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+;; Advent of Code 2020 - Day 1
+(require 'cl)
+(require 'ht)
+(require 'dash)
+
+(defmacro hash-set (&rest elements)
+  "Define a hash-table with empty values, for use as a set."
+  (cons 'ht (-map (lambda (x) (list x nil)) elements)))
+
+;; Puzzle 1:
+
+(defvar day1/input
+  (hash-set 1645 1995 1658 1062 1472 1710 1424 1823 1518 1656 1811 1511 1320 1521 1395
+            1996 1724 1666 1637 1504 1766 534 1738 1791 1372 1225 1690 1949 1495 1436 1166
+            1686 1861 1889 1887 997 1202 1478 833 1497 1459 1717 1272 1047 1751 1549 1204
+            1230 1260 1611 1506 1648 1354 1415 1615 1327 1622 1592 1807 1601 1026 1757 1376
+            1707 1514 1905 1660 1578 1963 1292 390 1898 1019 1580 1499 1830 1801 1881 1764
+            1442 1838 1088 1087 1040 1349 1644 1908 1697 1115 1178 1224 1810 1445 1594 1894
+            1287 1676 1435 1294 1796 1350 1685 1118 1488 1726 1696 1190 1538 1780 1806 1207
+            1346 1705 983 1249 1455 2002 1466 1723 1227 1390 1281 1715 1603 1862 1744 1774
+            1385 1312 1654 1872 1142 1273 1508 1639 1827 1461 1795 1533 1304 1417 1984 28
+            1693 1951 1391 1931 1179 1278 1400 1361 1369 1343 1416 1426 314 1510 1933 1239
+            1218 1918 1797 1255 1399 1229 723 1992 1595 1191 1916 1525 1605 1524 1869 1652
+            1874 1756 1246 1310 1219 1482 1429 1244 1554 1575 1123 1194 1408 1917 1613 1773
+            1809 1987 1733 1844 1423 1718 1714 1923 1503))
+
+(message "Solution to day1/1: %s"
+         (cl-loop for first being the hash-keys of day1/input
+                  for second = (- 2020 first)
+                  when (ht-contains? day1/input second)
+                  return (* first second)))
+
+;; Puzzle 2:
+
+(message "Solution to day1/1: %s"
+         (cl-loop for first being the hash-keys of day1/input
+                  for result =
+                  (cl-loop
+                   for second being the elements of (-drop 1 (ht-keys day1/input))
+                   for third = (- 2020 first second)
+                   when (ht-contains? day1/input third)
+                   return (* first second third))
+
+                  when result return result))