106 lines
3 KiB
Forth
106 lines
3 KiB
Forth
# Day 1: Report Repair
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After saving Christmas five years in a row, you've decided
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to take a vacation at a nice resort on a tropical island.
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Surely, Christmas will go on without you.
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The tropical island has its own currency and is entirely
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cash-only. The gold coins used there have a little picture
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of a starfish; the locals just call them stars. None of the
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currency exchanges seem to have heard of them, but somehow,
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you'll need to find fifty of these coins by the time you
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arrive so you can pay the deposit on your room.
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To save your vacation, you need to get all fifty stars by
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December 25th.
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Collect stars by solving puzzles. Two puzzles will be made
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available on each day in the Advent calendar; the second
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puzzle is unlocked when you complete the first. Each puzzle
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grants one star. Good luck!
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Before you leave, the Elves in accounting just need you to
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fix your expense report (your puzzle input); apparently,
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something isn't quite adding up.
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Specifically, they need you to find the two entries that
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sum to 2020 and then multiply those two numbers together.
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For example, suppose your expense report contained the
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following:
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1721
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979
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366
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299
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675
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1456
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In this list, the two entries that sum to 2020 are 1721
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and 299. Multiplying them together produces
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1721 \* 299 = 514579, so the correct answer is 514579.
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Of course, your expense report is much larger. Find the
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two entries that sum to 2020; what do you get if you
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multiply them together?
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----
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The first step is to get the input into something I
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can work with. My sample input has 200 entries, so I
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can fit them all on the stack, which means it's easy
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to create an array with them.
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Put your input into a file named `input-day-1`.
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~~~
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{ 'input-day-1 [ s:to-number ] file:for-each-line }
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'INPUT const
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~~~
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And then a simple approach is to just loop through the
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input twice, looking for any two values that add up to
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2020.
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----
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~~~
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:2020? dup-pair + #2020 eq? ;
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:display dup-pair * n:put nl ;
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INPUT [ INPUT [ 2020? [ display ] if drop ] a:for-each drop ] a:for-each
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~~~
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The second part is just slightly different:
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----
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The Elves in accounting are thankful for your help; one
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of them even offers you a starfish coin they had left
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over from a past vacation. They offer you a second one
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if you can find three numbers in your expense report that
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meet the same criteria.
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Using the above example again, the three entries that
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sum to 2020 are 979, 366, and 675. Multiplying them
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together produces the answer, 241861950.
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In your expense report, what is the product of the
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three entries that sum to 2020?
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----
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For this, I just add an additional iteration through the
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input. This can be *slow*, but does work. To save time,
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I `abort` after a match is found, so processing stops at
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the first match.
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~~~
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:dup-three 'abc 'abcabc reorder ;
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:2020? dup-three + + #2020 eq? ;
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:display dup-three * * n:put nl abort ;
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:inner INPUT [ 2020? [ display ] if drop ] a:for-each drop ;
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INPUT [ INPUT [ inner ] a:for-each drop ] a:for-each
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~~~
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