''' Dijkstra's shortest path covering (SVG) using priority queue. Firstly use this code to generate SVG frames. Then transform to bitmaps and convert to GIF. '''  # range size N = 500 margin = 20  def norm(px, py):     return ((px[0]-py[0])**2+(px[1]-py[1])**2)**.5  def saveToSVG(nFrames, points, edges, firmed, relaxing):     f = open('demo_'+'0'*(3-len(str(nFrames)))+str(nFrames)+'.svg', 'w')     f.write("<svg xmlns=\"http://www.w3.org/2000/svg\" version=\"1.1\">\n")     for p in points:         f.write("<circle cx=\"" +str(p[0]+margin)+ "\" cy=\""+ str(N-p[1]+margin) +"\" r=\"5\" fill=\"white\" stroke=\"black\"/>\n")     for i in range(len(edges)):         for j in edges[i]:             f.write("<line x1=\"" +str(points[i][0]+margin)+ "\" y1=\""+ str(N-points[i][1]+margin) +"\" x2=\"" + str(points[j][0]+margin) + "\" y2=\"" + str(N-points[j][1]+margin) + "\" stroke=\"grey\" stroke-width=\".5\"/>\n")     for L in firmed:         f.write("<line x1=\"" +str(L[0][0]+margin)+ "\" y1=\""+ str(N-L[0][1]+margin) +"\" x2=\"" + str(L[1][0]+margin) + "\" y2=\"" + str(N-L[1][1]+margin) + "\" stroke=\"red\" stroke-width=\"5\"/>\n")     for L in relaxing:         f.write("<line x1=\"" +str(L[0][0]+margin)+ "\" y1=\""+ str(N-L[0][1]+margin) +"\" x2=\"" + str(L[1][0]+margin) + "\" y2=\"" + str(N-L[1][1]+margin) + "\" stroke=\"blue\" stroke-width=\"5\"/>\n")     f.write("</svg>\n")     f.close()  def generatePoints(n):     import random as r     r.seed(10)          res = []     for i in range(n):         pt = [r.randint(0,N) for _ in [0, 1]]         if pt not in res:             res += [pt]     return res  # heuristic: neighbor with radius e.g. N/3 def generateEdges(n, points):     import random as r     r.seed(10)     edges = []     for i in range(n):         dst = []         for j in range(n):             if i!=j and norm(points[i], points[j]) < N/3:                 dst.append(j);         edges.append(dst)     return edges  def dijkstra(n, points, edges):     nframe = 0     dist = [float("inf") for i in range(n)]     prev = [-1 for _ in range(n)]     cover = []     import heapq     dist[0] = 0.0     heap = [[dist[i], i] for i in range(n)]     while len(heap)>0:         u = heap[0][1]         if prev[u]!=-1:             cover.append([points[prev[u]], points[u]])         saveToSVG(nframe, points, edges, cover, [])         nframe+=1         heapq.heappop(heap)         for i in edges[u]:             if i!=u and dist[i] > dist[u] + norm(points[i], points[u]):                 dist[i] = dist[u] + norm(points[i], points[u])                 prev[i] = u                 for j in range(len(heap)):                     if heap[j][1] == i:                         heap[j][0] = dist[i]                         break                 heapq.heapify(heap)                 saveToSVG(nframe, points, edges, cover, [[points[u], points[i]]])                 nframe+=1          return dist, prev  # test 50 points temporarily n = 50 pts = generatePoints(n) es = generateEdges(n, pts) dijkstra(n, pts, es)