tasks a-s

Task/Non-continuous-subsequences/Prolog/non-continuous-subsequences-1.pro

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+% fetch all the subsequences
+ncsubs(L, LNCSL) :-
+	setof(NCSL, one_ncsubs(L, NCSL), LNCSL).
+
+% how to build one subsequence
+one_ncsubs(L, NCSL) :-
+	extract_elem(L, NCSL);
+	(   sublist(L, L1),
+	    one_ncsubs(L1, NCSL)).
+
+% extract one element of the list
+% this element is neither the first nor the last.
+extract_elem(L, NCSL) :-
+	length(L, Len),
+	Len1 is Len - 2,
+	between(1, Len1, I),
+	nth0(I, L, Elem),
+	select(Elem, L, NCS1),
+	(   NCSL = NCS1; extract_elem(NCS1, NCSL)).
+
+% extract the first or the last element of the list
+sublist(L, SL) :-
+	(L = [_|SL];
+	reverse(L, [_|SL1]),
+	reverse(SL1, SL)).