This SRFI defines immutable deques. A deque is a double-ended queue, a sequence which allows elements to be added or removed efficiently from either end. A structure is immutable when all its operations leave the structure unchanged.
This implementation, by Shiro Kawai, is based on the Banker's Deque representation described by Chris Okasaki in Purely Functional Data Structures. The cost of the basic operations ideque-front, ideque-back, ideque-remove-front, and ideque-remove-back are amortized O(1).
This page includes excerpts from the SRFI document, but is primarily intended to document the forms exported by the egg. For a full description of the SRFI, see the full SRFI document.
Kevin Wortman & John Cowan (text), and Shiro Kawai (implementation).
Deques are disjoint from all other Scheme types.
- ideque element ...procedure
Returns an ideque containing the elements. The first element (if any) will be at the front of the ideque and the last element (if any) will be at the back. Takes O(n) time, where n is the number of elements.
- ideque-tabulate n procprocedure
Invokes the predicate proc on every exact integer from 0 (inclusive) to n (exclusive). Returns an ideque containing the results in order of generation. Takes O(n) time.
- ideque-unfold stop? mapper successor seedprocedure
Invokes the predicate stop? on seed. If it returns false, generate the next result by applying mapper to seed, generate the next seed by applying successor to seed, and repeat this algorithm with the new seed. If stop? returns true, return an ideque containing the results in order of accumulation. Takes O(n) time.
- ideque-unfold-right stop? mapper successor seedprocedure
Invokes the predicate stop? on seed. If it returns false, generate the next result by applying mapper to seed, generate the next seed by applying successor to seed, and repeat the algorithm with the new seed. If stop? returns true, return an ideque containing the results in reverse order of accumulation. Takes O(n) time.
- ideque? xprocedure
Returns #t if x is an ideque, and #f otherwise. Takes O(1) time.
- ideque-empty? idequeprocedure
Returns #t if ideque contains zero elements, and #f otherwise. Takes O(1) time.
- ideque= elt= ideque ...procedure
Determines ideque equality, given an element-equality procedure. Ideque A equals ideque B if they are of the same length, and their corresponding elements are equal, as determined by elt=. If the element-comparison procedure's first argument is from idequei, then its second argument is from idequei+1, i.e. it is always called as (elt= a b) for a an element of ideque A, and b an element of ideque B.
In the n-ary case, every idequei is compared to idequei+1 (as opposed, for example, to comparing ideque1 to every idequei, for i > 1). If there are zero or one ideque arguments, ideque= simply returns true. The name does not end in a question mark for compatibility with the SRFI-1 procedure list=.
Note that the dynamic order in which the elt= procedure is applied to pairs of elements is not specified. For example, if ideque= is applied to three ideques, A, B, and C, it may first completely compare A to B, then compare B to C, or it may compare the first elements of A and B, then the first elements of B and C, then the second elements of A and B, and so forth.
The equality procedure must be consistent with eq?. Note that this implies that two ideques which are eq? are always ideque=, as well; implementations may exploit this fact to "short-cut" the element-by-element comparisons.
- ideque-any pred idequeprocedure
- ideque-every pred idequeprocedure
Invokes pred on the elements of the ideque in order until one call returns a true/false value, which is then returned. If there are no elements, returns #f/#t. Takes O(n) time.
- ideque-front idequeprocedure
- ideque-back idequeprocedure
Returns the front/back element of ideque. It is an error for ideque to be empty. Takes O(1) time.
- ideque-remove-front idequeprocedure
- ideque-remove-back idequeprocedure
Returns an ideque with the front/back element of ideque removed. It is an error for ideque to be empty. Takes O(1) time.
- ideque-add-front ideque objprocedure
- ideque-add-back ideque objprocedure
Returns an ideque with obj pushed to the front/back of ideque. Takes O(1) time.
- ideque-ref ideque nprocedure
Returns the nth element of ideque. It is an error unless n is less than the length of ideque. Takes O(n) time.
- ideque-take ideque nprocedure
- ideque-take-right ideque nprocedure
Returns an ideque containing the first/last n elements of ideque. It is an error if n is greater than the length of ideque. Takes O(n) time.
- ideque-drop ideque nprocedure
- ideque-drop-right ideque nprocedure
Returns an ideque containing all but the first/last n elements of ideque. It is an error if n is greater than the length of ideque. Takes O(n) time.
