Day 18 Part 1

README.md

@@ -2,7 +2,7 @@
 
 My (attempted) solutions to [Advent of Code 2020](https://adventofcode.com/2020) in Elixir.
 
-<img width="978" alt="image" src="https://user-images.githubusercontent.com/498229/102344641-8e255400-3fdf-11eb-9231-7256ec6d747b.png">
+<img width="983" alt="image" src="https://user-images.githubusercontent.com/498229/102591989-31987500-4156-11eb-8d21-2ea88021c9b0.png">
 
 ## Strategy
 

day18/README

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+Day 18 Notes
+
++--------+
+| Part 1 |
++--------+
+
+$ elixir day18part1.exs
+<will add later>
+
+Thoughts:
+Since the order of precedence is the same, we can walk the token list, building up a left operand
+and operator as we go, then applying the operator as soon as we get the right operand.  For groups,
+use recursion to resolve the group, and push the result back onto the beginning of the token list.
+
+
++--------+
+| Part 2 |
++--------+
+
+$ elixir day18part2.exs
+
+Thoughts:
+
+Now the order of precedence is relevant, so can no longer just walk the token list, need to know the
+full context.
+Addition basically becomes implicit brackets.
+Will do this part later.
+
+
++------------------+
+| Overall Thoughts |
++------------------+
+
+

day18/day18part1.exs

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+defmodule Day18Part1 do
+  def run do
+    File.stream!("input")
+    |> Stream.map(&parse_line/1)
+    |> Stream.map(&evaluate/1)
+    |> Enum.sum()
+    |> IO.puts()
+  end
+
+  def parse_line(line) do
+    line
+    |> String.graphemes()
+    |> Stream.reject(fn char -> char in [" ", "\n"] end)
+    |> Enum.map(fn
+      "*" -> :multiply
+      "+" -> :add
+      "(" -> :start_group
+      ")" -> :end_group
+      num -> String.to_integer(num)
+    end)
+  end
+
+  def evaluate(command), do: evaluate(command, nil, 0)
+  def evaluate([], nil, acc), do: acc
+  def evaluate([:multiply | rest], nil, acc), do: evaluate(rest, :multiply, acc)
+  def evaluate([:add | rest], nil, acc), do: evaluate(rest, :add, acc)
+
+  def evaluate([:start_group | rest], op, acc) do
+    {remaining, group_result} = evaluate(rest, nil, 0)
+    evaluate([group_result | remaining], op, acc)
+  end
+
+  def evaluate([:end_group | rest], _op, acc), do: {rest, acc}
+  def evaluate([num | rest], :multiply, acc), do: evaluate(rest, nil, acc * num)
+  def evaluate([num | rest], :add, acc), do: evaluate(rest, nil, acc + num)
+  def evaluate([num | rest], nil, 0), do: evaluate(rest, nil, num)
+end
+
+Day18Part1.run()