Day 1

lib/day1/day1.ex

@@ -0,0 +1,73 @@
+defmodule Day1 do
+  @example_1 """
+  1721
+  979
+  366
+  299
+  675
+  1456
+  """
+
+  def part1(example \\ false) do
+    if(example, do: @example_1, else: File.read!("lib/day1/input.txt"))
+    |> String.trim()
+    |> String.split("\n")
+    |> Enum.map(&String.to_integer/1)
+    |> all_pairs()
+    |> find_sum_to_2020()
+  end
+
+  def part2(example \\ false) do
+    if(example, do: @example_1, else: File.read!("lib/day1/input.txt"))
+    |> String.trim()
+    |> String.split("\n")
+    |> Enum.map(&String.to_integer/1)
+    |> all_triples()
+    |> find_sum_to_2020()
+  end
+
+  def all_pairs(items) do
+    items
+    |> Enum.drop(-1)
+    |> Enum.with_index()
+    |> Enum.flat_map(fn {a, a_index} ->
+      {_, rest} = Enum.split(items, a_index + 1)
+
+      Enum.map(rest, fn b ->
+        {a, b}
+      end)
+    end)
+  end
+
+  def all_triples(items) do
+    items
+    |> Enum.drop(-1)
+    |> Enum.with_index()
+    |> Enum.flat_map(fn {a, a_index} ->
+      {_, rest} = Enum.split(items, a_index + 1)
+
+      rest
+      |> Enum.drop(-1)
+      |> Enum.with_index()
+      |> Enum.flat_map(fn {b, b_index} ->
+        {_, rest_rest} = Enum.split(rest, b_index + 1)
+
+        Enum.map(rest_rest, fn c ->
+          {a, b, c}
+        end)
+      end)
+    end)
+  end
+
+  def find_sum_to_2020(groups) do
+    Enum.find_value(groups, fn items ->
+      list = Tuple.to_list(items)
+
+      if Enum.reduce(list, 0, &(&1 + &2)) == 2020 do
+        Enum.reduce(list, 1, &(&1 * &2))
+      else
+        nil
+      end
+    end)
+  end
+end

lib/day1/input.txt

@@ -0,0 +1,200 @@
+1695
+1157
+1484
+1717
+622
+1513
+1924
+63
+1461
+1971
+1382
+1587
+1913
+1665
+1464
+1914
+1637
+1527
+1424
+1361
+1187
+272
+1909
+1448
+1623
+1164
+1931
+1646
+1096
+1655
+1962
+1961
+1694
+1792
+1989
+1616
+138
+1887
+1357
+1965
+1085
+308
+2007
+1254
+1179
+1124
+1719
+1467
+1928
+1630
+1676
+1359
+1241
+1511
+1413
+1656
+1818
+1919
+1422
+1745
+1208
+1609
+1544
+1775
+1154
+1057
+1440
+1242
+1202
+1266
+1305
+1836
+1760
+1730
+1396
+1315
+1496
+1964
+1300
+1195
+1583
+1607
+1743
+1682
+1453
+1848
+1320
+1601
+954
+1473
+1847
+1486
+1853
+1668
+1342
+1087
+1139
+1349
+1568
+1728
+1420
+1233
+1073
+1376
+1658
+1477
+1871
+1958
+1950
+1503
+1758
+1474
+1203
+1336
+1981
+1309
+1618
+1846
+1974
+1940
+1333
+1119
+1756
+1918
+961
+1307
+1375
+1346
+1611
+1284
+84
+1754
+1608
+2010
+1341
+1136
+1218
+1882
+1911
+1288
+1930
+1749
+1952
+1556
+1757
+1761
+1112
+1963
+1186
+1373
+1622
+1973
+1330
+1508
+1222
+1226
+1389
+1679
+1584
+1237
+1563
+1763
+1998
+1293
+1642
+95
+1661
+1674
+1100
+1262
+1895
+1548
+1400
+1205
+1435
+1156
+1034
+1577
+1701
+1198
+1173
+1500
+1858
+1809
+1780
+1412
+1982
+1070
+1523
+1776
+1598
+1113
+1144
+1777
+1313
+1102
+1999
+1405
+1784
+1196