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Mitigation for the cyclic-graph divergence (see tests/cyclic-graph.sx). Threads a `visited` accumulator through the recursion; each candidate next-step is gated by `nafc (membero z visited)`. Terminates on graphs with cycles, no Phase-7 tabling required for the simple acyclic-path query. Demonstrates a viable alternative to tabling for the common case where the user wants finite path enumeration over a graph with cycles. 3 new tests, 449/449 cumulative.
41 lines
1.3 KiB
Plaintext
41 lines
1.3 KiB
Plaintext
;; lib/minikanren/tests/path-cycle-free.sx — cycle-free reachability search.
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;;
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;; Threads a "visited" accumulator through the recursion, using nafc +
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;; membero to prevent revisiting nodes. Demonstrates how to make the
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;; cyclic-graph divergence problem (see tests/cyclic-graph.sx) tractable
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;; for graphs with cycles, without invoking Phase-7 tabling.
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(define
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cf-edges
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(list (list :a :b) (list :b :a) (list :b :c) (list :c :d) (list :d :a))) ; another cycle
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(define cf-edgeo (fn (from to) (membero (list from to) cf-edges)))
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(define
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patho-no-cycles
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(fn
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(x y visited path)
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(conde
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((cf-edgeo x y) (nafc (membero y visited)) (== path (list x y)))
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((fresh (z mid v-prime) (cf-edgeo x z) (nafc (membero z visited)) (conso z visited v-prime) (patho-no-cycles z y v-prime mid) (conso x mid path))))))
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(define cf-patho (fn (x y path) (patho-no-cycles x y (list x) path)))
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(mk-test
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"cycle-free-finds-finitely"
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(let
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((paths (run* q (cf-patho :a :d q))))
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(and
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(>= (len paths) 1)
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(every? (fn (p) (and (= (first p) :a) (= (last p) :d))) paths)))
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true)
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(mk-test
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"cycle-free-direct-edge"
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(run* q (cf-patho :a :b q))
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(list (list :a :b)))
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(mk-test "cycle-free-no-self-loop" (run* q (cf-patho :a :a q)) (list))
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(mk-tests-run!)
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