3 files changed, 365 insertions, 0 deletions

diff --git a/2020/day-01/README.org b/2020/day-01/README.org
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+#+SETUPFILE: ~/.emacs.d/org-templates/level-2.org
+#+HTML_LINK_UP: ../../writings/
+#+OPTIONS: toc:2
+#+EXPORT_FILE_NAME: index
+#+TITLE: Day 01 - Report Repair
+
+* Puzzle
+- This puzzle is taken from: https://adventofcode.com/2020/day/1
+
+After saving Christmas five years in a row, you've decided to take a
+vacation at a nice resort on a tropical island. Surely, Christmas will
+go on without you.
+
+The tropical island has its own currency and is entirely cash-only. The
+gold coins used there have a little picture of a starfish; the locals
+just call them stars. None of the currency exchanges seem to have heard
+of them, but somehow, you'll need to find fifty of these coins by the
+time you arrive so you can pay the deposit on your room.
+
+To save your vacation, you need to get all fifty stars by December 25th.
+
+Collect stars by solving puzzles. Two puzzles will be made available on
+each day in the Advent calendar; the second puzzle is unlocked when you
+complete the first. Each puzzle grants one star. Good luck!
+
+Before you leave, the Elves in accounting just need you to fix your
+expense report (your puzzle input); apparently, something isn't quite
+adding up.
+
+Specifically, they need you to find the two entries that sum to 2020 and
+then multiply those two numbers together.
+
+For example, suppose your expense report contained the following:
+#+BEGIN_SRC
+1721
+979
+366
+299
+675
+1456
+#+END_SRC
+
+In this list, the two entries that sum to 2020 are 1721 and 299.
+Multiplying them together produces 1721 * 299 = 514579, so the correct
+answer is 514579.
+
+Of course, your expense report is much larger. Find the two entries that
+sum to 2020; what do you get if you multiply them together?
+** Part 2
+The Elves in accounting are thankful for your help; one of them even
+offers you a starfish coin they had left over from a past vacation. They
+offer you a second one if you can find three numbers in your expense
+report that meet the same criteria.
+
+Using the above example again, the three entries that sum to 2020 are
+979, 366, and 675. Multiplying them together produces the
+answer, 241861950.
+
+In your expense report, what is the product of the three entries that
+sum to 2020?
+* Solution
+Slurping the input file to =@inputs=. =@inputs= will contain an array of
+integers from the input file which were seperated by a newline.
+#+BEGIN_SRC raku
+my @inputs;
+@inputs = "input".IO.lines>>.Int;
+#+END_SRC
+
+Iterate over all possible combinations of =@inputs=, so 2 nested =for= loop
+would've solved it. But a better solution would be to remove elements
+from =@inputs= while iterating over it.
+
+Consider this list: (=1, 2=). A nested =for= loop on it means we have to
+check for (=1, 1=, =1, 2=, =2, 1=, =2, 2=). But =1 + 2= is the same as =2 + 1= so we
+could've dropped either the 1st or the 2nd index from that array.
+
+We do just that by =pop='ing the value from =@inputs= while iterating. That
+way we just check for (=1, 1=, =1, 2=, =2, 2=). (=1, 2=) is skipped because =1=
+was =pop='ed out in =while= loop.
+
+#+BEGIN_SRC raku
+while pop @inputs -> $num_1 {
+    my Int $diff = 2020 - $num_1;
+    for @inputs -> $num_2 {
+        say "Part 1: ", $num_2 * $num_1 if $diff == $num_2;
+    }
+}
+#+END_SRC
+
+Also, =$diff= is calculated to be =2020 - $num_1= & checked for =$diff= being
+equal to =$num_2=. The check should be =$num_1 + $num_2= being equal to
+=2020=. But that would mean this addition would be run the number of times
+=for= loop is run. It is optimized by calculating =$diff= once for each =for=
+iteration.
