Day 3

lib/day3/day3.ex

@@ -0,0 +1,117 @@
+defmodule Day3 do
+  def paths do
+    File.read!("lib/day3/input.txt")
+    |> String.trim()
+    |> String.split("\n")
+    |> Enum.map(&parse_path/1)
+
+    # [parse_path("R8,U5,L5,D3"), parse_path("U7,R6,D4,L4")]
+
+    # [
+    #   parse_path("R75,D30,R83,U83,L12,D49,R71,U7,L72"),
+    #   parse_path("U62,R66,U55,R34,D71,R55,D58,R83")
+    # ]
+
+    # [
+    #   parse_path("R98,U47,R26,D63,R33,U87,L62,D20,R33,U53,R51"),
+    #   parse_path("U98,R91,D20,R16,D67,R40,U7,R15,U6,R7")
+    # ]
+  end
+
+  def part1 do
+    [path_a, path_b] = paths()
+
+    all_intersections(path_a, path_b)
+    |> List.delete({0, 0})
+    |> nearest_intersection()
+  end
+
+  def part2 do
+    [path_a, path_b] = paths()
+
+    all_intersections(path_a, path_b)
+    |> List.delete({0, 0})
+    |> Enum.map(fn intersection ->
+      {
+        intersection,
+        steps_to_reach_point(path_a, intersection) + steps_to_reach_point(path_b, intersection)
+      }
+    end)
+    |> Enum.min_by(fn {_intersection, steps} -> steps end)
+  end
+
+  def parse_path(path) do
+    {path, _final_point} =
+      path
+      |> String.split(",")
+      |> Enum.map_reduce({0, 0}, fn insn, {x, y} = point ->
+        case apply_insn(insn, point) do
+          {:horiz, new_x} ->
+            {
+              {:horiz, x, new_x, y},
+              {new_x, y}
+            }
+
+          {:vert, new_y} ->
+            {
+              {:vert, x, y, new_y},
+              {x, new_y}
+            }
+        end
+      end)
+
+    path
+  end
+
+  def apply_insn("L" <> digits, {x, _}), do: {:horiz, x - String.to_integer(digits)}
+  def apply_insn("R" <> digits, {x, _}), do: {:horiz, x + String.to_integer(digits)}
+  def apply_insn("U" <> digits, {_, y}), do: {:vert, y + String.to_integer(digits)}
+  def apply_insn("D" <> digits, {_, y}), do: {:vert, y - String.to_integer(digits)}
+
+  def intersection({:vert, a_x, a_y, a_y2}, {:horiz, b_x, b_x2, b_y}) do
+    if a_x >= min(b_x, b_x2) and a_x <= max(b_x, b_x2) and b_y >= min(a_y, a_y2) and
+         b_y <= max(a_y, a_y2) do
+      {a_x, b_y}
+    end
+  end
+
+  def intersection({:horiz, _, _, _} = a, {:vert, _, _, _} = b) do
+    intersection(b, a)
+  end
+
+  def intersection(_a, _b), do: nil
+
+  def all_intersections(path_a, path_b) do
+    Enum.flat_map(path_a, fn a_segment ->
+      path_b
+      |> Enum.map(fn b_segment ->
+        intersection(a_segment, b_segment)
+      end)
+      |> Enum.reject(&is_nil/1)
+    end)
+  end
+
+  def nearest_intersection(intersections) do
+    Enum.min_by(intersections, fn {x, y} ->
+      abs(x) + abs(y)
+    end)
+  end
+
+  def steps_to_reach_point(path, final_point, steps \\ 0)
+
+  def steps_to_reach_point([{:horiz, x, x2, y} | rest_path], {final_x, final_y}, steps) do
+    if final_x >= min(x, x2) and final_x <= max(x, x2) and final_y == y do
+      steps + abs(final_x - x)
+    else
+      steps_to_reach_point(rest_path, {final_x, final_y}, steps + abs(x - x2))
+    end
+  end
+
+  def steps_to_reach_point([{:vert, x, y, y2} | rest_path], {final_x, final_y}, steps) do
+    if final_x == x and final_y >= min(y, y2) and final_y <= max(y, y2) do
+      steps + abs(final_y - y)
+    else
+      steps_to_reach_point(rest_path, {final_x, final_y}, steps + abs(y - y2))
+    end
+  end
+end

