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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:
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Dependencies
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(require 'number)
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Library
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(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