+
+For example, consider this list: (=1, 2=). Without this optimization we go
+through these steps:
+#+BEGIN_SRC
+## first iteration through while loop
+if 1 + 1 == 2020 then print 1 * 1 # for loop 1
+if 1 + 2 == 2020 then print 1 * 2 # for loop 2
+
+## second iteration through while loop
+if 2 + 2 == 2020 then print 2 * 2 # for loop 1
+#+END_SRC
+
+But with the optimization we go through these steps:
+#+BEGIN_SRC
+## first iteration through while loop
+diff = 2020 - 1
+if 1 == diff then print 1 * 1 # for loop 1
+if 2 == diff then print 1 * 2 # for loop 2
+
+## second iteration through while loop
+diff = 2020 - 2
+if 2 == diff then print 2 * 2 # for loop 2
+#+END_SRC
+
+You see what changes? We replaced all that addition in each =for=
+iteration with a single subtraction. In our example list, instead of 3
+addition operations we just perform 2 subtraction operations but the
+optimization will better work on longer list.
+
+Now this is not the best solution, we could do some more optimization
+but I'll leave it at this.
+** Part 2
+Input is again slurped to =@inputs=. This time 3 nested for loops are
+implemented instead. This was the simplest solution that came to my
+mind.
+#+BEGIN_SRC raku
+@inputs = "input".IO.lines>>.Int;
+for @inputs -> $num_1 {
+    for @inputs -> $num_2 {
+        for @inputs -> $num_3 {
+            if 2020 == $num_1 + $num_2 + $num_3 {
+                say "Part 2: ", ($num_1 * $num_2 * $num_3);
+                exit;
+            }
+        }
+    }
+}
+#+END_SRC
diff --git a/2020/day-01/day-01.raku b/2020/day-01/day-01.raku
new file mode 100644
index 0000000..7ab6c49
--- /dev/null
+++ b/2020/day-01/day-01.raku
@@ -0,0 +1,23 @@
+#!/usr/bin/env raku
+
+my @inputs;
+@inputs = "input".IO.lines>>.Int;
+
+while pop @inputs -> $num_1 {
+    my Int $diff = 2020 - $num_1;
+    for @inputs -> $num_2 {
+        say "Part 1: ", $num_2 * $num_1 if $diff == $num_2;
+    }
+}
+
+@inputs = "input".IO.lines>>.Int;
+for @inputs -> $num_1 {
+    for @inputs -> $num_2 {
+        for @inputs -> $num_3 {
+            if 2020 == $num_1 + $num_2 + $num_3 {
+                say "Part 2: ", ($num_1 * $num_2 * $num_3);
+                exit;
+            }
+        }
+    }
+}
diff --git a/2020/day-01/input b/2020/day-01/input
new file mode 100644
index 0000000..cd21524
--- /dev/null
+++ b/2020/day-01/input
@@ -0,0 +1,200 @@
+1544
+1560
+1947
+1659
+1972
+1940
+1977
+1689
+1916
+1638
+1804
+1543
+1789
+545
+968
+1959
+1783
+1869
+1581
+1976
+1859
+1660
+1793
+69
+1653
+1866
+1541
+1920
+1751
+1681
+1829
+2009
+1752
+680
+1864
+1628
+1917
+1876
+2002
+1974
+1827
+1791
+1552
+1669
+1849
+1167
+1744
+1764
+1913
+1782
+1926
+1795
+1738
+1877
+1811
+1746
+1682
+1943
+1761
+1850
+983
+1617
+1901
+1750
+1842
+1588
+1679
+1759
+1994
+1847
+1657
+1981
+1648
+1996
+1572
+1953
+1555
+1665
+1680
+1872
+1826
+1316
+1962
+1893
+1545
+1535
+1895
+1819
+1891
+1919
+1853
+1831
+704
+1978
+1780
+1722
+1652
+1625
+478
+1030
+1985
+1720
+1817
+264
+1988
+1892
+1712
+1222
+1840
+1894
+1906
+1890
+1846
+1939
+1991
+1835
+1799
+1865
+1663
+1908
+1575
+1970
+1956
+1556
+1688
+1558
+1698
+1771
+1807
+1878
+1707
+1770
+1823
+1802
+1930
+1703
+1136
+1910
+1998
+1973
+1611
+1979
+1612
+1838
+1715
+1885
+1879
+1904
+1941
+1734
+1900
+1809
+1691
+1848
+1683
+1754
+1874
+1975
+1896
+1567
+1785
+1644
+1922
+1651
+1046
+1971
+1600
+1933
+1857
+1960
+1948
+1675
+1828
+1633
+1868
+1615
+1884
+1674
+1860
+1775
+995
+1596
+2006
+1737
+1649
+1997
+1767
+1784
+1705
+1664
+1766
+1839
+1533
+1935
+1796
+1781
+1589
+1594
+1987
+1769