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swag.lisp
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swag.lisp
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(defpackage :cp/swag
(:use :cl)
(:export #:swag-empty-error #:make-swag #:swag #:swag-push #:swag-pop #:swag-fold)
(:documentation "Provides sliding window aggregation over a semigroup."))
(in-package :cp/swag)
(define-condition swag-empty-error (error)
((swag :initarg :swag :reader swag-empty-error-swag))
(:report
(lambda (condition stream)
(format stream "Attempted to pop empty swag ~W"
(swag-empty-error-swag condition)))))
(defstruct (swag (:constructor make-swag ())
(:conc-name %swag-)
(:copier nil)
(:predicate nil))
;; FRONT and BACK stores a stack of cons cells: (object . cumulative sum)
(front nil :type list)
(back nil :type list))
(declaim (inline swag-push))
(defun swag-push (swag obj operation)
"Adds OBJ to the end of SWAG."
(push (cons obj (if (%swag-front swag)
(funcall operation (cdar (%swag-front swag)) obj)
obj))
(%swag-front swag)))
(declaim (inline swag-pop))
(defun swag-pop (swag operation)
"Removes the first element of SWAG."
(unless (%swag-back swag)
(unless (%swag-front swag)
(error 'swag-empty-error))
(loop for node in (%swag-front swag)
for value = (if (%swag-back swag)
(funcall operation (car node) (cdar (%swag-back swag)))
(car node))
do (push (rplacd node value) (%swag-back swag)))
(setf (%swag-front swag) nil))
(pop (%swag-back swag)))
(declaim (inline swag-fold))
(defun swag-fold (swag operation &optional identity)
"Folds the existing object by OPERATION, which must comprise a
semiring. Returns IDENTITY when an empty SWAG is folded."
(if (%swag-back swag)
(if (%swag-front swag)
(funcall operation
(cdar (%swag-back swag))
(cdar (%swag-front swag)))
(cdar (%swag-back swag)))
(if (%swag-front swag)
(cdar (%swag-front swag))
identity)))