From b0b6df2d4067390ecb23bf703ffff6f87e21b67f Mon Sep 17 00:00:00 2001
From: rhiannon morris <rhi@rhiannon.website>
Date: Fri, 9 Dec 2022 12:18:58 +0100
Subject: [PATCH] day9

---
 aoc.bqn  |   6 ++--
 aoc.m    |   2 ++
 basics.m |   8 +++++
 day9.m   | 101 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 114 insertions(+), 3 deletions(-)
 create mode 100644 day9.m

diff --git a/aoc.bqn b/aoc.bqn
index dc877a5..70e53c5 100644
--- a/aoc.bqn
+++ b/aoc.bqn
@@ -55,9 +55,9 @@ Day7 ⇐ {𝕩, "mercury only"!0}
 Day8 ⇐ {
   _rots ← {F‿x←𝔽‿𝕩, >{F⌾𝕏x}¨⟨⊢,⍉,⌽˘,⍉⌽⟩}
   A ← {+´⥊ ∨˝ ∧´∘(⊑>1⊸↓)¨ 𝕩}
-  B ← {⌈´⥊ ×˝ {1⊑⎊(1-˜≠𝕩)/ (⊑≤⊢) 𝕩}¨ 𝕩}
-  (A⋈B) ¯1⊸↓∘↓˘_rots '0'-˜ >•FLines 𝕩
+  B ← {⌈´⥊ ×˝ {1⊑⎊(1-˜≠𝕩) (/⊢≥⊑) 𝕩}¨ 𝕩}
+  (A⋈B) (¯1↓↓)˘_rots '0'-˜ >•FLines 𝕩
 }
 # ⟨ 1843 180000 ⟩
 
-# •Show Day8 ⊑•args
+Day9 ⇐ {𝕩, "mercury only"!0}
diff --git a/aoc.m b/aoc.m
index 3929ee9..8a9ffdc 100644
--- a/aoc.m
+++ b/aoc.m
@@ -46,6 +46,7 @@ run_day(Day, Part, Lines, Out) :-
 :- import_module day6.
 :- import_module day7.
 :- import_module day8.
+:- import_module day9.
 
 :- pred solution(int::in, sol::out(sol)) is semidet.
 solution(1, day1.run).
@@ -56,3 +57,4 @@ solution(5, day5.run).
 solution(6, day6.run).
 solution(7, day7.run).
 solution(8, day8.run).
+solution(9, day9.run).
diff --git a/basics.m b/basics.m
index 345b485..d5adbc6 100644
--- a/basics.m
+++ b/basics.m
@@ -6,6 +6,8 @@
 :- mode part(in,  out) is semidet.
 :- mode part(out, in)  is det.
 
+:- func part(part, T, T) = T.
+
 
 :- import_module io.
 :- import_module list.
@@ -33,6 +35,7 @@
 :- import_module int.
 
 :- func sum(list(int)) = int.
+:- func replicate(int, T) = list(T).
 
 
 :- import_module maybe.
@@ -45,6 +48,9 @@
 part("1", one).
 part("2", two).
 
+part(one, X, _) = X.
+part(two, _, Y) = Y.
+
 
 :- import_module exception.
 
@@ -71,6 +77,8 @@ read_lines_need(File, Lines, !IO) :-
 
 sum(Xs) = foldl(plus, Xs, 0).
 
+replicate(N, X) = Out :-
+  if N = 0 then Out = [] else Out = [X | replicate(N - 1, X)].
 
 to_int_may(Str) = Out :-
   if to_int(Str, N) then Out = yes(N) else Out = no.
diff --git a/day9.m b/day9.m
new file mode 100644
index 0000000..b570e9b
--- /dev/null
+++ b/day9.m
@@ -0,0 +1,101 @@
+:- module day9.
+:- interface.
+:- import_module basics.
+:- import_module univ.
+
+:- pred run(part::in, lines::in, univ::out) is cc_multi.
+
+:- implementation.
+:- import_module int.
+:- import_module string.
+:- import_module list.
+:- import_module set_bbbtree.
+
+:- type set(T) == set_bbbtree(T).
+
+
+:- type coord ---> c(x :: int, y :: int).
+:- type rope == list(coord).
+
+:- type direction ---> up; down; left; right.
+
+:- pred to_direction(string, direction).
+:- mode to_direction(in,  out) is semidet.
+:- mode to_direction(out, in)  is det.
+to_direction("U", up).
+to_direction("D", down).
+to_direction("L", left).
+to_direction("R", right).
+
+:- pred orth(direction::in, direction::in) is semidet.
+orth(A, B) :-
+  ((A = up   ; A = down),  (B = left ; B = right));
+  ((A = left ; A = right), (B = up   ; B = down)).
+
+:- pred line(string::in, list(direction)::out) is semidet.
+line(Str, replicate(N, D)) :-
+  words(Str) = [DD, NN],
+  to_direction(DD, D),
+  to_int(NN, N).
+
+:- pred move_head(direction::out, coord::in, coord::out) is multi.
+move_head(up,    c(X, Y), c(X,   Y+1)).
+move_head(down,  c(X, Y), c(X,   Y-1)).
+move_head(left,  c(X, Y), c(X-1, Y)).
+move_head(right, c(X, Y), c(X+1, Y)).
+
+:- pred ok(coord::in, coord::in) is semidet.
+ok(A, B) :- abs(A^x - B^x) =< 1, abs(A^y - B^y) =< 1.
+
+:- pred ok(rope::in) is semidet.
+ok([]). ok([_]).
+ok([A, B | Rest]) :- ok(A, B), ok([B | Rest]).
+
+:- pred same_rc(coord::in, coord::in) is semidet.
+same_rc(A, B) :- A^x = B^x ; A^y = B^y.
+
+:- pred move_tail1(coord::in, coord::in, coord::out) is nondet.
+move_tail1(H, !T) :- ok(H, !.T).
+move_tail1(H, !T) :-
+  if same_rc(H, !.T) then
+    move_head(_, !T),
+    ok(H, !.T)
+  else
+    move_head(D1, !T),
+    move_head(D2, !T),
+    orth(D1, D2),
+    ok(H, !.T).
+
+:- pred move_tail(coord::in, rope::in, rope::out) is nondet.
+move_tail(_, [],           []).
+move_tail(H, [!.T | !.Ts], [!:T | !:Ts]) :-
+  move_tail1(H, !T),
+  move_tail(!.T, !Ts).
+
+:- pred move(direction::in, rope::in, rope::out) is nondet.
+move(_, [],          []).
+move(D, [!.H | !.T], [!:H | !:T]) :-
+  move_head(D, !H),
+  move_tail(!.H, !T).
+
+
+:- type seen == set(coord).
+
+:- pred next(direction, rope, rope, seen, seen).
+:- mode next(in, in, out, in, out) is nondet.
+next(D, !Rope, !Seen) :-
+  move(D, !Rope),
+  last(!.Rope, Tail),
+  insert(Tail, !Seen).
+
+:- pred run0(part::in, lines::in, int::out) is cc_nondet.
+run0(Part, Lines, Out) :-
+  Start = replicate(part(Part, 2, 10), c(0, 0)),
+  map(line, Lines, Dirs),
+  foldl2(next, condense(Dirs), Start, _, init, End),
+  count(End, Out).
+
+run(Part, Lines, univ(Out)) :-
+  disable_warning [no_solution_disjunct] (
+    not run0(Part, Lines, Out) => die("bad input")
+  ).