Ruby's Hash is not a simple hash table — it uses a hybrid internal structure. For small hashes (≤8 entries), CRuby uses an AR (Array Representation) table stored inline with no heap allocation, using 1-byte hash hints for fast prefiltering before equality checks. Once a hash grows beyond the threshold, it automatically converts to a traditional ST table with open addressing and O(1) lookups. Key practical takeaways include: small hashes are extremely cheap, mutable keys cause unpredictable behavior, and custom key objects must implement both hash and eql? consistently. Ruby also guarantees insertion order since version 1.9.
Table of contents
The surprising part: small hashes aren’t hash tablesThis is not a naive linear scanWhy this is actually fastWhen it becomes a “real” hash tableHash lookup: the real contractThe rule you must never breakSubtle edge case: floatsOrdered hashes (since Ruby 1.9)A deeper insight from the sourceWhat this means for your code1) Small hashes are extremely cheap2) Don’t use mutable keys3) Always implement hash and eql? together4) Understand the performance modelThe takeawayShare this:Related298 Impressions