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(.module:
  [lux #*
   ["_" test (#+ Test)]
   [abstract/monad (#+ do)]
   [data
    ["." bit ("#@." equivalence)]
    [number
     ["n" nat]
     ["i" int]]]
   [math
    ["." random (#+ Random)]]]
  {1
   ["." / (#+ Equivalence)]})

(def: #export test
  Test
  (do random.monad
    [leftN random.nat
     rightN random.nat
     leftI random.int
     rightI random.int
     sample random.nat
     different (|> random.nat (random.filter (|>> (n.= sample) not)))]
    (<| (_.covering /._)
        ($_ _.and
            (_.cover [/.sum]
                     (let [equivalence (/.sum n.equivalence i.equivalence)]
                       (and (bit@= (:: n.equivalence = leftN leftN)
                                   (:: equivalence = (#.Left leftN) (#.Left leftN)))
                            (bit@= (:: n.equivalence = leftN rightN)
                                   (:: equivalence = (#.Left leftN) (#.Left rightN)))
                            (bit@= (:: i.equivalence = leftI leftI)
                                   (:: equivalence = (#.Right leftI) (#.Right leftI)))
                            (bit@= (:: i.equivalence = leftI rightI)
                                   (:: equivalence = (#.Right leftI) (#.Right rightI))))))
            (_.cover [/.product]
                     (let [equivalence (/.product n.equivalence i.equivalence)]
                       (and (bit@= (and (:: n.equivalence = leftN leftN)
                                        (:: i.equivalence = leftI leftI))
                                   (:: equivalence = [leftN leftI] [leftN leftI]))
                            (bit@= (and (:: n.equivalence = leftN rightN)
                                        (:: i.equivalence = leftI rightI))
                                   (:: equivalence = [leftN leftI] [rightN rightI])))))
            (_.cover [/.rec]
                     (let [equivalence (: (Equivalence (List Nat))
                                          (/.rec (function (_ equivalence)
                                                   (structure
                                                    (def: (= left right)
                                                      (case [left right]
                                                        [#.Nil #.Nil]
                                                        true

                                                        [(#.Cons leftH lefT) (#.Cons rightH rightT)]
                                                        (and (n.= leftH rightH)
                                                             (:: equivalence = lefT rightT))

                                                        _
                                                        false))))))]
                       (and (:: equivalence = (list sample sample) (list sample sample))
                            (not (:: equivalence = (list sample sample) (list sample)))
                            (not (:: equivalence = (list sample sample) (list different different))))))
            ))))

(def: #export (spec (^open "_@.") generator)
  (All [a] (-> (Equivalence a) (Random a) Test))
  (do random.monad
    [left generator
     right generator]
    (<| (_.with-cover [/.Equivalence])
        ($_ _.and
            (_.test "Reflexivity."
                    (_@= left left))
            (_.test "Symmetry."
                    (if (_@= left right)
                      (_@= right left)
                      (not (_@= right left))))))))