Implements reservoir sampler randomly sampling stream of features, cl…

…oses #7

robosat/osm/sampler.py

@@ -0,0 +1,72 @@
+import random
+
+
+class ReservoirSampler:
+    '''Randomly samples k items from a stream of unknown n items.
+    '''
+
+    def __init__(self, capacity):
+        '''Creates an new `ReservoirSampler` instance.
+
+        Args:
+          capacity: the number of items to randomly sample from a stream of unknown size.
+        '''
+
+        assert capacity > 0
+
+        self.capacity = capacity
+        self.reservoir = []
+        self.pushed = 0
+
+    def push(self, v):
+        '''Adds an item to the reservoir.
+
+        Args:
+          v: the item from the stream to add to the reservoir.
+        '''
+
+        size = len(self.reservoir)
+
+        if size < self.capacity:
+            self.reservoir.append(v)
+        else:
+            assert size == self.capacity
+            assert size <= self.pushed
+
+            p = self.capacity / self.pushed
+
+            if random.random() < p:
+                i = random.randint(0, size - 1)
+                self.reservoir[i] = v
+
+        self.pushed += 1
+
+    def __len__(self):
+        '''Returns the number of randomly sampled items.
+
+        Returns:
+          The number of randomly sampled items in the reservoir.
+        '''
+
+        return len(self.reservoir)
+
+    def __getitem__(self, k):
+        '''Returns a randomly sampled item in the reservoir.
+
+        Args:
+          k: the index for the kth item from the reservoir to return.
+
+        Returns:
+          The kth item in the reservoir of randomly sampled items.
+        '''
+
+        return self.reservoir[k]
+
+    def __repr__(self):
+        '''Returns the representation for this class.
+
+        Returns:
+          The string representation for this class.
+        '''
+
+        return '<{}: {}>'.format(self.__class__.__name__, list(self))