Day 10

lib/day10/day10.ex

@@ -0,0 +1,113 @@
+defmodule Day10 do
+  @example1 """
+  16
+  10
+  15
+  5
+  1
+  11
+  7
+  19
+  6
+  12
+  4
+  """
+
+  @example2 """
+  28
+  33
+  18
+  42
+  31
+  14
+  46
+  20
+  48
+  47
+  24
+  23
+  49
+  45
+  19
+  38
+  39
+  11
+  1
+  32
+  25
+  35
+  8
+  17
+  7
+  9
+  4
+  2
+  34
+  10
+  3
+  """
+
+  def part1(example \\ false) do
+    inputs = parse_input(example)
+
+    differences = get_differences(inputs)
+
+    Enum.count(differences, &(&1 == 1)) * Enum.count(differences, &(&1 == 3))
+  end
+
+  def part2(example \\ false) do
+    inputs = parse_input(example)
+
+    differences = get_differences(inputs)
+
+    count_sequences_of_ones(differences, [0])
+    |> Enum.map(fn x -> count_paths(x, 3) end)
+    |> Enum.reduce(1, &(&1 * &2))
+  end
+
+  def parse_input(example) do
+    case example do
+      false -> File.read!("lib/day10/input.txt")
+      1 -> @example1
+      2 -> @example2
+    end
+    |> String.trim()
+    |> String.split("\n")
+    |> Enum.map(&String.to_integer/1)
+    |> Enum.sort()
+  end
+
+  def get_differences(inputs) do
+    [0 | inputs]
+    |> Enum.zip(inputs ++ [List.last(inputs) + 3])
+    |> Enum.map(fn {a, b} -> b - a end)
+  end
+
+  def count_sequences_of_ones([cur_diff | rest_diffs], [cur_count | rest]) do
+    case cur_diff do
+      1 ->
+        count_sequences_of_ones(rest_diffs, [cur_count + 1 | rest])
+
+      _ ->
+        count_sequences_of_ones(rest_diffs, [0, cur_count | rest])
+    end
+  end
+
+  def count_sequences_of_ones([], counts) do
+    counts
+  end
+
+  def count_paths(length, _max_diff) when length <= 1 do
+    1
+  end
+
+  def count_paths(length, max_diff) do
+    IO.puts("count_paths(#{length}, #{max_diff})")
+
+    1..min(length, max_diff)
+    |> Enum.map(fn x ->
+      count_paths(length - x, max_diff)
+    end)
+    |> Enum.sum()
+  end
+end

lib/day10/input.txt

@@ -0,0 +1,92 @@
+56
+139
+42
+28
+3
+87
+142
+57
+147
+6
+117
+95
+2
+112
+107
+54
+146
+104
+40
+26
+136
+127
+111
+47
+8
+24
+13
+92
+18
+130
+141
+37
+81
+148
+31
+62
+50
+80
+91
+33
+77
+1
+96
+100
+9
+120
+27
+97
+60
+102
+25
+83
+55
+118
+19
+113
+49
+133
+14
+119
+88
+124
+110
+145
+65
+21
+7
+74
+72
+61
+103
+20
+41
+53
+32
+44
+10
+34
+121
+114
+67
+69
+66
+82
+101
+68
+84
+48
+73
+17
+43
+140