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+;;; set.el --- Working with mathematical sets -*- lexical-binding: t -*-
+
+;; Author: William Carroll <wpcarro@gmail.com>
+;; Version: 0.0.1
+;; Package-Requires: ((emacs "24.3"))
+
+;;; Commentary:
+;; The set data structure is a collection that deduplicates its elements.
+
+;;; Code:
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Dependencies
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(require 'cl-lib)
+(require 'dash)
+(require 'ht) ;; friendlier API for hash-tables
+(require 'struct)
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Wish List
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+;; - TODO: Support enum protocol for set.
+;; - TODO: Prefer a different hash-table library that doesn't rely on mutative
+;;   code.
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Library
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(cl-defstruct set xs)
+
+(defun set-from-list (xs)
+  "Create a new set from the list XS."
+  (make-set :xs (->> xs
+                     (-map (lambda (x) (cons x nil)))
+                     ht-from-alist)))
+
+(defun set-new (&rest args)
+  "Create a new set from ARGS."
+  (set-from-list args))
+
+(defun set-to-list (xs)
+  "Map set XS into a list."
+  (->> xs
+       set-xs
+       ht-keys))
+
+(defun set-add (x xs)
+  "Add X to set XS."
+  (struct-update set
+                 xs
+                 (lambda (table)
+                   (let ((table-copy (ht-copy table)))
+                     (ht-set table-copy x nil)
+                     table-copy))
+                 xs))
+
+;; TODO: Ensure all `*/reduce' functions share the same API.
+(defun set-reduce (acc f xs)
+  "Return a new set by calling F on each element of XS and ACC."
+  (->> xs
+       set-to-list
+       (-reduce-from (lambda (acc x) (funcall f x acc)) acc)))
+
+(defun set-intersection (a b)
+  "Return the set intersection between A and B."
+  (set-reduce (set-new)
+              (lambda (x acc)
+                (if (set-contains? x b)
+                    (set-add x acc)
+                  acc))
+              a))
+
+(defun set-count (xs)
+  "Return the number of elements in XS."
+  (->> xs
+       set-xs
+       ht-size))
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Predicates
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(defun set-empty? (xs)
+  "Return t if XS has no elements in it."
+  (= 0 (set-count xs)))
+
+(defun set-contains? (x xs)
+  "Return t if set XS has X."
+  (ht-contains? (set-xs xs) x))
+
+;; TODO: Prefer using `ht.el' functions for this.
+(defun set-equal? (a b)
+  "Return t if A and B share the name members."
+  (ht-equal? (set-xs a)
+             (set-xs b)))
+
+(defun set-distinct? (a b)
+  "Return t if A and B have no shared members."
+  (set-empty? (set-intersection a b)))
+
+(defun set-superset? (a b)
+  "Return t if A has all of the members of B."
+  (->> b
+       set-to-list
+       (-all? (lambda (x) (set-contains? x a)))))
+
+(defun set-subset? (a b)
+  "Return t if each member of set A is present in set B."
+  (set-superset? b a))
+
+(provide 'set)
+;;; set.el ends here