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commit 63bcc7603d6b31bd55b232f137e0e24c87aa2eeb
parent 82ebf9678c26287d141b8e57b558d33a3cda294b
Author: Luís Ferreira <contact@lsferreira.net>
Date:   Fri, 17 Dec 2021 23:23:34 +0000

posts: Add 'Solution in D for Advent of Code 2021, Day 6'

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

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+---
+title: 'Zettelkasten #7: Solution in D for Advent of Code 2021, Day 6'
+date: '2021-12-17T22:19:00+01:00'
+tags: ['zettelkasten', 'zet', 'dlang', 'aoc', 'aoc2021', 'adventofcode']
+description: "This post describes my solution in the D programming language for
+the 6th puzzle of the Advent of Code 2021."
+---
+
+## The challenge
+
+### Part 1
+
+> The sea floor is getting steeper. Maybe the sleigh keys got carried this way?
+>
+> A massive school of glowing lanternfish swims past. They must spawn quickly
+> to reach such large numbers - maybe exponentially quickly? You should model
+> their growth rate to be sure.
+>
+> Although you know nothing about this specific species of lanternfish, you
+> make some guesses about their attributes. Surely, each lanternfish creates a
+> new lanternfish once every 7 days.
+>
+> However, this process isn't necessarily synchronized between every
+> lanternfish - one lanternfish might have 2 days left until it creates another
+> lanternfish, while another might have 4. So, you can model each fish as a
+> single number that represents the number of days until it creates a new
+> lanternfish.
+>
+> Furthermore, you reason, a new lanternfish would surely need slightly longer
+> before it's capable of producing more lanternfish: two more days for its
+> first cycle.
+>
+> So, suppose you have a lanternfish with an internal timer value of 3:
+>
+> - After one day, its internal timer would become 2.
+> - After another day, its internal timer would become 1.
+> - After another day, its internal timer would become 0.
+> - After another day, its internal timer would reset to 6, and it would create
+>   a new lanternfish with an internal timer of 8.
+> - After another day, the first lanternfish would have an internal timer of 5,
+>   and the second lanternfish would have an internal timer of 7.
+>
+> A lanternfish that creates a new fish resets its timer to 6, not 7 (because 0
+> is included as a valid timer value). The new lanternfish starts with an
+> internal timer of 8 and does not start counting down until the next day.
+>
+> Realizing what you're trying to do, the submarine automatically produces a
+> list of the ages of several hundred nearby lanternfish (your puzzle input).
+>
+> [...]
+>
+> Find a way to simulate lanternfish. How many lanternfish would there be after
+> 80 days?
+
+You can read the challenge more in depth,
+[here](https://adventofcode.com/2021/day/6).
+
+### Part 2
+
+> Suppose the lanternfish live forever and have unlimited food and space. Would
+> they take over the entire ocean?
+>
+> [...]
+>
+> How many lanternfish would there be after 256 days?
+
+## Full solution
+
+```d
+auto input = stdin.byLineCopy().front.splitter(",").map!(to!ubyte).array; // input
+auto count = iota(0,9).map!(s => input.count!(a => a == s)).array;        // count fish lifes
+iota(0,256).each!((d){count[(d+7)%9]+=count[d%9];});                      // calculate number of fishes per day
+count.sum.writeln;                                                        // sum all fishes
+```