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+;;; cache.el --- Caching things -*- lexical-binding: t -*-
+;; Author: William Carroll <wpcarro@gmail.com>
+
+;;; Commentary:
+;; An immutable cache data structure.
+;;
+;; This is like a sideways stack, that you can pull values out from and re-push
+;; to the top.  It'd be like a stack supporting push, pop, pull.
+;;
+;; This isn't a key-value data-structure like you might expect from a
+;; traditional cache.  The name is subject to change, but the underlying idea of
+;; a cache remains the same.
+;;
+;; Think about prescient.el, which uses essentially an LRU cache integrated into
+;; counsel to help create a "clairovoyant", self-organizing list.
+;;
+;; Use-cases:
+;; - Keeps an cache of workspaces sorted as MRU with an LRU eviction strategy.
+
+;;; Code:
+
+(require 'prelude)
+(require 'struct)
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Library
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(cl-defstruct cache xs)
+
+;; TODO: Prefer another KBD for yasnippet form completion than company-mode's
+;; current KBD.
+
+(defun cache/from-list (xs)
+  "Turn list, XS, into a cache."
+  (make-cache :xs xs))
+
+(defun cache/contains? (x xs)
+  "Return t if X in XS."
+  (->> xs
+       cache-xs
+       (list/contains? x)))
+
+(defun cache/touch (x xs)
+  "Ensure value X in cache, XS, is front of the list.
+If X isn't in XS (using `equal'), insert it at the front."
+  (struct/update
+   cache
+   xs
+   (>> (list/reject (lambda (y) (equal x y)))
+       (list/cons x))
+   xs))
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Tests
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(progn
+  (let ((cache (cache/from-list '("chicken" "nugget"))))
+    ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+    ;; contains?/2
+    ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+    (prelude/refute
+     (cache/contains? "turkey" cache))
+    (prelude/assert
+     (cache/contains? "chicken" cache))
+    ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+    ;; touch/2
+    ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+    (prelude/assert
+     (equal
+      (cache/touch "nugget" cache)
+      (cache/from-list '("nugget" "chicken"))))
+    (prelude/assert
+     (equal
+      (cache/touch "spicy" cache)
+      (cache/from-list '("spicy" "chicken" "nugget"))))))
+
+(provide 'cache)
+;;; cache.el ends here