- ideque-split-at ideque nprocedure
Returns two values, the results of (ideque-take ideque n) and (ideque-drop ideque n) respectively, but may be more efficient. Takes O(n) time.
- ideque-length idequeprocedure
Returns the length of ideque as an exact integer. May take O(n) time, though O(1) is optimal.
- ideque-append ideque ...procedure
Returns an ideque with the contents of the ideque followed by the others, or an empty ideque if there are none. Takes O(kn) time, where k is the number of ideques and n is the number of elements involved.
- ideque-reverse idequeprocedure
Returns an ideque containing the elements of ideque in reverse order. Takes O(1) time.
- ideque-count pred idequeprocedure
pred is a procedure taking a single value and returning a single value. It is applied element-wise to the elements of ideque, and a count is tallied of the number of elements that produce a true value. This count is returned. Takes O(n) time. The dynamic order of calls to pred is unspecified.
- ideque-zip ideque1 ideque2 ...procedure
Returns an ideque of lists (not ideques) each of which contains the corresponding elements of ideques in the order specified. Terminates when all the elements of any of the ideques have been processed. Takes O(kn) time, where k is the number of ideques and n is the number of elements in the shortest ideque.
- ideque-map proc idequeprocedure
Applies proc to the elements of ideque and returns an ideque containing the results in order. The dynamic order of calls to proc is unspecified. Takes O(n) time.
- ideque-filter-map proc idequeprocedure
Applies proc to the elements of ideque and returns an ideque containing the true (i.e. non-#f) results in order. The dynamic order of calls to proc is unspecified. Takes O(n) time.
- ideque-for-each proc idequeprocedure
- ideque-for-each-right proc idequeprocedure
Applies proc to the elements of ideque in forward/reverse order and returns an unspecified result. Takes O(n) time.
- ideque-fold proc nil idequeprocedure
- ideque-fold-right proc nil idequeprocedure
Invokes proc on the elements of ideque in forward/reverse order, passing the result of the previous invocation as a second argument. For the first invocation, nil is used as the second argument. Returns the result of the last invocation, or nil if there was no invocation. Takes O(n) time.
- ideque-append-map proc idequeprocedure
Applies proc to the elements of ideque. It is an error if the result is not a list. Returns an ideque containing the elements of the lists in order. Takes O(n) time, where n is the number of elements in all the lists returned.
- ideque-filter pred idequeprocedure
- ideque-remove pred idequeprocedure
Returns an ideque containing the elements of ideque that do/do not satisfy pred. Takes O(n) time.
- ideque-partition proc idequeprocedure
Returns two values, the results of (ideque-filter pred ideque) and (ideque-remove pred ideque) respectively. Takes O(n) time.
- ideque-find-right pred ideque #!optional failureprocedure
Returns the first/last element of ideque that satisfies pred. If there is no such element, returns the result of invoking the thunk failure; the default thunk is (lambda () #f). Takes O(n) time.
- ideque-take-while pred idequeprocedure
- ideque-take-while-right pred idequeprocedure
Returns an ideque containing the longest initial/final prefix of elements in ideque all of which satisfy pred. Takes O(n) time.
- ideque-drop-while pred idequeprocedure
- ideque-drop-while-right pred idequeprocedure
Returns an ideque which omits the longest initial/final prefix of elements in ideque all of which satisfy pred, but includes all other elements of ideque. Takes O(n) time.
- ideque-span pred idequeprocedure
- ideque-break pred idequeprocedure
Returns two values, the initial prefix of the elements of ideque which do/do not satisfy pred, and the remaining elements. Takes O(n) time.
- list->ideque listprocedure
- ideque->list idequeprocedure
Conversion between ideque and list structures. FIFO order is preserved, so the front of a list corresponds to the front of an ideque. Each operation takes O(n) time.
- generator->ideque generatorprocedure
- ideque->generator idequeprocedure
Conversion between srfi-158 generators and ideques. Each operation takes O(n) time. A generator is a procedure that is called repeatedly with no arguments to generate consecutive values, and returns an end-of-file object when it has no more values to return.
Wolfgang Corcoran-Mathe <wcm at sigwinch dot xyzzy without the zy>
- (2020-11-30) Initial release.
- (2021-01-02) Apply upstream fixes for ideque-take and ideque-drop.
Copyright (C) John Cowan, Kevin Wortman (2015). All Rights Reserved.
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