n][1]
    for i in original_attributes.iteritems(): # for tuple i = (key, value)
      graph.node[n][i[0]] = i[1]

  # calculate Euclidian length of edges and write it as edges attribute
  if calculate_distance:
    edges = graph.edges()
    for i in xrange(len(edges)):
      edge = edges[i]
      node_1 = edge[0]
      node_2 = edge[1]
      x1, y1 = graph.node[node_1]['XY']
      x2, y2 = graph.node[node_2]['XY']
      dist = sqrt( pow( (x2 - x1), 2 ) + pow( (y2 - y1), 2 ) )
      dist = round(dist, 2)
      graph.edge[node_1][node_2]['distance'] = dist

  # plot graph
  if show:
    pointIDXY = dict(zip(range(len(point_graph)), point_graph))
    nx.draw(graph, pointIDXY)

  return graph