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commit 8cb020b3b128d5dbeb11ef758af80d2c44a12f9b
parent 63bcc7603d6b31bd55b232f137e0e24c87aa2eeb
Author: Luís Ferreira <contact@lsferreira.net>
Date:   Fri, 17 Dec 2021 23:35:06 +0000

posts: Add 'Solution in D for Advent of Code 2021, Day 7'

Signed-off-by: Luís Ferreira <contact@lsferreira.net>

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content/posts/zet-8-aoc-2021-07.md
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+---
+title: 'Zettelkasten #8: Solution in D for Advent of Code 2021, Day 7'
+date: '2021-12-17T22:25:00+01:00'
+tags: ['zettelkasten', 'zet', 'dlang', 'aoc', 'aoc2021', 'adventofcode']
+description: "This post describes my solution in the D programming language for
+the 7th puzzle of the Advent of Code 2021."
+---
+
+## The challenge
+
+> A giant whale has decided your submarine is its next meal, and it's much
+> faster than you are. There's nowhere to run!
+>
+> Suddenly, a swarm of crabs (each in its own tiny submarine - it's too deep
+> for them otherwise) zooms in to rescue you! They seem to be preparing to
+> blast a hole in the ocean floor; sensors indicate a massive underground cave
+> system just beyond where they're aiming!
+>
+> The crab submarines all need to be aligned before they'll have enough power
+> to blast a large enough hole for your submarine to get through. However, it
+> doesn't look like they'll be aligned before the whale catches you! Maybe you
+> can help?
+>
+> There's one major catch - crab submarines can only move horizontally.
+>
+> You quickly make a list of the horizontal position of each crab (your puzzle
+> input). Crab submarines have limited fuel, so you need to find a way to make
+> all of their horizontal positions match while requiring them to spend as
+> little fuel as possible.
+>
+> [...]
+>
+> Determine the horizontal position that the crabs can align to using the least
+> fuel possible. How much fuel must they spend to align to that position?
+
+You can read the challenge more in depth,
+[here](https://adventofcode.com/2021/day/7).
+
+## Part 1
+
+### Full solution
+
+```d
+auto input = stdin.byLineCopy().front.splitter(",").map!(to!long); // input
+input.map!(a => input.map!(b => abs(a-b)).sum).minElement.writeln; // calculate costs
+```
+
+## Part 2
+
+> The crabs don't seem interested in your proposed solution. Perhaps you
+> misunderstand crab engineering?
+>
+> As it turns out, crab submarine engines don't burn fuel at a constant rate.
+> Instead, each change of 1 step in horizontal position costs 1 more unit of
+> fuel than the last: the first step costs 1, the second step costs 2, the
+> third step costs 3, and so on.
+>
+> [...]
+>
+> Determine the horizontal position that the crabs can align to using the least
+> fuel possible so they can make you an escape route! How much fuel must they
+> spend to align to that position?
+
+### Full solution
+
+```d
+auto input = stdin.byLineCopy().front.splitter(",").map!(to!long).array; // input
+iota(input.maxElement).map!(a => input                                   // iter 0 to max element
+    .map!(b => abs(a-b)*(abs(a-b)+1)/2).sum).minElement.writeln;         // calculate costs
+```