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(define-module (srfi srfi-41 util)
#:use-module (srfi srfi-1)
#:use-module (srfi srfi-41)
#:use-module (util) ; let*, find-min
#:export (stream-car+cdr interleave-streams))
(define (stream-car+cdr stream)
(values (stream-car stream)
(stream-cdr stream)))
;; Merges a number of totally ordered streams into a single
;; totally ordered stream.
;; ((≺, stream)) → (≺, stream)
(define (interleave-streams < streams)
;; Drop all empty streams
(let ((streams (remove stream-null? streams)))
;; If all streams where empty, end the output stream
(if (null? streams)
stream-null
(let* ((min other (find-min < stream-car streams))
(m ms (stream-car+cdr min)))
(stream-cons m (interleave-streams < (cons ms other)))))))
(define-public (stream-insert < item s)
(interleave-streams < (list (stream item) s)))
;; Requires that stream is a total order in regards to what we filter
;; on. From there it knows that once it has found the first element
;; that satisfies our predicate all remaining elements satisfying pred
;; will be in direct succession.
(define-public (filter-sorted-stream pred stream)
(stream-take-while
pred (stream-drop-while
(negate pred) stream)))
;; Simmilar to the regular @code{filter-sorted-stream}, but once an
;; element satisfies @code{keep-remaning?} then the remaining tail
;; of the stream is all assumed to be good.
(define-public (filter-sorted-stream* pred? keep-remaining? stream)
(cond [(stream-null? stream) stream-null]
[(keep-remaining? (stream-car stream)) stream]
[(pred? (stream-car stream))
(stream-cons (stream-car stream)
(filter-sorted-stream*
pred? keep-remaining?
(stream-cdr stream)))]
[else (filter-sorted-stream* pred? keep-remaining?
(stream-cdr stream))]))
(define-public (stream-find pred stream)
(cond ((stream-null? stream) #f)
((pred (stream-car stream)) (stream-car stream))
(else (stream-find pred (stream-cdr stream)))))
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