diff --git a/aoc.bqn b/aoc.bqn
index 0c53d12..8ec101d 100644
--- a/aoc.bqn
+++ b/aoc.bqn
@@ -35,7 +35,7 @@ Day4 ⇐ {
 Day5 ⇐ {
   inp ← •FLines 𝕩
   Start ← {(𝕨⊸=⊑⎊0)¨⊸/ 𝕩}
-  stks ← ' '⊸≠⊸/¨ <˘ ⍉> (1⊸⊑¨ 4⊸Chunk)¨ '[' Start inp
+  stks ← ' '⊸≠⊸/¨ <˘⍉> (1⊸⊑¨ 4⊸Chunk)¨ '[' Start inp
   todo ← •BQN⚇1 (1‿3‿5⊸⊏ ' '⊸Split)¨ 'm' Start inp
   _Do1 ← {
     n‿s‿t ← 𝕩 - 0‿1‿1
@@ -47,4 +47,7 @@ Day5 ⇐ {
 }
 # ⟨ "QNNTGTPFN" "GGNPJBTTR" ⟩
 
-# •Show Day5 ⊑•args
+Day6 ⇐ {{𝕨+ ⊑/ ≡⟜⍷¨ <˘ 𝕨↕𝕩}⟜(⊑•FLines 𝕩)¨ 4‿14}
+# ⟨ 1929 3298 ⟩
+
+# •Show Day6 ⊑•args
diff --git a/aoc.m b/aoc.m
index c5df9e4..0011582 100644
--- a/aoc.m
+++ b/aoc.m
@@ -39,6 +39,8 @@ solution(3, day3.run).
 solution(4, day4.run).
 :- import_module day5.
 solution(5, day5.run).
+:- import_module day6.
+solution(6, day6.run).
 
 :- pred run_day(int, part, lines, univ).
 :- mode run_day(in, in, in, out) is cc_multi.
diff --git a/day6.m b/day6.m
new file mode 100644
index 0000000..75b115a
--- /dev/null
+++ b/day6.m
@@ -0,0 +1,43 @@
+:- module day6.
+:- interface.
+:- import_module basics.
+:- import_module univ.
+
+:- pred run(part::in, lines::in, univ::out) is cc_multi.
+
+:- implementation.
+:- import_module int.
+:- import_module char.
+:- import_module string.
+:- import_module list.
+:- import_module maybe.
+
+
+:- func uniq(list(T)) = list(T).
+uniq([]) = [].
+uniq([X | Xs]) = Ys :-
+  Ys0 = uniq(Xs),
+  (if member(X, Ys0) then Ys = Ys0 else Ys = [X | Ys0]).
+
+:- pred alldiff(list(T)::in) is semidet.
+alldiff(Xs) :- Xs = uniq(Xs).
+
+:- pred window(int::in, list(T)::in, int::out, list(T)::out) is nondet.
+window(Len, In,     0,     Out) :- take(Len, In, Out).
+window(Len, [_|In], I + 1, Out) :- window(Len, In, I, Out).
+
+:- pred start_index(int::in, list(T)::in, int::out) is cc_nondet.
+start_index(Len, Input, Index) :- window(Len, Input, Index, Out), alldiff(Out).
+
+:- func length(part) = int.
+length(one) = 4.
+length(two) = 14.
+
+:- pragma no_determinism_warning(run/3).
+run(Part, Lines, univ(Out)) :-
+  if Lines = [Line] then
+    Len = length(Part),
+    (if start_index(Len, to_char_list(Line), Out0)
+     then Out = Len + Out0
+     else die("not found"))
+  else die("bad input").