%Source: https://sites.google.com/site/prologsite/prolog-problems %query:encode_direct(g,f). % 1.13 (**): Run-length encoding of a list (direct solution) % encode_direct(L1,L2) :- the list L2 is obtained from the list L1 by % run-length encoding. Consecutive duplicates of elements are encoded % as terms [N,E], where N is the number of duplicates of the element E. % However, if N equals 1 then the element is simply copied into the % output list. % (list,list) (+,?) encode_direct([],[]). encode_direct([X|Xs],[Z|Zs]) :- count(X,Xs,Ys,1,Z), encode_direct(Ys,Zs). % count(X,Xs,Ys,K,T) Ys is the list that remains from the list Xs % when all leading copies of X are removed. T is the term [N,X], % where N is K plus the number of X's that can be removed from Xs. % In the case of N=1, T is X, instead of the term [1,X]. count(X,[],[],1,X). count(X,[],[],N,[N,X]) :- N > 1. count(X,[Y|Ys],[Y|Ys],1,X) :- X \= Y. count(X,[Y|Ys],[Y|Ys],N,[N,X]) :- N > 1, X \= Y. count(X,[X|Xs],Ys,K,T) :- K1 is K + 1, count(X,Xs,Ys,K1,T).
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