Secret Santra in core.logic

logic.clj

(ns secret-santa.logic
  (:refer-clojure :exclude [==])
  (:require [clojure.core.logic :refer :all]))

(defrel cannot-give ^:index p1 ^:index p2)

(fact cannot-give 'adam 'ashley)
(fact cannot-give 'bill 'brandon)
(fact cannot-give 'carl 'cynthia)

(def people '[adam ashley bill brandon carl cynthia])

(defn cangiveo
  [p1 p2]
  (conda
   [(cannot-give p1 p2) fail]
   [(cannot-give p2 p1) fail]
   [succeed succeed]))

(defn giveringo
  [q]
  (fresh [p1 l1]
    (conso p1 l1 q)
    (conde
     [(emptyo l1)]
     [(fresh [p2 l2]
        (conso p2 l2 l1)
        (cangiveo p1 p2)
        (giveringo l1))])))

(defn secret-santa
  [people]
  (let [options (vec (repeatedly (count people) lvar))
        ring (conj options (first options))]
    (run 1 [q]
      (== q options)
      (distincto options)
      (everyg #(membero % people) options)
      (giveringo ring))))

(secret-santa people)
;; => ([adam bill ashley carl brandon cynthia])