Count-Min Sketch for Frequency Estimation
In many real-world data engineering scenarios, you need to estimate the frequency of items in massive data streams — think of tracking trending hashtags on social media, counting queries in a search engine, or monitoring IP traffic in a data center. Trying to keep exact counts for every possible item quickly becomes impractical: the number of unique items may be enormous, memory is limited, and the data arrives too fast to store or process precisely. This is where the Count-Min Sketch comes in. Instead of storing every item, it uses a compact, probabilistic data structure to provide fast, approximate answers to frequency queries. While you sacrifice some accuracy, you gain dramatic improvements in speed and memory efficiency — making the Count-Min Sketch a powerful tool for streaming analytics.
1234567891011121314151617181920212223242526272829303132333435import hashlib class CountMinSketch: def __init__(self, width, depth): self.width = width self.depth = depth self.table = [[0] * width for _ in range(depth)] self.seeds = [i * 11 + 17 for i in range(depth)] def _hash(self, item, seed): # Use hashlib for reproducible hash values h = hashlib.md5((str(item) + str(seed)).encode()) return int(h.hexdigest(), 16) % self.width def update(self, item, count=1): for i in range(self.depth): idx = self._hash(item, self.seeds[i]) self.table[i][idx] += count def query(self, item): min_count = float('inf') for i in range(self.depth): idx = self._hash(item, self.seeds[i]) min_count = min(min_count, self.table[i][idx]) return min_count # Example usage: cms = CountMinSketch(width=100, depth=5) cms.update("apple") cms.update("banana", 3) cms.update("apple", 2) print("apple:", cms.query("apple")) # Output: 3 print("banana:", cms.query("banana")) # Output: 3 print("grape:", cms.query("grape")) # Output: 0
The Count-Min Sketch cleverly balances accuracy, memory usage, and speed. Its error is always one-sided: it may overestimate, but never underestimate, the true frequency of any item. The error margin and probability of exceeding it are determined by the width and depth of the sketch — wider tables reduce the maximum error, while deeper tables reduce the probability of a large error. This makes it ideal for high-speed environments like network monitoring, where you need to spot heavy hitters or detect anomalies without storing all data. In practice, you might use Count-Min Sketch to rank trending topics, throttle abusive users, or summarize massive logs for fast analytics. The trade-off is that you cannot retrieve the exact list of items or their true counts, but you can answer "how frequent is X?" for any X, with a guaranteed error bound and a tiny memory footprint.
The overestimation property of Count-Min Sketch means that frequency queries will never return a value less than the true count, but may return a value that is higher. This is due to hash collisions: multiple items may map to the same cell, causing their counts to be combined. The implication is that you can safely use the sketch to filter out items below a threshold (since you'll never miss a true heavy hitter), but you may need further checks to confirm high counts.
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In many real-world data engineering scenarios, you need to estimate the frequency of items in massive data streams — think of tracking trending hashtags on social media, counting queries in a search engine, or monitoring IP traffic in a data center. Trying to keep exact counts for every possible item quickly becomes impractical: the number of unique items may be enormous, memory is limited, and the data arrives too fast to store or process precisely. This is where the Count-Min Sketch comes in. Instead of storing every item, it uses a compact, probabilistic data structure to provide fast, approximate answers to frequency queries. While you sacrifice some accuracy, you gain dramatic improvements in speed and memory efficiency — making the Count-Min Sketch a powerful tool for streaming analytics.
1234567891011121314151617181920212223242526272829303132333435import hashlib class CountMinSketch: def __init__(self, width, depth): self.width = width self.depth = depth self.table = [[0] * width for _ in range(depth)] self.seeds = [i * 11 + 17 for i in range(depth)] def _hash(self, item, seed): # Use hashlib for reproducible hash values h = hashlib.md5((str(item) + str(seed)).encode()) return int(h.hexdigest(), 16) % self.width def update(self, item, count=1): for i in range(self.depth): idx = self._hash(item, self.seeds[i]) self.table[i][idx] += count def query(self, item): min_count = float('inf') for i in range(self.depth): idx = self._hash(item, self.seeds[i]) min_count = min(min_count, self.table[i][idx]) return min_count # Example usage: cms = CountMinSketch(width=100, depth=5) cms.update("apple") cms.update("banana", 3) cms.update("apple", 2) print("apple:", cms.query("apple")) # Output: 3 print("banana:", cms.query("banana")) # Output: 3 print("grape:", cms.query("grape")) # Output: 0
The Count-Min Sketch cleverly balances accuracy, memory usage, and speed. Its error is always one-sided: it may overestimate, but never underestimate, the true frequency of any item. The error margin and probability of exceeding it are determined by the width and depth of the sketch — wider tables reduce the maximum error, while deeper tables reduce the probability of a large error. This makes it ideal for high-speed environments like network monitoring, where you need to spot heavy hitters or detect anomalies without storing all data. In practice, you might use Count-Min Sketch to rank trending topics, throttle abusive users, or summarize massive logs for fast analytics. The trade-off is that you cannot retrieve the exact list of items or their true counts, but you can answer "how frequent is X?" for any X, with a guaranteed error bound and a tiny memory footprint.
The overestimation property of Count-Min Sketch means that frequency queries will never return a value less than the true count, but may return a value that is higher. This is due to hash collisions: multiple items may map to the same cell, causing their counts to be combined. The implication is that you can safely use the sketch to filter out items below a threshold (since you'll never miss a true heavy hitter), but you may need further checks to confirm high counts.
Merci pour vos commentaires !
