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;;; bag.el --- Working with bags (aka multi-sets) -*- lexical-binding: t -*-
;; Author: William Carroll <wpcarro@gmail.com>
;;; Commentary:
;; What is a bag? A bag should be thought of as a frequency table. It's a way
;; to convert a list of something into a set that allows duplicates. Isn't
;; allowing duplicates the whole thing with Sets? Kind of. But the interface
;; of Sets is something that bags resemble, so multi-set isn't as bag of a name
;; as it may first seem.
;;
;; If you've used Python's collections.Counter, the concept of a bag should be
;; familiar already.
;;
;; Interface:
;; - add :: x -> Bag(x) -> Bag(x)
;; - remove :: x -> Bag(x) -> Bag(x)
;; - union :: Bag(x) -> Bag(x) -> Bag(x)
;; - difference :: Bag(x) -> Bag(x) -> Bag(x)
;;; Code:
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;; Dependencies
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(require 'number)
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;; Library
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(cl-defstruct bag xs)
(defun bag/update (f xs)
"Call F on alist in XS."
(let ((ys (bag-xs xs)))
(setf (bag-xs xs) (funcall f ys))))
(defun bag/new ()
"Create an empty bag."
(make-bag :xs (alist/new)))
(defun bag/contains? (x xs)
"Return t if XS has X."
(alist/has-key? x (bag-xs xs)))
;; TODO: Tabling this for now since working with structs seems to be
;; disappointingly difficult. Where is `struct/update'?
;; (defun bag/add (x xs)
;; "Add X to XS.")
;; TODO: What do we name delete vs. remove?
;; (defun bag/remove (x xs)
;; "Remove X from XS.
;; This is a no-op is X doesn't exist in XS.")
(defun bag/from-list (xs)
"Map a list of `XS' into a bag."
(->> xs
(list/reduce
(bag/new)
(lambda (x acc)
(bag/add x 1 #'number/inc acc)))))
(provide 'bag)
;;; bag.el ends here
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