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+;;; random.el --- Functions for working with randomness -*- lexical-binding: t -*-
+;; Author: William Carroll <wpcarro@gmail.com>
+
+;;; Commentary:
+;; Functions for working with randomness.  Some of this code is not as
+;; functional as I'd like from.
+
+;;; Code:
+
+(require 'prelude)
+(require 'number)
+(require 'math)
+(require 'series)
+(require 'list)
+(require 'set)
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Library
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(defun random/int (x)
+  "Return a random integer from 0 to `X'."
+  (random x))
+
+;; TODO: Make this work with sequences instead of lists.
+(defun random/choice (xs)
+  "Return a random element of `XS'."
+  (let ((ct (list/length xs)))
+    (list/get
+     (random/int ct)
+     xs)))
+
+(defun random/boolean? ()
+  "Randonly return t or nil."
+  (random/choice (list t nil)))
+
+;; TODO: This may not work if any of these generate numbers like 0, 1, etc.
+(defun random/uuid ()
+  "Return a generated UUID string."
+  (let ((eight  (number/dec (math/triangle-of-power :base 16 :power 8)))
+        (four   (number/dec (math/triangle-of-power :base 16 :power 4)))
+        (twelve (number/dec (math/triangle-of-power :base 16 :power 12))))
+    (format "%x-%x-%x-%x-%x"
+            (random/int eight)
+            (random/int four)
+            (random/int four)
+            (random/int four)
+            (random/int twelve))))
+
+(defun random/token (length)
+  "Return a randomly generated hexadecimal string of LENGTH."
+  (->> (series/range 0 (number/dec length))
+       (list/map (lambda (_) (format "%x" (random/int 15))))
+       (list/join "")))
+
+;; TODO: Support random/sample
+(defun random/sample (n xs)
+  "Return a randomly sample of list XS of size N."
+  (prelude/assert (and (>= n 0) (< n (list/length xs))))
+  (cl-labels ((do-sample
+               (n xs y ys)
+               (if (= n (set/count ys))
+                   (->> ys
+                        set/to-list
+                        (list/map (lambda (i)
+                                    (list/get i xs))))
+                 (if (set/contains? y ys)
+                     (do-sample n xs (random/int (list/length xs)) ys)
+                   (do-sample n xs y (set/add y ys))))))
+    (do-sample n xs (random/int (list/length xs)) (set/new))))
+
+(provide 'random)
+;;; random.el ends here