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