Created a generalized algo to edmonds karp (TheAlgorithms#724)

Edmonds Karp algorithm is traditionally with only one source and one sink. What do you do if you have multiple sources and sinks? This algorithm is a generalized algorithm that regardless of however many sinks and sources you have, will allow you to use this algorithm. It does this by using the traditional algorithm but adding an artificial source and sink that allows with "infinite" weight.

Author: viraatdas

Graphs/edmonds_karp_Multiple_SourceAndSink.py

@@ -0,0 +1,182 @@
+class FlowNetwork:
+    def __init__(self, graph, sources, sinks):
+        self.sourceIndex = None
+        self.sinkIndex = None
+        self.graph = graph
+
+        self._normalizeGraph(sources, sinks)
+        self.verticesCount = len(graph)
+        self.maximumFlowAlgorithm = None
+
+    # make only one source and one sink
+    def _normalizeGraph(self, sources, sinks):
+        if sources is int:
+            sources = [sources]
+        if sinks is int:
+            sinks = [sinks]
+
+        if len(sources) == 0 or len(sinks) == 0:
+            return
+
+        self.sourceIndex = sources[0]
+        self.sinkIndex = sinks[0]
+
+        # make fake vertex if there are more
+        # than one source or sink
+        if len(sources) > 1 or len(sinks) > 1:
+            maxInputFlow = 0
+            for i in sources:
+                maxInputFlow += sum(self.graph[i])
+
+
+            size = len(self.graph) + 1
+            for room in self.graph:
+                room.insert(0, 0)
+            self.graph.insert(0, [0] * size)
+            for i in sources:
+                self.graph[0][i + 1] = maxInputFlow
+            self.sourceIndex  = 0
+
+            size = len(self.graph) + 1
+            for room in self.graph:
+                room.append(0)
+            self.graph.append([0] * size)
+            for i in sinks:
+                self.graph[i + 1][size - 1] = maxInputFlow
+            self.sinkIndex = size - 1
+
+
+    def findMaximumFlow(self):
+        if self.maximumFlowAlgorithm is None:
+            raise Exception("You need to set maximum flow algorithm before.")
+        if self.sourceIndex is None or self.sinkIndex is None:
+            return 0
+
+        self.maximumFlowAlgorithm.execute()
+        return self.maximumFlowAlgorithm.getMaximumFlow()
+
+    def setMaximumFlowAlgorithm(self, Algorithm):
+        self.maximumFlowAlgorithm = Algorithm(self)
+
+
+class FlowNetworkAlgorithmExecutor(object):
+    def __init__(self, flowNetwork):
+        self.flowNetwork = flowNetwork
+        self.verticesCount = flowNetwork.verticesCount
+        self.sourceIndex = flowNetwork.sourceIndex
+        self.sinkIndex = flowNetwork.sinkIndex
+        # it's just a reference, so you shouldn't change
+        # it in your algorithms, use deep copy before doing that
+        self.graph = flowNetwork.graph
+        self.executed = False
+
+    def execute(self):
+        if not self.executed:
+            self._algorithm()
+            self.executed = True
+
+    # You should override it
+    def _algorithm(self):
+        pass
+
+
+
+class MaximumFlowAlgorithmExecutor(FlowNetworkAlgorithmExecutor):
+    def __init__(self, flowNetwork):
+        super(MaximumFlowAlgorithmExecutor, self).__init__(flowNetwork)
+        # use this to save your result
+        self.maximumFlow = -1
+
+    def getMaximumFlow(self):
+        if not self.executed:
+            raise Exception("You should execute algorithm before using its result!")
+
+        return self.maximumFlow
+
+class PushRelabelExecutor(MaximumFlowAlgorithmExecutor):
+    def __init__(self, flowNetwork):
+        super(PushRelabelExecutor, self).__init__(flowNetwork)
+
+        self.preflow = [[0] * self.verticesCount for i in range(self.verticesCount)]
+
+        self.heights = [0] * self.verticesCount
+        self.excesses = [0] * self.verticesCount
+
+    def _algorithm(self):
+        self.heights[self.sourceIndex] = self.verticesCount
+
+        # push some substance to graph
+        for nextVertexIndex, bandwidth in enumerate(self.graph[self.sourceIndex]):
+            self.preflow[self.sourceIndex][nextVertexIndex] += bandwidth
+            self.preflow[nextVertexIndex][self.sourceIndex] -= bandwidth
+            self.excesses[nextVertexIndex] += bandwidth
+
+        # Relabel-to-front selection rule
+        verticesList = [i for i in range(self.verticesCount)
+                        if i != self.sourceIndex and i != self.sinkIndex]
+
+        # move through list
+        i = 0
+        while i < len(verticesList):
+            vertexIndex = verticesList[i]
+            previousHeight = self.heights[vertexIndex]
+            self.processVertex(vertexIndex)
+            if self.heights[vertexIndex] > previousHeight:
+                # if it was relabeled, swap elements
+                # and start from 0 index
+                verticesList.insert(0, verticesList.pop(i))
+                i = 0
+            else:
+                i += 1
+
+        self.maximumFlow = sum(self.preflow[self.sourceIndex])
+
+    def processVertex(self, vertexIndex):
+        while self.excesses[vertexIndex] > 0:
+            for neighbourIndex in range(self.verticesCount):
+                # if it's neighbour and current vertex is higher
+                if self.graph[vertexIndex][neighbourIndex] - self.preflow[vertexIndex][neighbourIndex] > 0\
+                        and self.heights[vertexIndex] > self.heights[neighbourIndex]:
+                    self.push(vertexIndex, neighbourIndex)
+
+            self.relabel(vertexIndex)
+
+    def push(self, fromIndex, toIndex):
+        preflowDelta = min(self.excesses[fromIndex],
+                           self.graph[fromIndex][toIndex] - self.preflow[fromIndex][toIndex])
+        self.preflow[fromIndex][toIndex] += preflowDelta
+        self.preflow[toIndex][fromIndex] -= preflowDelta
+        self.excesses[fromIndex] -= preflowDelta
+        self.excesses[toIndex] += preflowDelta
+
+    def relabel(self, vertexIndex):
+        minHeight = None
+        for toIndex in range(self.verticesCount):
+            if self.graph[vertexIndex][toIndex] - self.preflow[vertexIndex][toIndex] > 0:
+                if minHeight is None or self.heights[toIndex] < minHeight:
+                    minHeight = self.heights[toIndex]
+
+        if minHeight is not None:
+            self.heights[vertexIndex] = minHeight + 1
+
+if __name__ == '__main__':
+    entrances = [0]
+    exits = [3]
+    # graph = [
+    #     [0, 0, 4, 6, 0, 0],
+    #     [0, 0, 5, 2, 0, 0],
+    #     [0, 0, 0, 0, 4, 4],
+    #     [0, 0, 0, 0, 6, 6],
+    #     [0, 0, 0, 0, 0, 0],
+    #     [0, 0, 0, 0, 0, 0],
+    # ]
+    graph = [[0, 7, 0, 0], [0, 0, 6, 0], [0, 0, 0, 8], [9, 0, 0, 0]]
+
+    # prepare our network
+    flowNetwork = FlowNetwork(graph, entrances, exits)
+    # set algorithm
+    flowNetwork.setMaximumFlowAlgorithm(PushRelabelExecutor)
+    # and calculate
+    maximumFlow = flowNetwork.findMaximumFlow()
+
+    print("maximum flow is {}".format(maximumFlow))