omg I never committed d24p2

24/part2.pl

@@ -0,0 +1,35 @@
+:- use_module(library(pio)).
+:- use_module(library(dcg/basics)).
+:- initialization(main, main).
+
+main([FileName|_]) :-
+    input(FileName, Hails),
+    maplist(xyline, Hails, Lines),
+    findall(
+        [N1, N2, X, Y], (
+            member(N1-L1-X1assert, Lines), member(N2-L2-X2assert, Lines), N1 < N2,
+            intersect(L1, L2, X-Y), call(X1assert, X), call(X2assert, X),
+            X >= 200000000000000, X =< 400000000000000,
+            Y >= 200000000000000, Y =< 400000000000000),
+        Intersects),
+    length(Intersects, Answer),
+    write(Answer), nl.
+
+intersect(A1-B1-C1, A2-B2-C2, X-Y) :-
+    Det is A1*B2 - A2*B1, Det =\= 0,
+    X is (B2*C1 - B1*C2) / Det,
+    Y is (-A2*C1 + A1*C2) / Det.
+    
+xyline(N-(X-Y-_-Dx-Dy-_), N-(A-B-C)-Xassert) :-
+    A = Dy, B is -Dx, C is X*Dy - Y*Dx,
+    ( Dx >= 0 -> Xassert = =<(X); Xassert = >=(X) ).
+
+% input parsing stuff below. Brick indexing is for debugging.
+% assumption: no same hail. There are parallels, but no same
+input(FileName, Hails) :- phrase_from_file(hails(0, Hails), FileName).
+
+hails(_, []) --> eos, !.
+hails(N, [N-(X-Y-Z-Dx-Dy-Dz)|Hails]) -->
+    blanks, number(X), ",", blanks, number(Y), ",", blanks, number(Z), " @",
+    blanks, number(Dx), ",", blanks, number(Dy), ",", blanks, number(Dz), "\n",
+    {NextN is N + 1}, hails(NextN, Hails).