day 7: recursion and pattern matching

Signed-off-by: Lakshya Singh <[email protected]>

Author: king-11

README.md

@@ -29,3 +29,4 @@ Collect stars by solving puzzles. Two puzzles will be made available on each day
 | [Day 4: Ceres Search](./lib/advent_of_code/day_04.ex)       |    🌟    |    🌟    |
 | [Day 5: Print Queue](./lib/advent_of_code/day_05.ex)        |    🌟    |    🌟    |
 | [Day 6: Guard Gallivant](./lib/advent_of_code/day_06.ex)    |    🌟    |          |
+| [Day 7: Bridge Repair](./lib/advent_of_code/day_07.ex)      |    🌟    |    🌟    |

lib/advent_of_code/day_07.ex

@@ -0,0 +1,35 @@
+defmodule AdventOfCode.Day07 do
+  defp parse_input(input) do
+    String.split(input, "\n", trim: true)
+    |> Enum.map(fn line -> String.split(line, ":", trim: true) end)
+    |> Enum.map(fn [value, raw_list] ->
+      {String.to_integer(value),
+       String.split(raw_list, " ", trim: true)
+       |> Enum.map(fn raw_val -> String.to_integer(raw_val) end)}
+    end)
+  end
+
+  defp join_numbers(a, b), do: Integer.pow(10, length(Integer.digits(b, 10))) * a + b
+
+  @default_options [concat: true]
+  defp is_possible?(value, numbers, options \\ [])
+  defp is_possible?(value, [head1, head2 | tail], options) do
+    options = Keyword.merge(@default_options, options)
+    is_possible?(value, [head1 + head2 | tail], options) ||
+    is_possible?(value, [head1 * head2 | tail], options) ||
+    (options[:concat] && is_possible?(value, [join_numbers(head1, head2) | tail]))
+  end
+  defp is_possible?(value, [head1], _options), do: value == head1
+
+  def part1(input) do
+    parse_input(input)
+    |> Enum.filter(fn {value, numbers} -> is_possible?(value, numbers, concat: false) end)
+    |> Enum.reduce(0, fn {value, _}, acc -> value + acc end)
+  end
+
+  def part2(input) do
+    parse_input(input)
+    |> Enum.filter(fn {value, numbers} -> is_possible?(value, numbers) end)
+    |> Enum.reduce(0, fn {value, _}, acc -> value + acc end)
+  end
+end

test/day_07_test.exs

@@ -0,0 +1,33 @@
+defmodule AdventOfCode.Day07Test do
+  use ExUnit.Case, async: true
+
+  import AdventOfCode.Day07
+
+  setup do
+    [
+      input: """
+      190: 10 19
+      3267: 81 40 27
+      83: 17 5
+      156: 15 6
+      7290: 6 8 6 15
+      161011: 16 10 13
+      192: 17 8 14
+      21037: 9 7 18 13
+      292: 11 6 16 20
+      """
+    ]
+  end
+
+  test "part1", %{input: input} do
+    result = part1(input)
+
+    assert result == 3749
+  end
+
+  test "part2", %{input: input} do
+    result = part2(input)
+
+    assert result == 11387
+  end
+end