master

Chinese whispers algorithm

Author: amir-lahav

Chinese Whisper/Chinese Whisper.playground/Contents.swift

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+
+
+import Foundation
+
+struct SamplePair {
+    
+    let index1: Int
+    let index2: Int
+    let distance: Double
+    
+    init(idx1: Int, idx2: Int, distance: Double = 1) {
+        self.distance = distance
+        if (idx1 < idx2) {
+            index1 = idx1
+            index2 = idx2
+        } else {
+            index1 = idx2
+            index2 = idx1
+        }
+    }
+}
+
+struct OrderedSamplePair {
+    
+    let index1: Int
+    let index2: Int
+    let distance: Double
+    
+    init(idx1: Int, idx2: Int, distance: Double = 1) {
+        self.distance = distance
+        index1 = idx1
+        index2 = idx2
+    }
+}
+
+struct Pair {
+    let index1: Int
+    let index2: Int
+    
+    init(idx1: Int, idx2: Int) {
+        index1 = idx1
+        index2 = idx2
+    }
+}
+
+class ChineseWhispers {
+    
+    func convertUnorderedToOrdered(edges: [SamplePair]) -> [OrderedSamplePair] {
+        var orderdPairs: [OrderedSamplePair] = []
+        orderdPairs.reserveCapacity(edges.count*2)
+        
+        for (i, _) in edges.enumerated() {
+            orderdPairs.append(OrderedSamplePair(idx1: edges[i].index1, idx2: edges[i].index2, distance: edges[i].distance))
+            if edges[i].index1 != edges[i].index2 {
+                orderdPairs.append(OrderedSamplePair(idx1: edges[i].index2, idx2: edges[i].index1, distance: edges[i].distance))
+            }
+        }
+        return orderdPairs.sorted { (a, b) -> Bool in
+            a.index1 < b.index1 || (a.index1 == b.index1 && a.index2 < b.index2)
+        }
+    }
+    
+    func findNeighborRanges(edges: [OrderedSamplePair]) -> [Pair] {
+        let numNodes: Int = maxIndexPlusOne(pairs: edges)
+        var neighbors: [Pair] = Array(repeating: Pair(idx1: 0, idx2: 0), count: Int(numNodes))
+        var cur_node: Int = 0
+        var start_idx: Int = 0
+        
+        for (index, value) in edges.enumerated() {
+            if value.index1 != cur_node {
+                neighbors[Int(cur_node)] = Pair(idx1: start_idx,
+                                                idx2: index)
+                start_idx = index
+                cur_node = value.index1
+            }
+        }
+        if !neighbors.isEmpty {
+            neighbors[Int(cur_node)] = Pair(idx1: start_idx,
+                                            idx2: edges.count)
+        }
+        return neighbors
+    }
+    
+    func maxIndexPlusOne(pairs: [OrderedSamplePair]) -> Int {
+        if pairs.count == 0 {
+            return 0
+        }else {
+            var max_idx: Int = 0
+            for (_, value) in pairs.enumerated() {
+                if value.index1 > max_idx {
+                    max_idx = value.index1
+                }
+                if value.index2 > max_idx {
+                    max_idx = value.index2
+                }
+            }
+            return max_idx + 1
+        }
+        
+    }
+    
+    
+    func chinese_whispers<T>(
+        objects: [T],
+        distanceFunction: (T, T) -> Double,
+        eps: Double,
+        numIterations: Int) -> [Int] {
+        var edges: [SamplePair] = []
+        guard !objects.isEmpty else {
+            return []
+        }
+        for i in 0...objects.count-1 {
+            for j in i...objects.count-1 {
+                let length = distanceFunction(objects[i], objects[j])
+                if length < eps {
+                    edges.append(SamplePair(idx1: i, idx2: j))
+                }
+            }
+        }
+        return chinese_whispers(edges: edges, numIterations: numIterations)
+    }
+    
+    func chinese_whispers(
+        edges: [SamplePair],
+        numIterations: Int) -> [Int] {
+        let orderdEdges = convertUnorderedToOrdered(edges: edges)
+        return chinese_whispers(edges: orderdEdges, numIterations: numIterations)
+    }
+    
+    func chinese_whispers(
+        edges: [OrderedSamplePair],
+        numIterations: Int) -> [Int] {
+        let neighbors = findNeighborRanges(edges: edges)
+        var labels: [Int] = Array<Int>(repeating: 0, count: neighbors.count)
+        for i in 0...neighbors.count-1 {
+            labels[i] = i
+        }
+        
+        for _ in 0..<(neighbors.count * numIterations) {
+            
+            // Pick a random node.
+            let idx = Int.random(in: 0..<neighbors.count)
+            var labels_to_counts: [Int: Double] = [:]
+            let end: Int = Int(neighbors[idx].index2)
+            
+            for i in Int(neighbors[idx].index1)..<end {
+                
+                labels_to_counts[labels[Int(edges[i].index2)], default: 0] += Double(edges[i].distance)
+            }
+            
+            var bestScore: Double = -Double.infinity
+            var bestLabel = labels[idx]
+            let sorted_labels_to_counts = labels_to_counts.sorted { (a, b) -> Bool in
+                a.key < b.key
+            }
+            sorted_labels_to_counts.forEach { (i) in
+                if i.value > bestScore {
+                    bestScore = i.value
+                    bestLabel = i.key
+                }
+            }
+            labels[idx] = bestLabel
+        }
+        var label_remap: [Int: Int] = [:]
+        for (i, _) in labels.enumerated() {
+            let next_id = label_remap.count
+            if label_remap[labels[i]] == nil {
+                label_remap[labels[i]] = next_id
+            }
+        }
+        
+        for (i, _) in labels.enumerated() {
+            labels[i] = label_remap[labels[i]] ?? 0
+        }
+        return labels
+    }
+    
+    func group<T>(objects: [T], labels: [Int]) -> [[T]] {
+        var cluster: [Int : [T]] = [:]
+        for (i, value) in labels.enumerated() {
+            if cluster[value] != nil {
+                cluster[value]!.append(objects[i])
+            }else {
+                cluster[value] = [objects[i]]
+            }
+        }
+        return cluster.map { (_ , value) -> [T] in
+           value
+        }
+    }
+}
+
+
+
+
+let a = 1.1
+let b = 1.2
+let c = 1.3
+let d = 4.0
+
+let chineseWhispers = ChineseWhispers()
+let labels = chineseWhispers.chinese_whispers(objects: [a, b, c, d], distanceFunction: { (a, b) -> Double in
+    abs(a-b)
+}, eps: 0.4, numIterations: 100)
+let group = chineseWhispers.group(objects: [a, b, c, d], labels: labels)