All of the Clojure collections are immutable and persistent. In particular, the Clojure collections support efficient creation of 'modified' versions, by utilizing structural sharing, and make all of their performance bound guarantees for persistent use. The collections are efficient and inherently thread-safe. Collections are represented by abstractions, and there may be one or more concrete realizations. In particular, since 'modification' operations yield new collections, the new collection might not have the same concrete type as the source collection, but will have the same logical (interface) type.
All the collections support count for getting the size of the collection, conj for 'adding' to the collection, and seq to get a sequence that can walk the entire collection, though their specific behavior is slightly different for different types of collections.
Because collections support the seq function, all of the sequence functions can be used with any collection.
The Java collection interfaces specify algorithms for Lists, Sets, and Maps in calculating hashCode() values. All Clojure collections conform to these specifications in their hashCode() implementations.
Clojure provides its own hash computations that provide better hash properties for collections (and other types), known as the hasheq value.
The IHashEq interface marks collections that provide the hasheq() function to obtain the hasheq value. In Clojure, the hash function can be used to compute the hasheq value.
Ordered collections (vector, list, seq, etc) must use the following algorithm for calculating hasheq (where hash computes hasheq). Note that unchecked-add-int and unchecked-multiply-int are used to get integer overflow calculations.
(defn hash-ordered [collection]
(-> (reduce (fn [acc e] (unchecked-add-int
(unchecked-multiply-int 31 acc)
(hash e)))
1
collection)
(mix-collection-hash (count collection))))Unordered collections (maps, sets) must use the following algorithm for calculating hasheq. A map entry is treated as an ordered collection of key and value. Note that unchecked-add-int is used to get integer overflow calculations.
(defn hash-unordered [collection]
(-> (reduce unchecked-add-int 0 (map hash collection))
(mix-collection-hash (count collection))))The mix-collection-hash algorithm is an implementation detail subject to change.