Day 10 Part 2

README.md

@@ -2,7 +2,7 @@
 
 My (attempted) solutions to [Advent of Code 2020](https://adventofcode.com/2020) in Elixir.
 
-<img width="975" alt="image" src="https://user-images.githubusercontent.com/498229/101746064-b0663000-3b0e-11eb-81bd-92597dba329e.png">
+<img width="975" alt="image" src="https://user-images.githubusercontent.com/498229/101786711-97c33d80-3b41-11eb-831a-d6a9b3670f30.png">
 
 ## Strategy
 

day10/README

@@ -5,7 +5,7 @@ Day 10 Notes
 +--------+
 
 $ elixir day10part1.exs
-**solved - will add answer after part 2 is complete**
+2450
 
 Thoughts:
 
@@ -15,23 +15,47 @@ implementing exactly what the instructions say.
 In this case, we just need to sort the data, and compute the difference between all the pairs.
 
 
-+-------------------+
++--------+
-| Part 2 [UNSOLVED] |
+| Part 2 |
-+-------------------+
++--------+
 
 $ elixir day10part2.exs
-...
+32396521357312
 
 Thoughts:
 
-So far defeated on this one. I'm obviously missing a fundamental trick.
+Fudge me this took me a long time (hours, including a walk in the park...)
 
-I went down a massive rabbit hold calculating combinations, which didn't pay off.
+I spent far too long trying to find a mathematical answer based on combinations, and went down
-Will leave this for now, and maybe come back to it later.
+another rabbit hole identifying the "removable" items and then trying to calculate the answer
+based on that information.
 
+Turns out the trick I was missing is graph theory.
+
+Finally, after drawing out the example on paper, I realised that the number of combinations
+forms a directed graph, where the weight of each node inherits the weight of all its parents.
+For the example case (excuse my ascii drawing skills):
+
+0(1)-1(1)-4(1)-5(1)-|
+           |  ` |   |
+           |---6(2) |
+           |    |   |
+           |---7(4)-|
+                |
+               10(4)-11(4)
+                   ` |
+                     12(8)-18(8)-19(8)
+
+So just need to make an algorithm that builds this graph, by iterating the sorted list
+and checking the next 3 elements, keeping track of the weights as we go. It's actually simpler
+than that, because we don't need to keep the connections around, just the weightings for each node.
+
+Finally, the weight of the max joltage adapter is the answer.
 
 +------------------+
 | Overall Thoughts |
 +------------------+
 
-Not committing the input for now, in case somebody decides to @ me my answer or something.
+Part 1 was very easy, part 2 was fiendish. Glad this wasn't an interview question. Sometimes my
+brain just doesn't process certain types of problem. This was one of them. Once I made the
+connection to a weighted graph it was fine.

day10/day10part2.exs

@@ -0,0 +1,31 @@
+defmodule Day10Part2 do
+  def run do
+    File.read!("input")
+    |> String.split("\n", trim: true)
+    |> Enum.map(&String.to_integer/1)
+    |> count_arrangements()
+    |> IO.puts()
+  end
+
+  def count_arrangements(list), do: count_arrangements(%{0 => 1}, [0 | Enum.sort(list)])
+
+  def count_arrangements(arrs_to, [last]), do: arrs_to[last]
+
+  def count_arrangements(arrs_to, [current | rest]) do
+    rest
+    |> Enum.take(3)
+    |> count_reachable(current, arrs_to)
+    |> count_arrangements(rest)
+  end
+
+  def count_reachable([], _a, arrs_to), do: arrs_to
+
+  def count_reachable([b | rest], a, arrs_to) when b - a <= 3 do
+    arrs_to = Map.update(arrs_to, b, arrs_to[a], &(&1 + arrs_to[a]))
+    count_reachable(rest, a, arrs_to)
+  end
+
+  def count_reachable([_b | rest], a, arrs_to), do: count_reachable(rest, a, arrs_to)
+end
+
+Day10Part2.run()

day10/input

@@ -0,0 +1,104 @@
+2
+49
+78
+116
+143
+42
+142
+87
+132
+86
+67
+44
+136
+82
+125
+1
+108
+123
+46
+37
+137
+148
+106
+121
+10
+64
+165
+17
+102
+156
+22
+117
+31
+38
+24
+69
+131
+144
+162
+63
+171
+153
+90
+9
+107
+79
+7
+55
+138
+34
+52
+77
+152
+3
+158
+100
+45
+129
+130
+135
+23
+93
+96
+103
+124
+95
+8
+62
+39
+118
+164
+172
+75
+122
+20
+145
+14
+112
+61
+43
+141
+30
+85
+101
+151
+29
+113
+94
+68
+58
+76
+97
+28
+111
+128
+21
+11
+163
+161
+4
+168
+157
+27
+72

template/dayNpart1.exs

@@ -1,8 +1,7 @@
 defmodule DayREPLACE_MEPart1 do
   def run do
     File.read!("input")
-    |> String.trim()
+    |> String.split("\n", trim: true)
-    |> String.split("\n")
   end
 end