Added missing file tarjan_sort.py

others/tarjan_sort.py

@@ -0,0 +1,113 @@
+
+# Downloaded from http://www.logarithmic.net/pfh-files/blog/01208083168/sort.py
+
+"""
+   
+   Tarjan's algorithm and topological sorting implementation in Python
+   
+   by Paul Harrison
+   
+   Public domain, do with it as you will
+
+"""
+
+def strongly_connected_components(graph):
+    """ Find the strongly connected components in a graph using
+        Tarjan's algorithm.
+        
+        graph should be a dictionary mapping node names to
+        lists of successor nodes.
+        """
+
+    result = [ ]
+    stack = [ ]
+    low = { }
+
+    def visit(node):
+        if node in low: return
+
+        num = len(low)
+        low[node] = num
+        stack_pos = len(stack)
+        stack.append(node)
+
+        for successor in graph[node]:
+            visit(successor)
+            low[node] = min(low[node], low[successor])
+
+        if num == low[node]:
+            component = tuple(stack[stack_pos:])
+            del stack[stack_pos:]
+            result.append(component)
+            for item in component:
+                low[item] = len(graph)
+
+    for node in graph:
+        visit(node)
+
+    return result
+
+
+def topological_sort(graph):
+    count = { }
+    for node in graph:
+        count[node] = 0
+    for node in graph:
+        for successor in graph[node]:
+            count[successor] += 1
+
+    ready = [ node for node in graph if count[node] == 0 ]
+
+    result = [ ]
+    while ready:
+        node = ready.pop(-1)
+        result.append(node)
+
+        for successor in graph[node]:
+            count[successor] -= 1
+            if count[successor] == 0:
+                ready.append(successor)
+
+    return result
+
+
+def robust_topological_sort(graph):
+    """ First identify strongly connected components,
+        then perform a topological sort on these components. """
+
+    components = strongly_connected_components(graph)
+
+    node_component = { }
+    for component in components:
+        for node in component:
+            node_component[node] = component
+
+    component_graph = { }
+    for component in components:
+        component_graph[component] = [ ]
+
+    for node in graph:
+        node_c = node_component[node]
+        for successor in graph[node]:
+            successor_c = node_component[successor]
+            if node_c != successor_c:
+                component_graph[node_c].append(successor_c) 
+
+    return topological_sort(component_graph)
+
+
+if __name__ == '__main__':
+    d = {
+        0 : [1],
+        1 : [2],
+        2 : [1,3],
+        3 : [3],
+    }
+    #print d
+    #print robust_topological_sort(d)
+
+    d = {0 : [1, 2, 4], 1 : [3, 4], 2 : [0, 3], 3 : [], 4: [1]}
+    print d
+    print "scc", strongly_connected_components(d)
+    print "rts", robust_topological_sort(d)
+