Handle corner-case where first and last char are the same

(Folks on Slack ran into it. Test uses Evert's input.)

Author: maco

2021/elixir/lib/advent_of_code/day_14.ex

@@ -48,6 +48,7 @@ defmodule AdventOfCode.Day14 do
 
   def part2(args) do
     {start, key} = parse(args)
+    first_last = {List.first(start), List.last(start)}
 
     all_zeroes = Map.map(key, fn _ -> 0 end)
     counts = Enum.chunk_every(start, 2, 1, :discard)
@@ -56,7 +57,7 @@ defmodule AdventOfCode.Day14 do
     end)
 
     {min, max} = faster_polymer(counts, key, 1..40)
-    |> find_min_max()
+    |> find_min_max(first_last)
 
     max - min
   end
@@ -110,16 +111,37 @@ defmodule AdventOfCode.Day14 do
   Figures out the most and least common letters and how many
       iex> counts = %{{"N", "N"} => 0, {"N", "C"} => 1, {"C", "N"} => 1, {"N", "B"} => 1,
       ...> {"B","C"} => 1, {"C","H"} => 1, {"H","B"} => 1, {"C", "B"} => 0}
-      iex> AdventOfCode.Day14.find_min_max(counts)
+      iex> AdventOfCode.Day14.find_min_max(counts, {"B", "C"})
       {1, 2}
+
+      iex> counts = %{{"N", "N"} => 0, {"N", "C"} => 1, {"C", "N"} => 1, {"N", "B"} => 1,
+      ...> {"B","C"} => 1, {"C","H"} => 1, {"H","B"} => 1, {"C", "B"} => 0}
+      iex> AdventOfCode.Day14.find_min_max(counts, {"B", "B"})
+      {1, 3}
   """
-  def find_min_max(counts) do
+  def find_min_max(counts, first_last) do
     {{_, min}, {_, max}} = Enum.reduce(counts, %{}, fn {{first, second}, count}, total ->
       Map.update(total, first, count, &(&1 + count))
       |> Map.update(second, count, &(&1 + count))
     end)
     |> Map.map(fn {_, x} -> round(x / 2) end)
+    |> handle_first_last(first_last)
     |> Enum.min_max_by(fn {_letter, quant} -> quant  end)
     {min, max}
   end
+
+    @doc """
+      iex> AdventOfCode.Day14.handle_first_last(%{"B" => 3, "C" => 4, "H" => 5}, {"B", "C"})
+      %{"B" => 3, "C" => 4, "H" => 5}
+
+      iex> AdventOfCode.Day14.handle_first_last(%{"B" => 3, "C" => 4, "H" => 5}, {"B", "B"})
+      %{"B" => 4, "C" => 4, "H" => 5}
+  """
+  def handle_first_last(counts, {a, a}) do
+    as = counts[a]
+    Map.replace(counts, a, as + 1)
+  end
+
+  def handle_first_last(counts, {_first, _last}), do: counts
+
 end

2021/elixir/test/advent_of_code/day_14_test.exs

@@ -3,7 +3,7 @@ defmodule AdventOfCode.Day14Test do
   doctest AdventOfCode.Day14
 
   import AdventOfCode.Day14
-
+  @tag :skip
   test "part1" do
     input = """
     NNCB
@@ -30,6 +30,7 @@ defmodule AdventOfCode.Day14Test do
     assert result == 1588
   end
 
+  @tag :skip
   test "part2" do
     input = """
     NNCB
@@ -55,4 +56,115 @@ defmodule AdventOfCode.Day14Test do
 
     assert result == 2188189693529
   end
+
+  test "evert" do
+    input = """
+    PFVKOBSHPSPOOOCOOHBP
+
+    FV -> C
+    CP -> K
+    FS -> K
+    VF -> N
+    HN -> F
+    FF -> N
+    SS -> K
+    VS -> V
+    BV -> F
+    HC -> K
+    BP -> F
+    OV -> N
+    BF -> V
+    VH -> V
+    PF -> N
+    FC -> S
+    CS -> B
+    FK -> N
+    VK -> H
+    FN -> P
+    SH -> V
+    CV -> K
+    HP -> K
+    HO -> C
+    NO -> V
+    CK -> C
+    VB -> S
+    OC -> N
+    NS -> C
+    NF -> H
+    SF -> N
+    NK -> S
+    NP -> P
+    OO -> S
+    NH -> C
+    BC -> H
+    KS -> H
+    PV -> O
+    KO -> K
+    OK -> H
+    OH -> H
+    BH -> F
+    NB -> B
+    FH -> N
+    HV -> F
+    BN -> S
+    ON -> V
+    CB -> V
+    CF -> H
+    FB -> S
+    KF -> S
+    PS -> P
+    OB -> C
+    NN -> K
+    KV -> C
+    BK -> H
+    SN -> S
+    NC -> H
+    PK -> B
+    PC -> H
+    KN -> S
+    VO -> V
+    FO -> K
+    CH -> B
+    PH -> N
+    SO -> C
+    KH -> S
+    HB -> V
+    HH -> B
+    BB -> H
+    SC -> V
+    HS -> K
+    SP -> V
+    KB -> N
+    VN -> H
+    HK -> H
+    KP -> K
+    OP -> F
+    CO -> B
+    VP -> H
+    OS -> N
+    OF -> H
+    KK -> N
+    CC -> K
+    BS -> C
+    VV -> O
+    CN -> H
+    PB -> P
+    BO -> N
+    SB -> H
+    FP -> F
+    SK -> F
+    PO -> S
+    KC -> H
+    VC -> H
+    NV -> N
+    HF -> B
+    PN -> F
+    SV -> K
+    PP -> K
+    """
+    result = part2(input)
+
+    assert result == 3390034818249
+  end
+
 end