lib/day3/input.txt

@@ -0,0 +1,2 @@
+R995,D882,R144,U180,L638,U282,L907,D326,R731,D117,R323,U529,R330,U252,R73,U173,R345,U552,R230,U682,R861,U640,L930,U590,L851,D249,R669,D878,R951,D545,L690,U392,R609,D841,R273,D465,R546,U858,L518,U567,L474,D249,L463,D390,L443,U392,L196,U418,R433,U651,R520,D450,R763,U714,R495,D716,L219,D289,L451,D594,R874,U451,R406,U662,R261,D242,R821,D951,R808,D862,L871,U133,R841,D465,R710,U300,R879,D497,R85,U173,R941,U953,R705,U972,R260,D315,L632,U182,L26,D586,R438,U275,L588,U956,L550,D576,R738,U974,R648,D880,R595,D510,L789,U455,R627,U709,R7,D486,L184,U999,L404,U329,L852,D154,L232,U398,L587,U881,R938,D40,L657,D164,R45,D917,R106,U698,L824,D426,R879,U700,R847,D891,L948,U625,R663,D814,R217,U30,R610,D781,L415,D435,L904,U815,R152,U587,R287,U141,R866,D636,L290,D114,L751,D660,R6,U383,L263,U799,R330,U96,L6,U542,L449,D361,R486,U278,L990,U329,L519,U605,R501,D559,R916,U198,L499,D174,R513,U396,L473,D565,R337,D770,R211,D10,L591,D920,R367,D748,L330,U249,L307,D645,R661,U266,R234,D403,L513,U443,L989,D1,L674,D210,L537,D872,L607,D961,R894,U632,L195,U744,L426,U531,R337,D821,R113,U436,L700,U779,R555,U891,R268,D30,R958,U411,R904,U24,R760,D958,R231,U229,L561,D134,L382,D961,L237,U676,L223,U324,R663,D186,R833,U188,R419,D349,L721,U152,L912,U490,R10,D995,L98,U47,R140,D815,R826,U730,R808,U256,R479,D322,L504,D891,L413,D848,L732,U375,L307,U7,R682,U270,L495,D248,R691,D945,L70,U220,R635,D159,R269,D15,L161,U214,R814,D3,R354,D632,R469,D36,R85,U215,L243,D183,R140,U179,R812,U180,L905,U136,L34,D937,L875
+L999,D22,L292,U843,R390,U678,R688,D492,L489,U488,R305,U951,L636,U725,R402,U84,L676,U171,L874,D201,R64,D743,R372,U519,R221,U986,L393,D793,R72,D184,L553,D137,L187,U487,L757,U880,L535,U887,R481,U236,L382,D195,L388,D90,R125,U414,R512,D382,R972,U935,L172,D1,R957,U593,L151,D158,R396,D42,L30,D178,R947,U977,R67,D406,R744,D64,L677,U23,R792,U864,R259,U315,R314,U17,L37,D658,L642,U135,R624,U601,L417,D949,R203,D122,R76,D493,L569,U274,L330,U933,R815,D30,L630,D43,R86,U926,L661,D491,L541,D96,R868,D565,R664,D935,L336,D152,R63,U110,L782,U14,R172,D945,L732,D870,R404,U767,L907,D558,R748,U591,R461,D153,L635,D457,R241,U478,L237,U218,R393,U468,L182,D745,L388,D360,L222,D642,L151,U560,R437,D326,R852,U525,R717,U929,L470,U621,R421,U408,L540,D176,L69,U753,L200,U251,R742,U628,R534,U542,R85,D71,R283,U905,L418,D755,L593,U335,L114,D684,L576,D645,R652,D49,R86,D991,L838,D309,L73,U847,L418,U675,R991,U463,R314,D618,L433,U173,R869,D115,L18,U233,R541,U516,L570,U340,R264,D442,L259,U276,R433,D348,R524,D353,R336,D883,R580,U157,R79,D27,L134,D161,L748,D278,R322,D581,R654,D156,L930,D293,L156,U311,R807,D618,R408,U719,R366,D632,R307,D565,R478,D620,R988,D821,R365,D581,L946,D138,L943,U69,R620,U208,L407,U188,L122,U353,L751,U565,R849,D874,R668,D794,L140,D474,R289,D773,R344,D220,L55,D385,L394,U208,R305,U736,L896,D376,R331,D855,L466,U516,L741,U124,L825,U467,L525,D911,R76,U220,L610,U102,L261,D891,L585,U397,L152,U753,R822,D252,R106,U145,L7,U524,R343,U352,L357,D399,L446,D140,L723,U46,R687,D409,R884