improve d8p2 readability

08/part2.pl

@@ -1,5 +1,3 @@
-:- table direction_loop/1.
-
 :- op(700, xfx, l).
 :- op(700, xfx, r).
 From l To :- From to To-_.
@@ -14,51 +12,55 @@ collapse_routes([X], X).
 collapse_routes(Routes, Collapsed) :-
     writef('Collapsing: %t\n', [Routes]),
     sort(2, @=<, Routes, [Route1, Route2 | CdrRoutes]),
-    unify2(Route1, Route2, 0-0-0-0, [], NewRoute),
+    unify2(Route1, Route2, NewRoute),
     collapse_routes([NewRoute | CdrRoutes], Collapsed).
 
-unify2(Route1, Route2, N1-Term1-N2-Term2, Founds, NewRoute) :-
-    Route1 = A1-(Stride1-Offset1-Loop1),
-    Route2 = A2-(Stride2-Offset2-Loop2),
-    nth0(Term1, Loop1, C1-_),
-    nth0(Term2, Loop2, C2-_),
-    Z1 is Offset1 + Stride1*N1 + C1,
-    Z2 is Offset2 + Stride2*N2 + C2,
-    next(Route1, N1-Term1, NextN1-NextTerm1),
-    (   Z1 =:= Z2
-    ->  (   Founds = [OldZ]
-        ->  atom_concat(A1, A2, NewA),
-            NewStride is Z1 - OldZ,
-            NewRoute = NewA-(NewStride-OldZ-[0-NewA]),
+% unify2 combines 2 routes into one with its own stride-offsets-dests
+unify2(Route1, Route2, Route12) :- unify2(Route1, Route2, 0-0-0-0, [], Route12).
+unify2(Route1, Route2, N1-Dest1-N2-Dest2, Founds, NewRoute) :-
+    Route1 = A1-(Stride1-Offset1-Dests1),
+    Route2 = A2-(Stride2-Offset2-Dests2),
+    nth0(Dest1, Dests1, C1-Z1),
+    nth0(Dest2, Dests2, C2-Z2),
+    Len1 is Offset1 + Stride1*N1 + C1,
+    Len2 is Offset2 + Stride2*N2 + C2,
+    next(Dests1, N1-Dest1, NextN1-NextDest1),
+    (   Len1 =:= Len2
+    ->  (   Founds = [OldLen]
+        ->  atom_concat(A1, A2, NewA), atom_concat(Z1, Z2, NewZ),
+            NewStride is Len1 - OldLen,
+            NewRoute = NewA-(NewStride-OldLen-[0-NewZ]),
             !
-        ;   unify2(Route1, Route2, NextN1-NextTerm1-N2-Term2, [Z1], NewRoute)
+        ;   unify2(Route1, Route2, NextN1-NextDest1-N2-Dest2, [Len1], NewRoute)
         )
-    ;   Z1 < Z2
-    ->  unify2(Route1, Route2, NextN1-NextTerm1-N2-Term2, Founds, NewRoute)
-    ;   next(Route2, N2-Term2, NextN2-NextTerm2),
-        unify2(Route1, Route2, N1-Term1-NextN2-NextTerm2, Founds, NewRoute)
+    ;   Len1 < Len2
+    ->  unify2(Route1, Route2, NextN1-NextDest1-N2-Dest2, Founds, NewRoute)
+    ;   next(Dests2, N2-Dest2, NextN2-NextDest2),
+        unify2(Route1, Route2, N1-Dest1-NextN2-NextDest2, Founds, NewRoute)
     ).
 
-next(_-(_-_-Loop), NLoops-Term, NextNLoops-NextTerm) :-
-    length(Loop, LoopLen),
-    (   Term < LoopLen - 1
-    ->  NextNLoops is NLoops, NextTerm is Term + 1
-    ;   NextNLoops is NLoops + 1, NextTerm is 0).
+% next(Dests, NumberOfLoops-WhichInternalZ, NextNumberOfLoops-NextInternalZ)
+next(Dests, NLoops-Dest, NextNLoops-NextDest) :-
+    length(Dests, DestsLen),
+    (   Dest < DestsLen - 1
+    ->  NextNLoops is NLoops, NextDest is Dest + 1
+    ;   NextNLoops is NLoops + 1, NextDest is 0).
 
 routes(Starts, Routes) :-
-    maplist([S, S-Route]>>(zloop([], S, 0, [], Route)), Starts, Routes).
+    maplist([S, S-Route]>>(zloop(S, Route)), Starts, Routes).
 
-zloop(_, _, _, Zs, Stride-Offset-Loop) :-
+% zloop builds a route (Stride-Offset-Internals for a particular starting node.
+zloop(Node, Route) :- zloop([], Node, 0, [], Route).
+zloop(_Direction, _Node, _Index, Zs, Stride-Offset-Dests) :-
     Zs = [FirstZIndex-Z | _], reverse(Zs, [LastZIndex-Z | ReversedZs]),
     DeltaZ is LastZIndex - FirstZIndex, DeltaZ =\= 0,
     direction_len(Len),
     divmod(DeltaZ, Len, _, 0),
     Offset = FirstZIndex, Stride = DeltaZ,
     foldl([Idx-Z, NewIdx-Z, Off, Off]>>(NewIdx is Idx - Off),
-          ReversedZs, ReversedLoop, Offset, _),
-    reverse(ReversedLoop, Loop),
+          ReversedZs, ReversedDests, Offset, _),
+    reverse(ReversedDests, Dests),
     !.
-
 zloop(Directions, Node, Index, Zs, Route) :-
     (   is_end(Node)
     ->  append(Zs, [Index-Node], NewZs)