Day 19 Part 1

2020/day19/day19.ex

@@ -0,0 +1,126 @@
+defmodule RBNF do
+  def build_grammar(rules) do
+    rules
+    |> Enum.into(%{}, fn [name, rbnf] ->
+      {
+        name,
+        {:alt,
+         String.split(rbnf, " | ")
+         |> Enum.map(fn text ->
+           seq =
+             String.split(text, " ")
+             |> Enum.map(fn chars ->
+               cond do
+                 String.match?(chars, ~r/^\d+$/) -> {:ref, chars}
+                 String.match?(chars, ~r/^".*"$/) -> {:lit, String.slice(chars, 1..-2)}
+               end
+             end)
+
+           {:seq, seq}
+         end)}
+      }
+    end)
+  end
+
+  # Returns {matched_text, remaining_text}
+  def parse(grammar, rule, text) do
+    case rule do
+      {:lit, char} ->
+        if String.at(text, 0) == char do
+          String.split_at(text, 1)
+        else
+          {nil, text}
+        end
+
+      {:ref, name} ->
+        parse(grammar, Map.fetch!(grammar, name), text)
+
+      {:seq, rules} ->
+        {matches, remaining} =
+          Enum.map_reduce(rules, text, fn rule, buffer ->
+            if buffer == nil do
+              # An earlier match failed -> bail out
+              {nil, nil}
+            else
+              case parse(grammar, rule, buffer) do
+                {nil, extra} -> {nil, nil}
+                {matched, rest} -> {matched, rest}
+              end
+            end
+          end)
+
+        if Enum.all?(matches) do
+          {Enum.join(matches), remaining}
+        else
+          {nil, text}
+        end
+
+      {:alt, rules} ->
+        res =
+          Enum.reduce_while(rules, text, fn rule, _ ->
+            case parse(grammar, rule, text) do
+              {nil, _} -> {:cont, nil}
+              {matched, rest} -> {:halt, {matched, rest}}
+            end
+          end)
+
+        case res do
+          # Nothing matched
+          nil -> {nil, text}
+          {matched, rest} -> {matched, rest}
+        end
+    end
+  end
+end
+
+defmodule Day19 do
+  defp read_input do
+    Path.expand('input', Path.dirname(__ENV__.file))
+    |> File.read!()
+  end
+
+  defp test_input do
+    """
+    0: 4 1 5
+    1: 2 3 | 3 2
+    2: 4 4 | 5 5
+    3: 4 5 | 5 4
+    4: "a"
+    5: "b"
+
+    ababbb
+    bababa
+    abbbab
+    aaabbb
+    aaaabbb
+    """
+  end
+
+  defp parse_input(text) do
+    [rules, messages] = String.split(text, "\n\n", trim: true)
+    {parse_rules(rules), parse_messages(messages)}
+  end
+
+  defp parse_rules(text) do
+    String.split(text, "\n", trim: true)
+    |> Enum.map(&String.split(&1, ": "))
+  end
+
+  defp parse_messages(text) do
+    String.split(text, "\n", trim: true)
+  end
+
+  def part1 do
+    {rules, messages} = read_input() |> parse_input()
+    grammar = RBNF.build_grammar(rules)
+
+    Enum.count(messages, fn msg ->
+      case RBNF.parse(grammar, {:ref, "0"}, msg) do
+        {msg, ""} -> true
+        _ -> false
+      end
+    end)
+  end
+end
+
+Day19.part1() |> IO.inspect()