Merge pull request #554 from spamegg1/patch-17

Solution article for 2024 Day 01

Author: gkepka

docs/2024/puzzles/day01.md

@@ -2,10 +2,71 @@ import Solver from "../../../../../website/src/components/Solver.js"
 
 # Day 1: Historian Hysteria
 
+by [@spamegg1](https://github.com/spamegg1)
+
 ## Puzzle description
 
 https://adventofcode.com/2024/day/1
 
+## Solution Summary
+
+1. Parse the input to split it into two lists (left/right), each sorted in increasing order.
+2. Find the distance scores (for `part1`) and the similarity scores (for `part2`).
+3. Sum the scores.
+
+### Parsing
+
+Our parser iterates over the lines, extracts the pair of numbers from each line,
+then splits them into two lists (lefts and rights), and separately sorts the lists.
+Therefore it looks like this:
+
+```scala
+def parse(input: String): (Seq[Long], Seq[Long]) =
+  // Extract pairs of numbers from each line
+  val pairs = input
+    .linesIterator
+    .map(line => line.split("   ").map(_.toLong))
+    .toSeq
+
+  // Group the left and right members from each pair, sort them
+  val lefts = pairs.map(_.head).toSeq.sorted
+  val rights = pairs.map(_.last).toSeq.sorted
+  (lefts, rights)
+```
+
+### Part 1
+
+Now that the lefts and rights are sorted in increasing order, we can zip them,
+so that the first smallest on the left is paired with the first smallest on the right,
+the second smallest on the left is paired with the second smallest on the right, and so on.
+Then we can find the distances between them, and sum the distances:
+
+```scala
+def part1(input: String): Long =
+  val (lefts, rights) = parse(input)
+  lefts
+    .zip(rights)
+    .map((left, right) => math.abs(left - right)) // distances
+    .sum
+end part1
+```
+
+### Part 2
+
+Very similar, but instead of distances, we find a left number's similarity on the right list.
+We do this by counting how many times the left number occurs on the right list,
+then multiply that count by the number itself.
+Finally we sum the similarity scores of all the left numbers:
+
+```scala
+def part2(input: String): Long =
+  val (lefts, rights) = parse(input)
+  lefts
+    .map(left => rights.count(_ == left) * left) // similarity scores
+    .sum
+end part2
+```
+
 ## Solutions from the community
 
 - [Solution](https://github.com/rmarbeck/advent2024/tree/main/day1) by [Raphaël Marbeck](https://github.com/rmarbeck)