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Hash Array Mapped Tries in MIT Scheme ------------------------------------- Hash Array Mapped Tries (abbreviated as HAMTs) are a data-structure that offer extremely fast persistent mappings for keys that map onto fixnum's; this frequently means eq? style associations. More details can be found by consulting Philip Bagwell's paper on the data structure.  - procedure: make-hamt Creates and returns a newly allocated hash array mapped trie. The trie contains no associations. The resulting trie uses eq? to determine if keys are equal. - procedure: hamt? object Returns #t if object is a hash array mapped trie, otherwise returns #f. - procedure: hamt/insert hamt key datum Returns a new trie containing all the associations in hamt as well as the association of datum with key. If hamt already had an association for key, the new association overwrites the old. The average time required by this operation is bounded by the logarithm of the number of associations in the trie. - procedure: hamt/get hamt key default Returns the datum associated with key in hamt. If there is no association for key, default is returned. The average time required by this operation is bounded by the logarithm of the number of associations in the trie. - procedure: hamt/delete hamt key Returns a new tree containing all the associations in hamt, except that if hamt contains an association for key, it is removed from the result. The average time required by this operation is proportional to the logarithm of the number of associations in the trie. - procedure: hamt/lookup hamt key if-found if-not-found If-found must be a procedure of one argument, and if-not-found must be a procedure of no arguments. If hamt contains an assocation for key, if-found is invoked on the datum of the assocation. Otherwise, if-not-found is invoked with no arguments. In either case, the result yielded by the invoked procedure is returned as the result of hamt/lookup (hamt/lookup reduces into the invoked procedure, i.e. calls it tail-recursively). The average time required by this operation is bounded by the logarithm of the number of associations in the trie.  http://lampwww.epfl.ch/papers/idealhashtrees.pdf