docs(2025): day 06 writeup

Author: scarf005

docs/2025/puzzles/day06.md

@@ -2,10 +2,245 @@ import Solver from "../../../../../website/src/components/Solver.js"
 
 # Day 6: Trash Compactor
 
+by [@scarf005](https://github.com/scarf005)
+
 ## Puzzle description
 
 https://adventofcode.com/2025/day/6
 
+## Solution Summary
+
+Processing data row-by-row is usually straightforward, but handling columns can be tricky.
+Here comes [transpose](https://www.scala-lang.org/api/3.7.4/scala/collection/IterableOps.html#transpose-5d3) to the rescue!
+The `transpose` method switches the rows and columns of a 2D collection, which is exactly what we need for this puzzle.
+
+## Part 1
+
+```
+123 328  51 64 
+ 45 64  387 23 
+  6 98  215 314
+*   +   *   +
+```
+
+To make working with columns easier, we'd like the inputs to be like this:
+
+```
+123  45   6 *
+328  64  98 +
+ 51 387 215 *
+ 64  23 314 +
+```
+
+First, let's split the input into a 2D grid:
+
+```scala
+def part1(input: String) = input.linesIterator.toVector // split input into lines
+  .map(_.trim.split(raw"\s+")) // split lines into words by whitespaces
+
+// Vector(
+//   Array(123, 328, 51, 64),
+//   Array(45, 64, 387, 23),
+//   Array(6, 98, 215, 314), 
+//   Array(*, +, *, +)
+// )
+```
+
+After we transpose it, we get the desired output:
+
+```scala
+def part1(input: String) = input.linesIterator.toVector
+  .map(_.trim.split(raw"\s+"))
+  .transpose
+
+// Vector(
+//   Vector(123, 45, 6, *),
+//   Vector(328, 64, 98, +),
+//   Vector(51, 387, 215, *),
+//   Vector(64, 23, 314, +)
+// )
+```
+
+Now it's a matter of processing each column, which is fairly straightforward.
+Let's define an [extension method](https://docs.scala-lang.org/scala3/reference/contextual/extension-methods.html) to improve readability.
+
+```scala
+extension (xs: IterableOnce[(symbol: String, nums: IterableOnce[String])])
+  inline def calculate: Long = xs.iterator.collect {
+    case ("*", nums) => nums.iterator.map(_.toLong).product
+    case ("+", nums) => nums.iterator.map(_.toLong).sum
+  }.sum
+```
+
+Finally:
+
+```scala
+def part1(input: String): Long = input.linesIterator.toVector
+  .map(_.trim.split(raw"\s+"))
+  .transpose
+  .iterator
+  .map { col => (col.last, col.init) }
+  .calculate
+```
+
+## Part 2
+
+This time it's tricky, but what if we transpose the entire input string?
+
+```
+123 328  51 64 
+ 45 64  387 23 
+  6 98  215 314
+*   +   *   +
+```
+
+would become
+
+```
+1  *
+24
+356
+
+369+
+248
+8
+
+ 32*
+581
+175
+
+623+
+431
+  4
+```
+
+Which is exactly what we need to calculate cephalopod math!
+
+```scala
+def part2(input: String) =
+  val lines = input.linesIterator.toVector // get list of lines
+  val ops = lines.last.split(raw"\s+").toVector // we'll use them later
+  lines
+    .init // transposing requires all rows to be of equal length, so remove symbols from last line for simplicity
+    .transpose
+
+// Vector(
+//   Vector(1,  ,  ),
+//   Vector(2, 4,  ),
+//   Vector(3, 5, 6),
+//   Vector( ,  ,  ),
+//   Vector(3, 6, 9),
+//   Vector(2, 4, 8),
+//   Vector(8,  ,  ),
+//   Vector( ,  ,  ),
+//   Vector( , 3, 2),
+//   Vector(5, 8, 1),
+//   Vector(1, 7, 5),
+//   Vector( ,  ,  ),
+//   Vector(6, 2, 3),
+//   Vector(4, 3, 1),
+//   Vector( ,  , 4)
+// )
+```
+
+Now we can easily convert each column into cephalopod number strings:
+
+```scala
+def part2(input: String) =
+  val lines = input.linesIterator.toVector // get list of lines
+  val ops = lines.last.split(raw"\s+").toVector // we'll use them later
+  lines.init.transpose.map(_.mkString.trim)
+
+// Vector(
+//   "1",
+//   "24",
+//   "356",
+//   "",
+//   "369",
+//   "248",
+//   "8",
+//   "",
+//   "32",
+//   "581",
+//   "175",
+//   "",
+//   "623",
+//   "431",
+//   "4",
+// )
+```
+
+The only thing left is to split this Vector by separator `""`. Sadly, the scala
+standard library doesn't have an `Iterable.splitBy` method (yet!), so we'll define our own:
+
+```scala
+extension [A](xs: IterableOnce[A])
+  // we're using Vector.newBuilder to build the result efficiently
+  inline def splitBy(sep: A) =
+    val (b, cur) = (Vector.newBuilder[Vector[A]], Vector.newBuilder[A]) // b stores the result, cur stores the current chunk
+    for e <- xs.iterator do
+      if e != sep then cur += e // if current element is not the separator, add it to the current chunk
+      else { b += cur.result(); cur.clear() } // else, append the current chunk to result and clear it
+    (b += cur.result()).result() // finally, append the last chunk and return the result
+```
+
+```scala
+def part2(input: String) =
+  val lines = input.linesIterator.toVector // get list of lines
+  val ops = lines.last.split(raw"\s+").toVector // we'll use them later
+  lines.init.transpose.map(_.mkString.trim).splitBy("")
+
+// Vector(
+//   Vector(1, 24, 356),
+//   Vector(369, 248, 8),
+//   Vector(32, 581, 175),
+//   Vector(623, 431, 4)
+// )
+```
+
+Reusing the `calculate` extension method from part 1, we can now finish part 2:
+
+```scala
+def part2(input: String): Long =
+  val lines = input.linesIterator.toVector
+  val ops = lines.last.split(raw"\s+").toVector
+  val xss = lines.init.transpose.map(_.mkString.trim).splitBy("")
+
+  (ops lazyZip xss) // zip the operations with the chunks, lazily for efficiency
+    .calculate
+```
+
+## Final Code
+
+```scala
+extension [A](xs: IterableOnce[A])
+  inline def splitBy(sep: A) =
+    val (b, cur) = (Vector.newBuilder[Vector[A]], Vector.newBuilder[A])
+    for e <- xs.iterator do
+      if e != sep then cur += e else { b += cur.result(); cur.clear() }
+    (b += cur.result()).result()
+
+extension (xs: IterableOnce[(symbol: String, nums: IterableOnce[String])])
+  inline def calculate: Long = xs.iterator.collect {
+    case ("*", nums) => nums.iterator.map(_.toLong).product
+    case ("+", nums) => nums.iterator.map(_.toLong).sum
+  }.sum
+
+def part1(input: String): Long = input.linesIterator.toVector
+  .map(_.trim.split(raw"\s+"))
+  .transpose
+  .iterator
+  .map { col => (col.last, col.view.init) }
+  .calculate
+
+def part2(input: String): Long =
+  val lines = input.linesIterator.toVector
+  val ops = lines.last.split(raw"\s+").toVector
+  val xss = lines.init.transpose.map(_.mkString.trim).splitBy("")
+
+  (ops lazyZip xss).calculate
+```
+
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