## SRFI-134: Immutable deques

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).

## SRFI Description

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.

## Authors

Kevin Wortman & John Cowan (text), and Shiro Kawai (implementation).

## Specification

Deques are disjoint from all other Scheme types.

## Procedures

### Constructors

*[procedure]*

`(ideque element ...)`

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.

*[procedure]*

`(ideque-tabulate n proc)`

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.

*[procedure]*

`(ideque-unfold stop? mapper successor seed)`

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.

*[procedure]*

`(ideque-unfold-right stop? mapper successor seed)`

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.

### Predicates

*[procedure]*

`(ideque? x)`

Returns `#t` if *x* is an ideque, and `#f` otherwise. Takes O(1) time.

*[procedure]*

`(ideque-empty? ideque)`

Returns `#t` if *ideque* contains zero elements, and `#f` otherwise. Takes O(1) time.

*[procedure]*

`(ideque= elt= ideque ...)`

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.

*[procedure]*

`(ideque-any pred ideque)`

*[procedure]*

`(ideque-every pred ideque)`

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.

### Queue operations

*[procedure]*

`(ideque-front ideque)`

*[procedure]*

`(ideque-back ideque)`

Returns the front/back element of *ideque*. It is an error for *ideque* to be empty. Takes O(1) time.

*[procedure]*

`(ideque-remove-front ideque)`

*[procedure]*

`(ideque-remove-back ideque)`

Returns an ideque with the front/back element of *ideque* removed. It is an error for *ideque* to be empty. Takes O(1) time.

*[procedure]*

`(ideque-add-front ideque obj)`

*[procedure]*

`(ideque-add-back ideque obj)`

Returns an ideque with *obj* pushed to the front/back of *ideque*. Takes O(1) time.

### Other accessors

*[procedure]*

`(ideque-ref ideque n)`

Returns the *nth* element of *ideque*. It is an error unless *n* is less than the length of *ideque*. Takes O(*n*) time.

*[procedure]*

`(ideque-take ideque n)`

*[procedure]*

`(ideque-take-right ideque n)`

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.

*[procedure]*

`(ideque-drop ideque n)`

*[procedure]*

`(ideque-drop-right ideque n)`

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.

*[procedure]*

`(ideque-split-at ideque n)`

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.

### The whole ideque

*[procedure]*

`(ideque-length ideque)`

Returns the length of *ideque* as an exact integer. May take O(*n*) time, though O(1) is optimal.

*[procedure]*

`(ideque-append ideque ...)`

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 *ideque*s and *n* is the number of elements involved.

*[procedure]*

`(ideque-reverse ideque)`

Returns an ideque containing the elements of *ideque* in reverse order. Takes O(1) time.

*[procedure]*

`(ideque-count pred ideque)`

*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.

*[procedure]*

`(ideque-zip ideque1 ideque2 ...)`

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.

### Mapping

*[procedure]*

`(ideque-map proc ideque)`

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.

*[procedure]*

`(ideque-filter-map proc ideque)`

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.

*[procedure]*

`(ideque-for-each proc ideque)`

*[procedure]*

`(ideque-for-each-right proc ideque)`

Applies *proc* to the elements of *ideque* in forward/reverse order and returns an unspecified result. Takes O(*n*) time.

*[procedure]*

`(ideque-fold proc nil ideque)`

*[procedure]*

`(ideque-fold-right proc nil ideque)`

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.

*[procedure]*

`(ideque-append-map proc ideque)`

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.

### Filtering

*[procedure]*

`(ideque-filter pred ideque)`

*[procedure]*

`(ideque-remove pred ideque)`

Returns an ideque containing the elements of *ideque* that do/do not satisfy *pred*. Takes O(*n*) time.

*[procedure]*

`(ideque-partition proc ideque)`

Returns two values, the results of `(ideque-filter pred ideque)` and `(ideque-remove pred ideque)` respectively. Takes O(*n*) time.

### Searching

*[procedure]*

`(ideque-find pred ideque [ failure ])`

*[procedure]*

`(ideque-find-right pred ideque [ failure ])`

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.

*[procedure]*

`(ideque-take-while pred ideque)`

*[procedure]*

`(ideque-take-while-right pred ideque)`

Returns an ideque containing the longest initial/final prefix of elements in *ideque* all of which satisfy *pred*. Takes O(*n*) time.

*[procedure]*

`(ideque-drop-while pred ideque)`

*[procedure]*

`(ideque-drop-while-right pred ideque)`

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.

*[procedure]*

`(ideque-span pred ideque)`

*[procedure]*

`(ideque-break pred ideque)`

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.

### Conversion

*[procedure]*

`(list->ideque list)`

*[procedure]*

`(ideque->list ideque)`

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.

*[procedure]*

`(generator->ideque generator)`

*[procedure]*

`(ideque->generator ideque)`

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.

## About this egg

### Dependencies

The srfi-1, srfi-158, and srfi-145 eggs are required.

### Maintainer

Wolfgang Corcoran-Mathe <wcm at sigwinch dot xyzzy without the zy>

### Repository

### Version history

- 0.1
- (2020-11-30) Initial release.
- 0.2
- (2021-01-02) Apply upstream fixes for
`ideque-take`and`ideque-drop`.

## SRFI license

Copyright (C) John Cowan, Kevin Wortman (2015). All Rights Reserved.

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

## Implementation license

Copyright (c) 2015 Shiro Kawai <shiro@acm.org>

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.

2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

3. Neither the name of the authors nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.