From a2eca138aa3e179655428460d1b006512a55695e Mon Sep 17 00:00:00 2001
From: Dory <dory@dory.moe>
Date: Sat, 9 Dec 2023 22:01:20 -0800
Subject: [PATCH] idiomatic but slow d8p2

---
 08/part2.idiomaticbutslow.pl | 86 ++++++++++++++++++++++++++++++++++++
 1 file changed, 86 insertions(+)
 create mode 100644 08/part2.idiomaticbutslow.pl

diff --git a/08/part2.idiomaticbutslow.pl b/08/part2.idiomaticbutslow.pl
new file mode 100644
index 0000000..b5eadab
--- /dev/null
+++ b/08/part2.idiomaticbutslow.pl
@@ -0,0 +1,86 @@
+% :- table step_at/2.
+% :- table node_at/3.
+:- table reachable_endnode/2.
+
+:- op(700, xfx, l).
+:- op(700, xfx, r).
+From l To :- From to To-_.
+From r To :- From to _-To.
+
+answer(Answer) :-
+    starts(Starts),
+    routes(Starts, [Route1 | RestOfRoutes]),
+    foldl(unify2, RestOfRoutes, Route1, _-(_-Answer-_)).
+
+% unify2 combines 2 routes into one with its own stride-offsets-dests
+unify2(Route1, Route2, NewRoute) :-
+    once(findnsols(2, S, converge(Route1, Route2, S), SolutionPair)),
+    SolutionPair = [NewA-LenA-NewZ, _-LenB-_],
+    NewStride is LenB - LenA,
+    NewRoute = NewA-(NewStride-LenA-[0-NewZ]).
+
+% Len = Stride1*X1 + Offset1 + Dest1 = Stride2*X2 + Offset2 + Dest2
+% For performance, Route1's Stride should =< Route2's Stride
+converge(Route1, Route2, NewA-Len-NewZ) :-
+    writef('Combining %t - %t\n', [Route1, Route2]),
+    Route1 = A1-(Stride1-Offset1-Dests1),
+    Route2 = A2-(Stride2-Offset2-Dests2),
+    natnum(X2),
+    pick([Dests1, Dests2], [Dest1-Z1, Dest2-Z2]),
+    0 is (Stride2*X2 + Offset2 + Dest2 - Offset1 - Dest1) mod Stride1,
+    Len is Stride2*X2 + Offset2 + Dest2,
+    atom_concat(A1, A2, NewA), atom_concat(Z1, Z2, NewZ).
+
+routes(Starts, Routes) :-
+    maplist([S, S-Route]>>(route(S, Route)), Starts, Routes).
+
+% route builds a route (Stride-Offset-Internals) for a particular starting node.
+route(Start, Stride-FirstN-[0-FirstDest | Dests]) :-
+    direction_len(DirLen),
+    once(reachable_endnode(Start, FirstN-FirstDest)),
+    findall(ShiftedN-Dest,
+            (   reachable_endnode(Start, N-Dest),
+                ShiftedN is N - FirstN,
+                ShiftedN =\= 0,
+                (0 is ShiftedN mod DirLen -> !; true)),
+            TmpDests),
+    last(TmpDests, Stride-LastDest),
+    append(Dests, [Stride-LastDest], TmpDests).
+    
+% Dest is reachable from Start after N steps
+reachable_endnode(Start, N-Dest) :-
+    natnum(N),
+    node_at(N, Start, Dest),
+    is_end(Dest).
+
+starts(Starts) :- findall(X, X to _, Nodes), include(is_start, Nodes, Starts).
+
+is_start(Node) :- atom_chars(Node, [_, _, a]).
+is_end(Node) :- atom_chars(Node, [_, _, z]).
+
+% node_at(N, Dir, From, Dest) :- Dest is reached after moving N step from From.
+node_at(0, Start, Start).
+node_at(N, From, Dest) :-
+    N > 0,
+    PrevN is N - 1,
+    step_at(PrevN, PrevStep),
+    G =.. [PrevStep, PrevNode, Dest], G,
+    node_at(PrevN, From, PrevNode).
+
+% Step is the N-th step (counting starts from 0).
+step_at(N, Step) :-
+    direction_list(Dir),
+    length(Dir, DirLen),
+    divmod(N, DirLen, _, Remainder),
+    nth0(Remainder, Dir, Step).
+
+direction_list(Dir) :- direction(Str), atom_chars(Str, Dir).
+direction_len(Len) :- direction_list(D), length(D, Len).
+
+% pick one item from each sublist of ListOfLists & put them into Items in order.
+% [[1,2,3], [4], [5,6]] -> [1,4,5]; [1,4,6]; [2,4,5]; [2,4,6]; [3,4,5]; [3,4,6].
+pick(ListOfLists, Items) :-
+    maplist([SubList, X]>>(member(X, SubList)), ListOfLists, Items).
+
+natnum(0).
+natnum(N) :- natnum(N0), N is N0 + 1.