How Do Go Maps Achieve Constant-Time Key Lookups on Average?

How Go Maps Efficiently Search for Keys

Despite the vast size of Go maps, retrieving their key-value pairs is claimed to require a "constant number of key comparisons on average." To understand how this is possible, let's delve into their internal implementation.

Go maps are implemented as hash tables where data is distributed across an array of buckets. Each bucket can accommodate up to 8 key-value pairs, with the low-order bits of the hash function determining the bucket assignment. To further distinguish entries within a bucket, the hash function's high-order bits are stored.

When the number of hashed keys exceeds 8 in a single bucket, extra buckets are chained together. This approach ensures that the number of key comparisons required to find a specific key remains constant, regardless of the map's size.

In other words, finding a key in a map with 2,000 keys doesn't involve sequentially searching through all 2,000 keys. Instead, it leverages the hash function to directly access the appropriate bucket and perform a limited number of comparisons within that bucket. This approach provides a significant performance advantage, especially for large maps.

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