;; 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))