Day 2 Part 2

README.md

@@ -2,7 +2,7 @@
 
 My (attempted) solutions to [Advent of Code 2020](https://adventofcode.com/2020) in Elixir.
 
-<img width="976" alt="image" src="https://user-images.githubusercontent.com/498229/100701084-45b44680-33e2-11eb-9fb9-11c6161a49ef.png">
+<img width="973" alt="image" src="https://user-images.githubusercontent.com/498229/100832848-05b99600-34ac-11eb-931f-4b3a95d428f7.png">
 
 ## Strategy
 

day2/README

@@ -0,0 +1,28 @@
+Day 2 Notes
+
+------
+Part1:
+------
+
+$ elixir day2part1.exs
+414
+
+Thoughts:
+
+Use a regex to parse the input
+Huh, can't iterate a string with Enum. Split into a list with Enum.graphemes/1
+
+------
+Part1:
+------
+
+$ elixir day2part2.exs
+413
+
+Thoughts:
+
+Convert indices to be zero-based.
+No xor operator without importing Bitwise? Just write it manually.
+Initially misread the problem - thought first position must match, and second position must not match,
+which caused some confusion.
+Probably rebound to the same variable names a bit too much - should work on clearer variable names.

day2/day2part2.exs

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+defmodule Day2Part2 do
+  def run do
+    regex = ~r/\A(\d+)-(\d+) (\w): (\w+)\z/
+
+    File.read!("input")
+    |> String.trim()
+    |> String.split("\n")
+    |> Enum.map(&Regex.run(regex, &1, capture: :all_but_first))
+    |> Enum.map(fn [first, second, char, password] ->
+      {
+        String.to_integer(first) - 1,
+        String.to_integer(second) - 1,
+        char,
+        String.graphemes(password)
+      }
+    end)
+    |> Enum.filter(fn {first, second, char, password} ->
+      first = Enum.at(password, first) == char
+      second = Enum.at(password, second) == char
+      (first && !second) || (!first && second)
+    end)
+    |> Enum.count()
+    |> IO.puts()
+  end
+end
+
+Day2Part2.run()